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* Smalltalk: (gst).             The GNU Smalltalk user's guide.
END-INFO-DIR-ENTRY

     Permission is granted to copy, distribute and/or modify this
     document under the terms of the GNU Free Documentation License,
     Version 1.2 or any later version published by the Free Software
     Foundation; with no Invariant Sections, with no Front-Cover Texts,
     and with no Back-Cover Texts.  A copy of the license is included
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File: gst.info,  Node: Top,  Up: (DIR)

GNU Smalltalk User's Guide
**************************

GNU Smalltalk User's Guide

   This document describes installing and operating the GNU Smalltalk
programming environment.

     Permission is granted to copy, distribute and/or modify this
     document under the terms of the GNU Free Documentation License,
     Version 1.2 or any later version published by the Free Software
     Foundation; with no Invariant Sections, with no Front-Cover Texts,
     and with no Back-Cover Texts.  A copy of the license is included
     in the section entitled "GNU Free Documentation License".

* Menu:


* Overview::            What GNU Smalltalk is.
* Using GNU Smalltalk:: Running GNU Smalltalk.
* Features::            A description of GNU Smalltalk's special features.
* Packages::            An easy way to install Smalltalk code into an image.
* Emacs::               GNU Smalltalk and Emacs.
* C and Smalltalk::     GNU Smalltalk's C/Smalltalk interoperability features.
* Tutorial::            An introduction to Smalltalk and OOP.

--- The detailed node listing ---

Using GNU Smalltalk:
* Invocation::          What you can specify on the command line.
* Operation::           A step-by-step description of the
                        startup process and a short description of
                        how to interact with GNU Smalltalk.
* Syntax::              A description of the input file syntax
* Test suite::          How to run the test suite system.
* Legal concerns::      Licensing of GNU Smalltalk

Operation:
* Command-line processing::        Picking an image path and a kernel path.
* Loading or creating an image::   Loading an image or creating a new one.
* Starting the system::            After the image is created or restored.

Legal concerns:
* GPL::                 Complying with the GNU GPL.
* LGPL::                Complying with the GNU LGPL.

Features:
* Extended streams::    Extensions to streams, and generators
* Regular expressions:: String matching extensions
* Namespaces::          Avoiding clashes between class names.
* Disk file-IO::        Methods for reading and writing disk files.
* Object dumping::      Methods that read and write objects in binary format.
* Dynamic loading::     Picking external libraries and modules at run-time.
* Documentation::       Automatic documentation generation.
* Memory access::       The direct memory accessing classes and methods, plus
                        broadcasts from the virtual machine.
* GC::                  The GNU Smalltalk memory manager.
* Security::            Sandboxing and access control.
* Special objects::     Methods to assign particular properties to objects.

Packages
* GTK and VisualGST:                     GUI.
* Parser, STInST, Compiler:              Smalltalk-in-Smalltalk.
* DBI:                                   Database.
* I18N:                                  Locales.
* Seaside:                               Seaside.
* Swazoo:                                Swazoo.
* SUnit:                                 SUnit.
* Sockets, WebServer, NetClients:        Network support.
* XML, XPath, XSL:                       XML.
* Other packages:                        Other packages.

Emacs
* Editing::           Autoindent and more for GNU Smalltalk.
* Interactor::        Smalltalk interactor mode.

C and Smalltalk:
* External modules::        Linking your libraries to the virtual machine
* C callout::               Calls from Smalltalk to C
* C data types::            Manipulating C data from Smalltalk
* Smalltalk types::         Manipulating Smalltalk data from C
* Smalltalk callin::        Calls from C to Smalltalk
* Object representation::   Manipulating your own Smalltalk objects
* Incubator::               Protecting newly created objects from garbage
                            collections
* Other C functions::       Handling and creating OOPs
* Using Smalltalk::         The Smalltalk environment as an extension library

Tutorial:
* Getting started::              Starting to explore GNU Smalltalk
* Some classes::                 Using some of the Smalltalk classes
* The hierarchy::                The Smalltalk class hierarchy
* Creating classes::             Creating a new class of objects
* Creating subclasses::          Adding subclasses to another class
* Code blocks (I)::              Control structures in Smalltalk
* Code blocks (II)::             Guess what? More control structures
* Debugging::                    Things go bad in Smalltalk too!
* More subclassing::             Coexisting in the class hierarchy
* Streams::                      Something really powerful
* Exception handling::           More sophisticated error handling
* Behind the scenes::            Some nice stuff from the Smalltalk innards
* And now::                      Some final words
* The syntax::                   For the most die-hard computer scientists


File: gst.info,  Node: Overview,  Next: Using GNU Smalltalk,  Prev: Top,  Up: Top

Introduction
************

GNU Smalltalk is an implementation that closely follows the Smalltalk-80
language as described in the book `Smalltalk-80: the Language and its
Implementation' by Adele Goldberg and David Robson, which will
hereinafter be referred to as `the Blue Book'.

   The Smalltalk programming language is an object oriented programming
language.  This means, for one thing, that when programming you are
thinking of not only the data that an object contains, but also of the
operations available on that object.  The object's data representation
capabilities and the operations available on the object are
"inseparable"; the set of things that you can do with an object is
defined precisely by the set of operations, which Smalltalk calls
"methods", that are available for that object: each object belongs to a
"class" (a datatype and the set of functions that operate on it) or,
better, it is an "instance" of that class. You cannot even examine the
contents of an object from the outside--to an outsider, the object is a
black box that has some state and some operations available, but that's
all you know: when you want to perform an operation on an object, you
can only send it a "message", and the object picks up the method that
corresponds to that message.

   In the Smalltalk language, everything is an object.  This includes
not only numbers and all data structures, but even classes, methods,
pieces of code within a method ("blocks" or "closures"), stack frames
("contexts"), etc.  Even `if' and `while' structures are implemented as
methods sent to particular objects.

   Unlike other Smalltalks (including Smalltalk-80), GNU Smalltalk
emphasizes Smalltalk's rapid prototyping features rather than the
graphical and easy-to-use nature of the programming environment (did
you know that the first GUIs ever ran under Smalltalk?).  The
availability of a large body of system classes, once you master them,
makes it pretty easy to write complex programs which are usually a task
for the so called "scripting languages".  Therefore, even though we
have a GUI environment based on GTK (*note GTK and VisualGST: GUI.),
the goal of the GNU Smalltalk project is currently to produce a
complete system to be used to write your scripts in a clear,
aesthetically pleasing, and philosophically appealing programming
language.

   An example of what can be obtained with Smalltalk in this novel way
can be found in *note Class reference: (gst-libs)Top.  That part of the
manual is entirely generated by a Smalltalk program, starting from the
source code for the class libraries distributed together with the
system.


File: gst.info,  Node: Using GNU Smalltalk,  Next: Features,  Prev: Overview,  Up: Top

1 Using GNU Smalltalk
*********************

* Menu:

* Invocation::          What you can specify on the command line.
* Operation::           A step-by-step description of the
                        startup process and a short description of
                        how to interact with GNU Smalltalk.
* Syntax::              A description of the input file syntax
* Test suite::          How to run the test suite system.
* Legal concerns::      Licensing of GNU Smalltalk


File: gst.info,  Node: Invocation,  Next: Operation,  Up: Using GNU Smalltalk

1.1 Command line arguments
==========================

The GNU Smalltalk virtual machine may be invoked via the following
command:
     gst [ flags ... ] [ file ... ]

   When you invoke GNU Smalltalk, it will ensure that the binary image
file (called `gst.im') is up to date; if not, it will build a new one as
described in *note Loading an image or creating a new one: Loading or
creating an image.  Your first invocation should look something like
this:

     "Global garbage collection... done"
     GNU Smalltalk ready

     st>

   If you specify one or more FILEs, they will be read and executed in
order, and Smalltalk will exit when end of file is reached.  If you
don't specify FILE, GNU Smalltalk reads standard input, issuing a `st>'
prompt if the standard input is a terminal.  You may specify `-' for
the name of a file to invoke an explicit read from standard input.

   To exit while at the `st>' prompt, use `Ctrl-d', or type
`ObjectMemory quit' followed by <RET>.  Use `ObjectMemory snapshot'
first to save a new image that you can reload later, if you wish.

   As is standard for GNU-style options, specifying `--' stops the
interpretation of options so that every argument that follows is
considered a file name even if it begins with a `-'.

   You can specify both short and long flags; for example, `--version'
is exactly the same as `-v', but is easier to remember.  Short flags
may be specified one at a time, or in a group.  A short flag or a group
of short flags always starts off with a single dash to indicate that
what follows is a flag or set of flags instead of a file name; a long
flag starts off with two consecutive dashes, without spaces between
them.

   In the current implementation the flags can be intermixed with file
names, but their effect is as if they were all specified first.  The
various flags are interpreted as follows:

`-a'
`--smalltalk-args'
     Treat all options afterward as arguments to be given to Smalltalk
     code retrievable with `Smalltalk arguments', ignoring them as
     arguments to GNU Smalltalk itself.

     Examples:
     command line         Options seen by GNU Smalltalk   `Smalltalk arguments'
     (empty)              (none)                          `#()'
     `-Via foo bar'       `-Vi'                           `#('foo' 'bar')'
     `-Vai test'          `-Vi'                           `#('test')'
     `-Vaq'               `-Vq'                           `#()'
     `--verbose -aq -c  ' `--verbose -q'                  `#('-c')'

`-c'
`--core-dump'
     When a fatal signal occurs, produce a core dump before terminating.
     Without this option, only a backtrace is provided.

`-D'
`--declaration-trace'
     Print the class name, the method name, and the byte codes that the
     compiler generates as it compiles methods.  Only applies to files
     that are named explicitly on the command line, unless the flag is
     given multiple times on the command line.

`-E'
`--execution-trace'
     Print the byte codes being executed as the interpreter operates.
     Only works for statements explicitly issued by the user (either
     interactively or from files given on the command line), unless the
     flag is given multiple times on the command line.

`--kernel-directory'
     Specify the directory from which the kernel source files will be
     loaded.  This is used mostly while compiling GNU Smalltalk itself.
     Smalltalk code can retrieve this information with `Directory
     kernel'.

`--no-user-files'
     Don't load any files from `~/.st/' (*note Loading an image or
     creating a new one: Loading or creating an image.).(1) This is
     used mostly while compiling GNU Smalltalk itself, to ensure that
     the installed image is built only from files in the source tree.

`-K FILE'
`--kernel-file FILE'
     Load FILE in the usual way, but look for it relative to the kernel
     directory's parent directory, which is usually
     `/usr/local/share/smalltalk/'.  See `--kernel-dir' above.

`-f'
`--file'
     The following two command lines are equivalent:

          gst -f FILE `args...'
          gst -q FILE -a `args...'

     This is meant to be used in the so called "sharp-bang" sequence at
     the beginning of a file, as in

          #! /usr/bin/gst -f

          ... Smalltalk source code ...

     GNU Smalltalk treats the first line as a comment, and the `-f'
     option ensures that the arguments are passed properly to the
     script.  Use this instead to avoid hard-coding the path to
     `gst':(2)

          #! /bin/sh
          "exec" "gst" "-f" "$0" "$@"

          ... Smalltalk source code ...

`-g'
`--no-gc-messages'
     Suppress garbage collection messages.

`-h'
`--help'
     Print out a brief summary of the command line syntax of GNU
     Smalltalk, including the definitions of all of the option flags,
     and then exit.

`-i'
`--rebuild-image'
     Always build and save a new image file; see *note Loading an image
     or creating a new one: Loading or creating an image.

`--maybe-rebuild-image'
     Perform the image checks and rebuild as described in *note Loading
     an image or creating a new one: Loading or creating an image.
     This is the default when `-I' is not given.

`-I FILE'
`--image-file FILE'
     Use the image file named FILE as the image file to load instead of
     the default location, and set FILE's directory part as the image
     path.  This option completely bypasses checking the file dates on
     the kernel files; use `--maybe-rebuild-image' to restore the usual
     behavior, writing the newly built image to FILE if needed.

`-q'
`--quiet'
`--silent'
     Suppress the printing of answered values from top-level expressions
     while GNU Smalltalk runs.

`-r'
`--regression-test'
     This is used by the regression testing system and is probably not
     of interest to the general user.  It controls printing of certain
     information.

`-S'
`--snapshot'
     Save the image after loading files from the command line.  Of
     course this "snapshot" is not saved if you include - (stdin) on
     the command line and exit by typing `Ctrl-c'.

`-v'
`--version'
     Print out the GNU Smalltalk version number, then exit.

`-V'
`--verbose'
     Print various diagnostic messages while executing (the name of
     each file as it's loaded, plus messages about the beginning of
     execution or how many byte codes were executed).

   ---------- Footnotes ----------

   (1) The directory would be called `_st/' under MS-DOS.  Under OSes
that don't use home directories, it would be looked for in the current
directory.

   (2) The words in the shell command `exec' are all quoted, so GNU
Smalltalk parses them as five separate comments.


File: gst.info,  Node: Operation,  Next: Syntax,  Prev: Invocation,  Up: Using GNU Smalltalk

1.2 Startup sequence
====================

*Caveat*: _The startup sequence is pretty complicated.  If you are not
interested in its customization, you can skip the first two sections
below.  These two sections also don't apply when using the command-line
option `-I', unless also using `--maybe-rebuild-image'._

   You can abort GNU Smalltalk at any time during this procedure with
`Ctrl-c'.

* Menu:

* Command-line processing::        Picking an image path and a kernel path.
* Loading or creating an image::   Loading an image or creating a new one.
* Starting the system::            After the image is created or restored.


File: gst.info,  Node: Command-line processing,  Next: Loading or creating an image,  Up: Operation

1.2.1 Picking an image path and a kernel path
---------------------------------------------

When GNU Smalltalk is invoked, it first chooses two paths, the "image
path" and the "kernel path".  The image path is set by considering these
paths in succession:
   * the directory part of the `--image-file' option if it is   given;

   * the value of the `SMALLTALK_IMAGE' environment variable   if it is
     defined and readable; this step will disappear in a future
     release;

   * the path compiled in the binary (usually, under Unix systems,
     `/usr/local/var/lib/smalltalk' or a similar path under `/var')
     if it exists and it is readable;

   * the current directory.  The current directory is also used if
     the image has to be rebuilt but you cannot write to a directory
     chosen according to the previous criteria.

   The "kernel path" is the directory in which to look for Smalltalk
code compiled into the base image.  The possibilities in this case are:
   * the argument to the `--kernel-dir' option if it is given;

   * the value of the `SMALLTALK_KERNEL' environment variable   if it
     is defined and readable; this step will disappear in a future
     release;

   * the path compiled in the binary (usually, under Unix systems,
     `/usr/local/share/smalltalk/kernel' or a similar data file path)
     if it exists and it is readable;

   * a subdirectory named `kernel' of the image path.


File: gst.info,  Node: Loading or creating an image,  Next: Starting the system,  Prev: Command-line processing,  Up: Operation

1.2.2 Loading an image or creating a new one
--------------------------------------------

GNU Smalltalk can load images created on any system with the same
pointer size as its host system by approximately the same version of
GNU Smalltalk, even if they have different endianness.  For example,
images created on 32-bit PowerPC can be loaded with a 32-bit x86 `gst'
VM, provided that the GNU Smalltalk versions are similar enough.  Such
images are called "compatible images".  It cannot load images created on
systems with different pointer sizes; for example, our x86 `gst' cannot
load an image created on x86-64.

   Unless the `-i' flag is used, GNU Smalltalk first tries to load the
file named by `--image-file', defaulting to `gst.im' in the image path.
If this is found, GNU Smalltalk ensures the image is "not stale",
meaning its write date is newer than the write dates of all of the
kernel method definition files.  It also ensures that the image is
"compatible", as described above.  If both tests pass, GNU Smalltalk
loads the image and continues with *note After the image is created or
restored: Starting the system.

   If that fails, a new image has to be created.  The image path may
now be changed to the current directory if the previous choice is not
writeable.

   To build an image, GNU Smalltalk loads the set of files that make up
the kernel, one at a time.  The list can be found in `libgst/lib.c', in
the `standard_files' variable.  You can override kernel files by
placing your own copies in `~/.st/kernel/'.(1)  For example, if you
create a file `~/.st/kernel/Builtins.st', it will be loaded instead of
the `Builtins.st' in the kernel path.

   To aid with image customization and local bug fixes, GNU Smalltalk
loads two more files (if present) before saving the image.  The first is
`site-pre.st', found in the parent directory of the kernel directory.
Unless users at a site change the kernel directory when running `gst',
`/usr/local/share/smalltalk/site-pre.st' provides a convenient place
for site-wide customization.  The second is `~/.st/pre.st', which can
be different for each user's home directory.(2).

   Before the next steps, GNU Smalltalk takes a snapshot of the new
memory image, saving it over the old image file if it can, or in the
current directory otherwise.

   ---------- Footnotes ----------

   (1) The directory is called `_st/kernel' under MS-DOS.  Under OSes
that don't use home directories, it is looked for in the current
directory.

   (2) The file is looked up as `_st/pre.st' under MS-DOS and again,
under OSes that don't use home directories it is looked for as `pre.st'
in the current directory.


File: gst.info,  Node: Starting the system,  Prev: Loading or creating an image,  Up: Operation

1.2.3 After the image is created or restored
--------------------------------------------

Next, GNU Smalltalk sends the `returnFromSnapshot' event to the
dependents of the special class `ObjectMemory' (*note Memory access::).
Afterwards, it loads `~/.st/init.st' if available.(1)

   You can remember the difference between `pre.st' and `init.st' by
remembering that `pre.st' is the _pre_-snapshot file and `init.st' is
the post-image-load _init_ialization file.

   Finally, GNU Smalltalk loads files listed on the command line, or
prompts for input at the terminal, as described in *note Command line
arguments: Invocation.

   ---------- Footnotes ----------

   (1) The same considerations made above hold here too.  The file is
called `_st/init.st' under MS-DOS, and is looked for in the current
directory under OSes that don't use home directories.


File: gst.info,  Node: Syntax,  Next: Test suite,  Prev: Operation,  Up: Using GNU Smalltalk

1.3 Syntax of GNU Smalltalk
===========================

The language that GNU Smalltalk accepts is basically the same that
other Smalltalk environment accept and the same syntax used in the
"Blue Book", also known as `Smalltalk-80: The Language and Its
Implementation'.  The return operator, which is represented in the Blue
Book as an up-arrow, is mapped to the ASCII caret symbol `^'; the
assignment operator (left-arrow) is usually represented as `:='(1).

   Actually, the grammar of GNU Smalltalk is slightly different from
the grammar of other Smalltalk environments in order to simplify
interaction with the system in a command-line environment as well as in
full-screen editors.

   Statements are executed one by one; multiple statements are
separated by a period.  At end-of-line, if a valid statement is
complete, a period is implicit.  For example,

     8r300. 16rFFFF
   prints out the decimal value of octal `300' and hex `FFFF', each
followed by a newline.

   Multiple statements share the same local variables, which are
automatically declared.  To delete the local variables, terminate a
statement with `!' rather than `.' or newline.  Here,

     a := 42
     a!
     a
   the first two `a's are printed as `42', but the third one is
uninitialized and thus printed as `nil'.

   In order to evaluate multiple statements in a single block, wrap
them into an "eval block" as follows:
     Eval [
         a := 42.  a printString
     ]
   This won't print the intermediate result (the integer 42), only the
final result (the string `'42'').

     ObjectMemory quit
   exits from the system.  You can also type a `C-d' to exit from
Smalltalk if it's reading statements from standard input.

   GNU Smalltalk provides three extensions to the language that make it
simpler to write complete programs in an editor.  However, it is also
compatible with the "file out" syntax as shown in the "Green Book"
(also known as `Smalltalk-80: Bits of History, Words of Advice' by
Glenn Krasner).

   A new class is created using this syntax:

     SUPERCLASS-NAME subclass: NEW-CLASS-NAME [
     | INSTANCE VARIABLES |
     PRAGMAS
     MESSAGE-PATTERN-1 [ STATEMENTS ]
     MESSAGE-PATTERN-2 [ STATEMENTS ]
     ...
     CLASS-VARIABLE-1 := EXPRESSION.
     CLASS-VARIABLE-2 := EXPRESSION.
     ...
     ]

   In short:
   * Instance variables are defined with the same syntax as method
     temporary   variables.

   * Unlike other Smalltalks, method statements are inside brackets.

   * Class variables are defined the same as variable assignments.

   * Pragmas define class comment, class category, imported
     namespaces, and the shape of indexed instance variables.

          <comment: 'Class comment'>
          <category: 'Examples-Intriguing'>
          <import: SystemExceptions>
          <shape: #pointer>

   A similar syntax is used to define new methods in an existing class.

     CLASS-EXPRESSION extend [
     ...
     ]

   The CLASS-EXPRESSION is an expression that evaluates to a class
object, which is typically just the name of a class, although it can be
the name of a class followed by the word `class', which causes the
method definitions that follow to apply to the named class itself,
rather than to its instances.

     Number extend [
         radiusToArea [
             ^self squared * Float pi
         ]
         radiusToCircumference [
             ^self * 2 * Float pi
         ]
     ]

   A complete treatment of the Smalltalk syntax and of the class library
can be found in the included tutorial and class reference (*note Class
Reference: (gst-base)Top.).

   More information on the implementation of the language can be found
in the `Blue Book'; the relevant parts are available, scanned, at
`http://stephane.ducasse.free.fr/FreeBooks/BlueBook/Bluebook.pdf'.

   ---------- Footnotes ----------

   (1) It   also bears mentioning that there are two assignment
operators:   `_' and `:='.  Both are usable interchangeably, provided
that   they are surrounded by spaces.  The GNU Smalltalk kernel code
uses the   `:=' form exclusively, but `_' is supported a) for
compatibility with previous versions of GNU Smalltalk b) because this
is the   correct mapping between the assignment operator mentioned in
the Blue   Book and the current ASCII definition.  In the ancient days
(like the   middle 70's), the ASCII underscore character was also
printed as a   back-arrow, and many terminals would display it that
way, thus its   current usage.  Anyway, using `_' may lead to
portability problems.


File: gst.info,  Node: Test suite,  Next: Legal concerns,  Prev: Syntax,  Up: Using GNU Smalltalk

1.4 Running the test suite
==========================

GNU Smalltalk comes with a set of files that provides a simple
regression test suite.

   To run the test suite, you should be connected to the top-level
Smalltalk directory.  Type
     make check

   You should see the names of the test suite files as they are
processed, but that's it.  Any other output indicates some problem.


File: gst.info,  Node: Legal concerns,  Prev: Test suite,  Up: Using GNU Smalltalk

1.5 Licensing of GNU Smalltalk
==============================

Different parts of GNU Smalltalk comes under two licenses: the virtual
machine and the development environment (compiler and browser) come
under the GNU General Public License, while the system class libraries
come under the Lesser General Public License.

* Menu:

* GPL::        Complying with the GNU GPL.
* LGPL::       Complying with the GNU LGPL.


File: gst.info,  Node: GPL,  Next: LGPL,  Up: Legal concerns

1.5.1 Complying with the GNU GPL
--------------------------------

The GPL licensing of the virtual machine means that all derivatives of
the virtual machine must be put under the same license.  In other
words, it is strictly forbidden to distribute programs that include the
GNU Smalltalk virtual machine under a license that is not the GPL.
This also includes any bindings to external libraries.  For example,
the bindings to Gtk+ are released under the GPL.

   In principle, the GPL would not extend to Smalltalk programs, since
these are merely input data for the virtual machine.  On the other
hand, using bindings that are under the GPL via dynamic linking would
constitute combining two parts (the Smalltalk program and the bindings)
into one program.  Therefore, we added a special exception to the GPL
in order to avoid gray areas that could adversely hit both the project
and its users:

     In addition, as a special exception, the Free Software Foundation
     give you permission to combine GNU Smalltalk with free software
     programs or libraries that are released under the GNU LGPL and with
     independent programs running under the GNU Smalltalk virtual
     machine.

     You may copy and distribute such a system following the terms of
     the GNU GPL for GNU Smalltalk and the licenses of the other code
     concerned, provided that you include the source code of that other
     code when and as the GNU GPL requires distribution of source code.

     Note that people who make modified versions of GNU Smalltalk are
     not obligated to grant this special exception for their modified
     versions; it is their choice whether to do so.  The GNU General
     Public License gives permission to release a modified version
     without this exception; this exception also makes it possible to
     release a modified version which carries forward this exception.


File: gst.info,  Node: LGPL,  Prev: GPL,  Up: Legal concerns

1.5.2 Complying with the GNU LGPL
---------------------------------

Smalltalk programs that run under GNU Smalltalk are linked with the
system classes in GNU Smalltalk class library.  Therefore, they must
respect the terms of the Lesser General Public License(1).

   The interpretation of this license for architectures different from
that of the C language is often difficult; the accepted one for
Smalltalk is as follows.  The image file can be considered as an object
file, falling under Subsection 6a of the license, as long as it allows
a user to load an image, upgrade the library or otherwise apply
modifications to it, and save a modified image: this is most
conveniently obtained by allowing the user to use the read-eval-print
loop that is embedded in the GNU Smalltalk virtual machine.

   In other words, provided that you leave access to the loop in a
documented way, or that you provide a way to file in arbitrary files in
an image and save the result to a new image, you are obeying Subsection
6a of the Lesser General Public License, which is reported here:

     a) Accompany the work with the complete corresponding
     machine-readable source code for the Library including whatever
     changes were used in the work (which must be distributed under
     Sections 1 and 2 above); and, if the work is an executable linked
     with the Library, with the complete machine-readable "work that
     uses the Library", as object code and/or source code, so that the
     user can modify the Library and then relink to produce a modified
     executable containing the modified Library.  (It is understood
     that the user who changes the contents of definitions files in the
     Library will not necessarily be able to recompile the application
     to use the modified definitions.)

   In the future, alternative mechanisms similar to shared libraries may
be provided, so that it is possible to comply with the GNU LGPL in
other ways.

   ---------- Footnotes ----------

   (1) Of course, they may   be more constrained by usage of GPL class
libraries.


File: gst.info,  Node: Features,  Next: Packages,  Prev: Using GNU Smalltalk,  Up: Top

2 Features of GNU Smalltalk
***************************

In this section, the features which are specific to GNU Smalltalk are
described.  These features include support for calling C functions from
within Smalltalk, accessing environment variables, and controlling
various aspects of compilation and execution monitoring.

   Note that, in general, GNU Smalltalk is much more powerful than the
original Smalltalk-80, as it contains a lot of methods that are common
in today's Smalltalk implementation and are present in the ANSI
Standard for Smalltalk, but were absent in the Blue Book.  Examples
include Collection's `allSatisfy:' and `anySatisfy:' methods and many
methods in SystemDictionary (the Smalltalk dictionary's class).

* Menu:

* Extended streams::    Extensions to streams, and generators
* Regular expressions:: String matching extensions
* Namespaces::          Avoiding clashes between class names.
* Disk file-IO::        Methods for reading and writing disk files.
* Object dumping::      Methods that read and write objects in binary format.
* Dynamic loading::     Picking external libraries and modules at run-time.
* Documentation::       Automatic documentation generation.
* Memory access::       The direct memory accessing classes and methods, plus
                        broadcasts from the virtual machine.
* GC::                  The GNU Smalltalk memory manager.
* Security::            Sandboxing and access control.
* Special objects::     Methods to assign particular properties to objects.


File: gst.info,  Node: Extended streams,  Next: Regular expressions,  Up: Features

2.1 Extended streams
====================

The basic image in GNU Smalltalk includes powerful extensions to the
_Stream_ hierarchy found in ANSI Smalltalk (and Smalltalk-80).  In
particular:

   * Read streams support all the iteration protocols available for
     collections.  In some cases (like `fold:', `detect:',
     `inject:into:') these are completely identical.  For messages that
     return a new stream, such as `select:' and `collect:', the blocks
     are evaluated lazily, as elements are requested from the stream
     using `next'.

   * Read streams can be concatenated using `,' like
     SequenceableCollections.

   * "Generators" are supported as a quick way to create a Stream.  A
     generator is a kind of pluggable stream, in that a user-supplied
     blocks defines which values are in a stream.

     For example, here is an empty generator and two infinite
     generators:

          "Returns an empty stream"
          Generator on: [ :gen | ]

          "Return an infinite stream of 1's"
          Generator on: [ :gen | [ gen yield: 1 ] repeat ]

          "Return an infinite stream of integers counting up from 1"
          Generator inject: 1 into: [ :value | value + 1 ]

     The block is put "on hold" and starts executing as soon as `#next'
     or `#atEnd' are sent to the generator.  When the block sends
     `#yield:' to the generator, it is again put on hold and the
     argument becomes the next object in the stream.

     Generators use "continuations", but they shield the users from
     their complexity by presenting the same simple interface as
     streams.


File: gst.info,  Node: Regular expressions,  Next: Namespaces,  Prev: Extended streams,  Up: Features

2.2 Regular expression matching
===============================

_Regular expressions_, or "regexes", are a sophisticated way to
efficiently match patterns of text.  If you are unfamiliar with regular
expressions in general, see *note Syntax of Regular Expressions:
(emacs)Regexps, for a guide for those who have never used regular
expressions.

   GNU Smalltalk supports regular expressions in the core image with
methods on `String'.

   The GNU Smalltalk regular expression library is derived from GNU
libc, with modifications made originally for Ruby to support Perl-like
syntax.  It will always use its included library, and never the ones
installed on your system; this may change in the future in
backwards-compatible ways.  Regular expressions are currently 8-bit
clean, meaning they can work with any ordinary String, but do not
support full Unicode, even when package `I18N' is loaded.

   Broadly speaking, these regexes support Perl 5 syntax; register
groups `()' and repetition `{}' must not be given with backslashes, and
their counterpart literal characters should.  For example, `\{{1,3}'
matches `{', `{{', `{{{'; correspondingly, `(a)(\()' matches `a(', with
`a' and `(' as the first and second register groups respectively.  GNU
Smalltalk also supports the regex modifiers `imsx', as in Perl.  You
can't put regex modifiers like `im' after Smalltalk strings to specify
them, because they aren't part of Smalltalk syntax.  Instead, use the
inline modifier syntax.  For example, `(?is:abc.)' is equivalent to
`[Aa][Bb][Cc](?:.|\n)'.

   In most cases, you should specify regular expressions as ordinary
strings.  GNU Smalltalk always caches compiled regexes, and uses a
special high-efficiency caching when looking up literal strings (i.e.
most regexes), to hide the compiled `Regex' objects from most code.
For special cases where this caching is not good enough, simply send
`#asRegex' to a string to retrieved a compiled form, which works in all
places in the public API where you would specify a regex string.  You
should always rely on the cache until you have demonstrated that using
Regex objects makes a noticeable performance difference in your code.

   Smalltalk strings only have one escape, the `'' given by `''', so
backslashes used in regular expression strings will be understood as
backslashes, and a literal backslash can be given directly with `\\'(1).

   The methods on the compiled Regex object are private to this
interface.  As a public interface, GNU Smalltalk provides methods on
String, in the category `regex'.  There are several methods for
matching, replacing, pattern expansion, iterating over matches, and
other useful things.

   The fundamental operator is `#searchRegex:', usually written as
`#=~', reminiscent of Perl syntax.  This method will always return a
`RegexResults', which you can query for whether the regex matched, the
location Interval and contents of the match and any register groups as
a collection, and other features.  For example, here is a simple
configuration file line parser:

     | file config |
     config := LookupTable new.
     file := (File name: 'myapp.conf') readStream.
     file linesDo: [:line |
         (line =~ '(\w+)\s*=\s*((?: ?\w+)+)') ifMatched: [:match |
             config at: (match at: 1) put: (match at: 2)]].
     file close.
     config printNl.

   As with Perl, `=~' will scan the entire string and answer the
leftmost match if any is to be found, consuming as many characters as
possible from that position.  You can anchor the search with variant
messages like `#matchRegex:', or of course `^' and `$' with their usual
semantics if you prefer.

   You shouldn't modify the string while you want a particular
RegexResults object matched on it to remain valid, because changes to
the matched text may propagate to the RegexResults object.

   Analogously to the Perl `s' operator, GNU Smalltalk provides
`#replacingRegex:with:'.  Unlike Perl, GNU Smalltalk employs the
pattern expansion syntax of the `#%' message here.  For example, `'The
ratio is 16/9.' replacingRegex: '(\d+)/(\d+)' with: '$%1\over%2$''
answers `'The ratio is $16\over9$.''.  In place of the `g' modifier,
use the `#replacingAllRegex:with:' message instead.

   One other interesting String message is `#onOccurrencesOfRegex:do:',
which invokes its second argument, a block, on every successful match
found in the receiver.  Internally, every search will start at the end
of the previous successful match.  For example, this will print all the
words in a stream:

     stream contents onOccurrencesOfRegex: '\w+'
                     do: [:each | each match printNl]

   ---------- Footnotes ----------

   (1) Whereas it must be given as `\\\\' in a literal Emacs Lisp
string, for example.


File: gst.info,  Node: Namespaces,  Next: Disk file-IO,  Prev: Regular expressions,  Up: Features

2.3 Namespaces
==============

[This section (and the implementation of namespaces in GNU Smalltalk)
is based on the paper `Structured Symbolic Name Spaces in Smalltalk',
by Augustin Mrazik.]

2.3.1 Introduction
------------------

The Smalltalk-80 programming environment, upon which GNU Smalltalk is
historically based, supports symbolic identification of objects in one
global namespace--in the `Smalltalk' system dictionary. This means that
each global variable in the system has its unique name which is used
for symbolic identification of the particular object in the source code
(e.g. in expressions or methods).  The most important of these global
variables are classes defining the behavior of objects.

   In development dealing with modelling of real systems, "polymorphic
symbolic identification" is often needed.  By this, we mean that it
should be possible to use the same name for different classes or other
global variables.  Selection of the proper variable binding should be
context-specific.  By way of illustration, let us consider class
`Statement' as an example which would mean totally different things in
different domains:

GNU Smalltalk or other programming language
     An expression in the top level of a code body, possibly with
     special syntax available such as assignment or branching.

Bank
     A customer's trace report of recent transactions.

AI, logical derivation
     An assertion of a truth within a logical system.

   This issue becomes inevitable if we start to work persistently, using
`ObjectMemory snapshot' to save after each session for later
resumption.  For example, you might have the class `Statement' already
in your image with the "Bank" meaning above (e.g. in the live bank
support systems we all run in our images) and you might decide to start
developing YAC [Yet Another C].  Upon starting to write parse nodes for
the compiler, you would find that `#Statement' is boundk in the banking
package.  You could replace it with your parse node class, and the
bank's `Statement' could remain in the system as an unbound class with
full functionality; however, it could not be accessed anymore at the
symbolic level in the source code.  Whether this would be a problem or
not would depend on whether any of the bank's code refers to the class
`Statement', and when these references occur.

   Objects which have to be identified in source code by their names are
included in `Smalltalk', the sole instance of `SystemDictionary'.  Such
objects may be identified simply by writing their names as you would
any variable names.  The code is compiled in the default environment,
and if the variable is found in `Smalltalk', without being shadowed by
a class pool or local variables, its value is retrieved and used as the
value of the expression.  In this way `Smalltalk' represents the sole
symbolic namespace.  In the following text the symbolic namespace, as a
concept, will be called simply "environment" to make the text more
clear.

2.3.2 Concepts
--------------

To support polymorphic symbolical identification several environments
will be needed.  The same name may exist concurrently in several
environments as a key, pointing to diverse objects in each.

   Symbolic navigation between these environments is needed.  Before
approaching the problem of the syntax and semantics to be implemented,
we have to decide on structural relations to be established between
environments.

   Since the environment must first be symbolically identified to direct
access to its global variables, it must first itself be a global
variable in another environment.  `Smalltalk' is a great choice for the
root environment, from which selection of other environments and their
variables begins. From `Smalltalk' some of the existing
sub-environments may be seen; from these other sub-environments may be
seen, etc.  This means that environments represent nodes in a graph
where symbolic selections from one environment to another one represent
branches.

   The symbolic identification should be unambiguous, although it will
be polymorphic. This is why we should avoid cycles in the environment
graph. Cycles in the graph could cause also other problems in the
implementation, e.g. inability to use trivially recursive algorithms.
Thus, in general, the environments must build a directed acyclic graph;
GNU Smalltalk currently limits this to an n-ary tree, with the extra
feature that environments can be used as pool dictionaries.

   Let us call the partial ordering relation which occurs between
environments "inheritance".  Sub-environments inherit from their
super-environments.  The feature of inheritance in the meaning of
object-orientation is associated with this relation: all associations of
the super-environment are valid also in its sub-environments, unless
they are locally redefined in the sub-environment.

   A super-environment includes all its sub-enviroments as
`Association's under their names.  The sub-environment includes its
super-environment under the symbol `#Super'.  Most environments inherit
from `Smalltalk', the standard root environment, but they are not
required to do so; this is similar to how most classes derive from
`Object', yet one can derive a class directly from `nil'.  Since they
all inherit `Smalltalk''s global variables, it is not necessary to
define `Smalltalk' as pointing to `Smalltalk''s `Smalltalk' in each
environment.

   The inheritance links to the super-environments are used in the
lookup for a potentially inherited global variable.  This includes
lookups by a compiler searching for a variable binding and lookups via
methods such as `#at:' and `#includesKey:'.

2.3.3 Syntax
------------

Global objects of an environment, be they local or inherited, may be
referenced by their symbol variable names used in the source code, e.g.
                 John goHome

if the `#John -> aMan' association exists in the particular environment
or one of its super-environments, all along the way to the root
environment.

   If an object must be referenced from another environment (i.e. which
is not one of its sub-environments) it has to be referenced either
_relatively_ to the position of the current environment, using the
`Super' symbol, or _absolutely_, using the "full pathname" of the
object, navigating from the tree root (usually `Smalltalk') through the
tree of sub-environments.

   For the identification of global objects in another environment, we
use a "pathname" of symbols.  The symbols are separated by periods; the
"look" to appear is that of
     Smalltalk.Tasks.MyTask

and of
     Super.Super.Peter.

   As is custom in Smalltalk, we are reminded by capitalization that we
are accessing global objects.  Another syntax returns the "variable
binding", the `Association' for a particular global.  The first example
above is equivalently:
     #{Smalltalk.Tasks.MyTask} value

   The latter syntax, a "variable binding", is also valid inside
literal arrays.

2.3.4 Implementation
--------------------

A superclass of `SystemDictionary' called `RootNamespace' is defined,
and many of the features of the Smalltalk-80 `SystemDictionary' will be
hosted by that class.  `Namespace' and `RootNamespace' are in turn
subclasses of `AbstractNamespace'.

   To handle inheritance, the following methods have to be defined or
redefined in Namespace (_not_ in RootNamespace):

Accessors like `#at:ifAbsent:' and `#includesKey:'
     Inheritance must be implemented.  When `Namespace', trying to read
     a variable, finds an association in its own dictionary or a
     super-environment dictionary, it uses that; for `Dictionary''s
     writes and when a new association must be created, `Namespace'
     creates it in its own dictionary.  There are special methods like
     `#set:to:' for cases in which you want to modify a binding in a
     super-environment if that is the relevant variable's binding.

Enumerators like `#do:' and `#keys'
     This should return *all* the objects in the namespace, including
     those which are inherited.

Hierarchy access
     `AbstractNamespace' will also implement a new set of methods that
     allow one to navigate through the namespace hierarchy; these
     parallel those found in `Behavior' for the class hierarchy.

   The most important task of the `Namespace' class is to provide
organization for the most important global objects in the Smalltalk
system--for the classes.  This importance becomes even more crucial in
a structure of multiple environments intended to change the semantics of
code compiled for those classes.

   In Smalltalk the classes have the instance variable `name' which
holds the name of the class. Each "defined class" is included in
`Smalltalk', or another environment, under this name.  In a framework
with several environments the class should know the environment in
which it has been created and compiled.  This is a new property of
`Class' which must be defined and properly used in relevant methods.
In the mother environment the class shall be included under its name.

   Any class, as with any other object, may be included concurrently in
several environments, even under different symbols in the same or in
diverse environments.  We can consider these "alias names" of the
particular class or other value.  A class may be referenced under the
other names or in other environments than its mother environment, e.g.
for the purpose of instance creation or messages to the class, but it
should not compile code in these environments, even if this compilation
is requested from another environment.  If the syntax is not correct in
the mother environment, a compilation error occurs.  This follows from
the existence of class "mother environments", as a class is responsible
for compiling its own methods.

   An important issue is also the name of the class answered by the
class for the purpose of its identification in diverse tools (e.g. in a
browser). This must be changed to reflect the environment in which it is
shown, i.e. the method `nameIn: environment' must be implemented and
used in proper places.

   Other changes must be made to the Smalltalk system to achieve the
full functionality of structured environments.  In particular, changes
have to be made to the behavior classes, the user interface, the
compiler, and a few classes supporting persistance.  One small detail
of note is that evaluation in the REPL or `Workspace', implemented by
compiling methods on `UndefinedObject', make more sense if
`UndefinedObject''s environment is the "current environment" as
reachable by `Namespace current', even though its mother environment by
any other sensibility is `Smalltalk'.

2.3.5 Using namespaces
----------------------

Using namespaces is often merely a matter of adding a `namespace'
option to the GNU Smalltalk XML package description used by
`PackageLoader', or wrapping your code like this:
         Namespace current: NewNS [
             ...
         ]

   Namespaces can be imported into classes like this:
         Stream subclass: EncodedStream [
             <import: Encoders>
         ]

Alternatively, paths to classes (and other objects) in the namespaces
will have to be specified completely.  Importing a namespace into a
class is similar to C++'s `using namespace' declaration within the
class proper's definition.

   Finally, be careful when working with fundamental system classes.
Although you can use code like
         Namespace current: NewNS [
             Smalltalk.Set subclass: Set [
                 <category: 'My application-Extensions'>
                 ...
             ]
         ]

this approach won't work when applied to core classes.  For example,
you might be successful with a `Set' or `WriteStream' object, but
subclassing `SmallInteger' this way can bite you in strange ways:
integer literals will still belong to the `Smalltalk' dictionary's
version of the class (this holds for `Array's, `String's, etc. too),
primitive operations will still answer standard Smalltalk
`SmallIntegers', and so on.  Similarly, word-shaped will recognize
32-bit `Smalltalk.LargeInteger' objects, but not `LargeInteger's
belonging to your own namespace.

   Unfortunately, this problem is not easy to solve since Smalltalk has
to know the OOPs of determinate class objects for speed--it would not
be feasible to lookup the environment to which sender of a message
belongs every time the `+' message was sent to an Integer.

   So, GNU Smalltalk namespaces cannot yet solve 100% of the problem of
clashes between extensions to a class--for that you'll still have to
rely on prefixes to method names.  But they _do_ solve the problem of
clashes between class names, or between class names and pool dictionary
names.

   Namespaces are unrelated from packages; loading a package does not
import the corresponding namespace.


File: gst.info,  Node: Disk file-IO,  Next: Object dumping,  Prev: Namespaces,  Up: Features

2.4 Disk file-IO primitive messages
===================================

Four classes (`FileDescriptor', `FileStream', `File', `Directory')
allow you to create files and access the file system in a fully
object-oriented way.

   `FileDescriptor' and `FileStream' are much more powerful than the
corresponding C language facilities (the difference between the two is
that, like the C `stdio' library, `FileStream' does buffering).  For one
thing, they allow you to write raw binary data in a portable
endian-neutral format.  But, more importantly, these classes
transparently implement virtual filesystems and asynchronous I/O.

   Asynchronous I/O means that an input/output operation blocks the
Smalltalk Process that is doing it, but not the others, which makes them
very useful in the context of network programming.  Virtual file systems
mean that these objects can transparently extract files from archives
such as `tar' and `gzip' files, through a mechanism that can be
extended through either shell scripting or Smalltalk programming.  For
more information on these classes, look in the class reference, under
the `VFS' namespace.  URLs may be used as file names; though, unless
you have loaded the `NetClients' package (*note Network support::),
only `file' URLs will be accepted.

   In addition, the three files, `stdin', `stdout', and `stderr' are
declared as global instances of `FileStream' that are bound to the
proper values as passed to the C virtual machine.  They can be accessed
as either `stdout' and `FileStream stdout'--the former is easier to
type, but the latter can be clearer.

   Finally, `Object' defines four other methods: `print' and `printNl',
`store' and `storeNl'.  These do a `printOn:' or `storeOn:' to the
"Transcript" object; this object, which is the sole instance of class
`TextCollector', normally delegates write operations to `stdout'.  If
you load the VisualGST GUI, instead, the Transcript Window will be
attached to the Transcript object (*note GTK and VisualGST: GUI.).

   The `fileIn:' message sent to the FileStream class, with a file name
as a string argument, will cause that file to be loaded into Smalltalk.

   For example,
     FileStream fileIn: 'foo.st' !
   will cause `foo.st' to be loaded into GNU Smalltalk.


File: gst.info,  Node: Object dumping,  Next: Dynamic loading,  Prev: Disk file-IO,  Up: Features

2.5 The GNU Smalltalk ObjectDumper
==================================

Another GNU Smalltalk-specific class, the `ObjectDumper' class, allows
you to dump objects in a portable, endian-neutral, binary format. Note
that you can use the `ObjectDumper' on ByteArrays too, thanks to another
GNU Smalltalk-specific class, `ByteStream', which allows you to treat
ByteArrays the same way you would treat disk files.

   For more information on the usage of the `ObjectDumper', look in the
class reference.


File: gst.info,  Node: Dynamic loading,  Next: Documentation,  Prev: Object dumping,  Up: Features

2.6 Dynamic loading
===================

The `DLD' class enhances the C callout mechanism to automatically look
for unresolved functions in a series of program-specified libraries.  To
add a library to the list, evaluate code like the following:
          DLD addLibrary: 'libc'

   The extension (`.so', `.sl', `.a', `.dll' depending on your
operating system) will be added automatically.  You are advised not to
specify it for portability reasons.

   You will then be able to use the standard C call-out mechanisms to
define all the functions in the C run-time library.  Note that this is
a potential security problem (especially if your program is SUID root
under Unix), so you might want to disable dynamic loading when using
GNU Smalltalk as an extension language.  To disable dynamic loading,
configure GNU Smalltalk passing the `--disable-dld' switch.

   Note that a `DLD' class will be present even if dynamic loading is
disabled (either because your system is not supported, or by the
`--disable-dld' configure switch) but any attempt to perform dynamic
linking will result in an error.


File: gst.info,  Node: Documentation,  Next: Memory access,  Prev: Dynamic loading,  Up: Features

2.7 Automatic documentation generator
=====================================

GNU Smalltalk includes an automatic documentation generator invoked via
the `gst-doc' command.  The code is actually part of the
`ClassPublisher' package, and `gst-doc' takes care of reading the code
to be documented and firing a `ClassPublisher'.

   Currently, `gst-doc' can only generate output in Texinfo format,
though this will change in future releases.

   `gst-doc' can document code that is already in the image, or it can
load external files and packages.  Note that the latter approach will
not work for files and packages that programmatically create code or
file in other files/packages.

   `gst-doc' is invoked as follows:

     gst-doc [ FLAG ... ] CLASS ...

   The following options are supported:

`-p PACKAGE'
`--package=PACKAGE'
     Produce documentation for the classes inside the PACKAGE package.

`-f FILE'
`--file=FILE'
     Produce documentation for the classes inside the FILE file.

`-I'
`--image-file'
     Produce documentation for the code that is already in the given
     image.

`-o'
`--output=FILE'
     Emit documentation in the named file.

   CLASS is either a class name, or a namespace name followed by `.*'.
Documentation will be written for classes that are specified in the
command line.  CLASS can be omitted if a `-f' or `-p' option is given.
In this case, documentation will be written for all the classes in the
package.


File: gst.info,  Node: Memory access,  Next: GC,  Prev: Documentation,  Up: Features

2.8 Memory accessing methods
============================

GNU Smalltalk provides methods to query its own internal data
structures.  You may determine the real memory address of an object or
the real memory address of the OOP table that points to a given object,
by using messages to the `Memory' class, described below.

 -- Method on Object: asOop
     Returns the index of the OOP for anObject.  This index is immume
     from garbage collection and is the same value used by default as
     an hash value for anObject (it is returned by Object's
     implementation of `hash' and `identityHash').

 -- Method on Integer: asObject
     Converts the given OOP _index_ (not address) back to an object.
     Fails if no object is associated to the given index.

 -- Method on Integer: asObjectNoFail
     Converts the given OOP _index_ (not address) back to an object.
     Returns nil if no object is associated to the given index.

   Other methods in ByteArray and Memory allow to read various C types
(`doubleAt:', `ucharAt:', etc.).  These are mostly obsoleted by
`CObject' which, in newer versions of GNU Smalltalk, supports manually
managed heap-backed memory as well as garbage collected
ByteArray-backed memory.

   Another interesting class is ObjectMemory.  This provides a few
methods that enable one to tune the virtual machine's usage of memory;
many methods that in the past were instance methods of Smalltalk or
class methods of Memory are now class methods of ObjectMemory.  In
addition, and that's what the rest of this section is about, the
virtual machines signals events to its dependents exactly through this
class.

   The events that can be received are
"returnFromSnapshot"
     This is sent every time an image is restarted, and substitutes the
     concept of an "init block" that was present in previous versions.

"aboutToQuit"
     This is sent just before the interpreter is exiting, either because
     `ObjectMemory quit' was sent or because the specified files were
     all filed in.  Exiting from within this event might cause an
     infinite loop, so be careful.

"aboutToSnapshot"
     This is sent just before an image file is created.  Exiting from
     within this event will leave any preexisting image untouched.

"finishedSnapshot"
     This is sent just after an image file is created.  Exiting from
     within this event will not make the image unusable.


File: gst.info,  Node: GC,  Next: Security,  Prev: Memory access,  Up: Features

2.9 Memory management in GNU Smalltalk
======================================

The GNU Smalltalk virtual machine is equipped with a garbage collector,
a facility that reclaims the space occupied by objects that are no
longer accessible from the system roots.  The collector is composed of
several parts, each of which can be invoked by the virtual machine
using various tunable strategies, or invoked manually by the programmer.

   These parts include a "generation scavenger", a "mark & sweep"
collectory with an incremental sweep phase, and a "compactor".  All
these facilities work on different memory spaces and differs from the
other in its scope, speed and disadvantages (which are hopefully
balanced by the availability of different algorithms).  What follows is
a description of these algorithms and of the memory spaces they work in.

   "NewSpace" is the memory space where young objects live.  It is
composed of three sub-spaces: an object-creation space ("Eden") and two
"SurvivorSpaces". When an object is first created, it is placed in
Eden. When Eden starts to fill up (i.e., when the number of used bytes
in Eden exceeds the scavenge threshold), objects that are housed in
Eden or in the occupied SurvivorSpace and that are still reachable from
the system roots are copied to the unoccupied SurvivorSpace.  As an
object survives different scavenging passes, it will be shuffled by the
scavenger from the occupied SurvivorSpace to the unoccupied one.  When
the number of used bytes in SurvivorSpace is high enough that the
scavenge pause might be excessively long, the scavenger will move some
of the older surviving objects from NewSpace to "OldSpace".  In the
garbage collection jargon, we say that such objects are being "tenured"
to OldSpace.

   This garbage collection algorithm is designed to reclaim short-lived
objects, that is those objects that expire while residing in NewSpace,
and to decide when enough data is residing in NewSpace that it is
useful to move some of it in OldSpace.  A "copying" garbage collector
is particularly efficient in an object population whose members are
more likely to die than survive, because this kind of scavenger spends
most of its time copying survivors, who will be few in number in such
populations, rather than tracing corpses, who will be many in number.
This fact makes copying collection especially well suited to NewSpace,
where a percentage of 90% or more objects often fails to survive across
a single scavenge.

   The particular structure of NewSpace has many advantages.  On one
hand, having a large Eden and two small SurvivorSpaces has a smaller
memory footprint than having two equally big semi-spaces and allocating
new objects directly from the occupied one (by default, GNU Smalltalk
uses 420=300+60*2 kilobytes of memory, while a simpler configuration
would use 720=360*2 kilobytes).  On the other hand, it makes tenuring
decisions particularly simple: the copying order is such that
short-lived objects tend to be copied last, while objects that are
being referred from OldSpace tend to be copied first: this is because
the tenuring strategy of the scavenger is simply to treat the
destination SurvivorSpace as a circular buffer, tenuring objects with a
First-In-First-Out policy.

   An object might become part of the scavenger root set for several
reasons: objects that have been tenured are roots if their data lives
in an OldSpace page that has been written to since the last scavenge
(more on this later), plus all objects can be roots if they are known
to be referenced from C code or from the Smalltalk stacks.

   In turn, some of the old objects can be made to live in a special
area, called "FixedSpace".  Objects that reside in FixedSpace are
special in that their body is guaranteed to remain at a fixed address
(in general, GNU Smalltalk only ensures that the header of the object
remains at a fixed address in the Object Table).  Because the garbage
collector can and does move objects, passing objects to foreign code
which uses the object's address as a fixed key, or which uses a
ByteArray as a buffer, presents difficulties.  One can use `CObject' to
manipulate C data on the `malloc' heap, which indeed does not move, but
this can be tedious and requires the same attentions to avoid memory
leaks as coding in C.  FixedSpace provides a much more convenient
mechanism: once an object is deemed fixed, the object's body will never
move through-out its life-time; the space it occupies will however
still be returned automatically to the FixedSpace pool when the object
is garbage collected.  Note that because objects in FixedSpace cannot
move, FixedSpace cannot be compacted and can therefore suffer from
extensive fragmentation.  For this reason, FixedSpace should be used
carefully.  FixedSpace however is rebuilt (of course) every time an
image is brought up, so a kind of compaction of FixedSpace can be
achieved by saving a snapshot, quitting, and then restarting the newly
saved image.

   Memory for OldSpace and FixedSpace is allocated using a variation of
the system allocator `malloc': in fact, GNU Smalltalk uses the same
allocator for its own internal needs, for OldSpace and for FixedSpace,
but it ensures that a given memory page never hosts objects that reside
in separate spaces.  New pages are mapped into the address space as
needed and devoted to OldSpace or FixedSpace segments; similarly, when
unused they may be subsequently unmapped, or they might be left in
place waiting to be reused by `malloc' or by another Smalltalk data
space.

   Garbage that is created among old objects is taken care of by a mark
& sweep collector which, unlike the scavenger which only reclaims
objects in NewSpace, can only reclaim objects in OldSpace.  Note that
as objects are allocated, they will not only use the space that was
previously occupied in the Eden by objects that have survived, but they
will also reuse the entries in the global Object Table that have been
freed by object that the scavenger could reclaim.  This quest for free
object table entries can be combined with the sweep phase of the
OldSpace collector, which can then be done incrementally, limiting the
disruptive part of OldSpace garbage collection to the mark phase.

   Several runs of the mark & sweep collector can lead to fragmentation
(where objects are allocated from several pages, and then become
garbage in an order such that a bunch of objects remain in each page
and the system is not able to recycle them).  For this reason, the
system periodically tries to compact OldSpace.  It does so simply by
looping through every old object and copying it into a new OldSpace.
Since the OldSpace allocator does not suffer from fragmentation until
objects start to be freed nor after all objects are freed, at the end
of the copy all the pages in the fragmented OldSpace will have been
returned to the system (some of them might already have been used by
the compacted OldSpace), and the new, compacted OldSpace is ready to be
used as the system OldSpace.  Growing the object heap (which is done
when it is found to be quite full even after a mark & sweep collection)
automatically triggers a compaction.

   You can run the compactor without marking live objects.  Since the
amount of garbage in OldSpace is usually quite limited, the overhead
incurred by copying potentially dead objects is small enough that the
compactor still runs considerably faster than a full garbage
collection, and can still give the application some breathing room.

   Keeping OldSpace and FixedSpace in the same heap would then make
compaction of OldSpace (whereby it is rebuilt from time to time in
order to limit fragmentation) much less effective.  Also, the `malloc'
heap is not used for FixedSpace objects because GNU Smalltalk needs to
track writes to OldSpace and FixedSpace in order to support efficient
scavenging of young objects.

   To do so, the grey page table(1) contains one entry for each page in
OldSpace or FixedSpace that is thought to contain at least a reference
to an object housed in NewSpace.  Every page in OldSpace is created as
grey, and is considered grey until a scavenging pass finds out that it
actually does not contain pointers to NewSpace.  Then the page is
recolored black(2), and will stay black until it is written to or
another object is allocated in it (either a new fixed object, or a
young object being tenured).  The grey page table is expanded and
shrunk as needed by the virtual machine.

   Drawing an histogram of object sizes shows that there are only a few
sources of large objects on average (i.e., objects greater than a page
in size), but that enough of these objects are created dynamically that
they must be handled specially.  Such objects should not be allocated
in NewSpace along with ordinary objects, since they would fill up
NewSpace prematurely (or might not even fit in it), thus accelerating
the scavenging rate, reducing performance and resulting in an increase
in tenured garbage.  Even though this is not an optimal solution
because it effectively tenures these objects at the time they are
created, a benefit can be obtained by allocating these objects directly
in FixedSpace.  The reason why FixedSpace is used is that these objects
are big enough that they don't result in fragmentation(3); and using
FixedSpace instead of OldSpace avoids that the compactor copies them
because this would not provide any benefit in terms of reduced
fragmentation.

   Smalltalk activation records are allocated from another special heap,
the context pool.  This is because it is often the case that they can
be deallocated in a Last-In-First-Out (stack) fashion, thereby saving
the work needed to allocate entries in the object table for them, and
quickly reusing the memory that they use.  When the activation record
is accessed by Smalltalk, however, the activation record must be turned
into a first-class `OOP'(4).  Since even these objects are usually very
short-lived, the data is however not copied to the Eden: the eviction
of the object bodies from the context pool is delayed to the next
scavenging, which will also empty the context pool just like it empties
Eden.  If few objects are allocated and the context pool turns full
before the Eden, a scavenging is also triggered; this is however quite
rare.

   Optionally, GNU Smalltalk can avoid the overhead of interpretation by
executing a given Smalltalk method only after that method has been
compiled into the underlying microprocessor's machine code. This
machine-code generation is performed automatically, and the resulting
machine code is then placed in `malloc'-managed memory.  Once executed,
a method's machine code is left there for subsequent execution.
However, since it would require way too much memory to permanently
house the machine-code version of every Smalltalk method, methods might
be compiled more than once: when a translation is not used at the time
that two garbage collection actions are taken (scavenges and global
garbage collections count equally), the incremental sweeper discards
it, so that it will be recomputed if and when necessary.

   ---------- Footnotes ----------

   (1) The denomination "grey" comes from the lexicon of "tri-color
marking", which is an   abstraction of every possible garbage
collection algorithm: in   tri-color marking, grey objects are those
that are known to be   reachable or that we are not interested in
reclaiming, yet have not   been scanned to mark the objects that they
refer to as reachable.

   (2) Black   objects are those that are known to be reachable or that
we are not   interested in reclaiming, and are known to have references
only to   other black or grey objects (in case you're curious, the
tri-color   marking algorithm goes on like this: object not yet known
to be   reachable are white, and when all objects are either black or
white,   the white ones are garbage).

   (3) Remember that free pages are shared among the   three heaps,
that is, OldSpace, FixedSpace and the `malloc'   heap.  When a large
object is freed, the memory that it used can be   reused by `malloc' or
by OldSpace allocation

   (4) This is short for "Ordinary   Object Pointer".


File: gst.info,  Node: Security,  Next: Special objects,  Prev: GC,  Up: Features

2.10 Security in GNU Smalltalk
==============================


File: gst.info,  Node: Special objects,  Prev: Security,  Up: Features

2.11 Special kinds of objects
=============================

A few methods in Object support the creation of particular objects.
This include:

   * finalizable objects

   * weak and ephemeron objects (i.e. objects whose contents are
     considered specially, during the heap scanning phase of garbage
     collection).

   * read-only objects (like literals found in methods)

   * fixed objects (guaranteed not to move across garbage collections)


   They are:

 -- Method on Object: makeWeak
     Marks the object so that it is considered weak in subsequent
     garbage collection passes.  The garbage collector will consider
     dead an object which has references only inside weak objects, and
     will replace references to such an "almost-dead" object with nils,
     and then send the `mourn' message to the object.

 -- Method on Object: makeEphemeron
     Marks the object so that it is considered specially in subsequent
     garbage collection passes.  Ephemeron objects are sent the message
     `mourn' when the first instance variable is not referenced or is
     referenced _only through another instance variable in the
     ephemeron_.

     Ephemerons provide a very versatile base on which complex
     interactions with the garbage collector can be programmed (for
     example, finalization which is described below is implemented with
     ephemerons).

 -- Method on Object: addToBeFinalized
     Marks the object so that, as soon as it becomes unreferenced, its
     `finalize' method is called.  Before `finalize' is called, the VM
     implicitly removes the objects from the list of finalizable ones.
     If necessary, the `finalize' method can mark again the object as
     finalizable, but by default finalization will only occur once.

     Note that a finalizable object is kept in memory even when it has
     no references, because tricky finalizers might "resuscitate" the
     object; automatic marking of the object as not to be finalized has
     the nice side effect that the VM can simply delay the releasing of
     the memory associated to the object, instead of being forced to
     waste memory even after finalization happens.

     An object must be explicitly marked as to be finalized _every time
     the image is loaded_; that is, finalizability is not preserved by
     an image save.  This was done because in most cases finalization
     is used together with operating system resources that would be
     stale when the image is loaded again.  For `CObject's, in
     particular, freeing them would cause a segmentation violation.

 -- Method on Object: removeToBeFinalized
     Removes the to-be-finalized mark from the object.  As I noted
     above, the finalize code for the object does not have to do this
     explicitly.

 -- Method on Object: finalize
     This method is called by the VM when there are no more references
     to the object (or, of course, if it only has references inside
     weak objects).

 -- Method on Object: isReadOnly
     This method answers whether the VM will refuse to make changes to
     the objects when methods like `become:', `basicAt:put:', and
     possibly `at:put:' too (depending on the implementation of the
     method).  Note that GNU Smalltalk won't try to intercept
     assignments to fixed instance variables, nor assignments via
     `instVarAt:put:'. Many objects (Characters, `nil', `true',
     `false', method literals) are read-only by default.

 -- Method on Object: makeReadOnly: aBoolean
     Changes the read-only or read-write status of the receiver to that
     indicated by `aBoolean'.

 -- Method on Object: basicNewInFixedSpace
     Same as `#basicNew', but the object won't move across garbage
     collections.

 -- Method on Object: basicNewInFixedSpace:
     Same as `#basicNew:', but the object won't move across garbage
     collections.

 -- Method on Object: makeFixed
     Ensure that the receiver won't move across garbage collections.
     This can be used either if you decide after its creation that an
     object must be fixed, or if a class does not support using `#new'
     or `#new:' to create an object

   Note that, although particular applications will indeed have a need
for fixed, read-only or finalizable objects, the `#makeWeak' primitive
is seldom needed and weak objects are normally used only indirectly,
through the so called "weak collections".  These are easier to use
because they provide additional functionality (for example, `WeakArray'
is able to determine whether an item has been garbage collected, and
`WeakSet' implements hash table functionality); they are:

   * `WeakArray'

   * `WeakSet'

   * `WeakKeyDictionary'

   * `WeakValueLookupTable'

   * `WeakIdentitySet'

   * `WeakKeyIdentityDictionary'

   * `WeakValueIdentityDictionary'

   Versions of GNU Smalltalk preceding 2.1 included a
`WeakKeyLookupTable' class which has been replaced by
`WeakKeyDictionary'; the usage is completely identical, but the
implementation was changed to use a more efficient approach based on
ephemeron objects.


File: gst.info,  Node: Packages,  Next: Emacs,  Prev: Features,  Up: Top

3 Packages
**********

GNU Smalltalk includes a packaging system which allows one to file in
components (often called "goodies" in Smalltalk lore) without caring of
whether they need other goodies to be loaded first.

   The packaging system is implemented by a Smalltalk class,
`PackageLoader', which looks for information about packages in various
places:
   * the kernel directory's parent directory; this is where   an
     installed `packages.xml' resides, in a system-wide data
     directory such as `/usr/local/share/smalltalk';

   * the above directory's `site-packages' subdirectory, for   example
     `/usr/local/share/smalltalk/site-packages';

   * in the file `.st/packages.xml', hosting per-user packages;

   * finally, there can be a `packages.xml' in the same directory   as
     the current image.

   Each of this directories can contain package descriptions in an XML
file named (guess what) `packages.xml', as well as standalone packages
in files named `*.star' (short for `Smalltalk archive').  Later in this
section you will find information about `gst-package', a program that
helps you create `.star' files.

   There are two ways to load something using the packaging system. The
first way is to use the PackageLoader's `fileInPackage:' and
`fileInPackages:' methods.  For example:

         PackageLoader fileInPackages: #('DBD-MySQL' 'DBD-SQLite').
         PackageLoader fileInPackage: 'Sockets'.

   The second way is to use the `gst-load' script which is installed
together with the virtual machine.  For example, you can do:

   gst-load DBD-MySQL DBD-SQLite DBI

and GNU Smalltalk will automatically file in:

   * DBI, loaded first because it is needed by the other two packages

   * Sockets and Digest, not specified, but needed by DBD-MySQL

   * DBD-MySQL

   * DBD-SQLite

Notice how DBI has already been loaded.

   Then it will save the Smalltalk image, and finally exit.

   `gst-load' supports several options:

`-I'
`--image-file'
     Load the packages inside the given image.

`-i'
`--rebuild-image'
     Build an image from scratch and load the package into it.  Useful
     when the image specified with `-I' does not exist yet.

`-q'
`--quiet'
     Hide the script's output.

`-v'
`--verbose'
     Show which files are loaded, one by one.

`-f'
`--force'
     If a package given on the command-line is already present, reload
     it.  This does not apply to automatically selected prerequisites.

`-t'
`--test'
     Run the package testsuite before installing, and exit with a
     failure if the tests fail.  Currently, the testsuites are placed
     in the image together with the package, but this may change in
     future versions.

`-n'

`--dry-run'
     Do not save the image after loading.

`--start[=ARG]'
     Start the services identified by the package.  If an argument is
     given, only one package can be specified on the command-line.  If
     at least one package specifies a startup script, `gst-load' won't
     exit.

   To provide support for this system, you have to give away with your
GNU Smalltalk goodies a small file (usually called `package.xml') which
looks like this:
     <package>
       <name>DBD-SQLite</name>
       <namespace>DBI.SQLite</namespace>

       <!-- The `prereq' tag identifies packages that
            must be loaded before this one. -->
       <prereq>DBI</prereq>

       <!-- The `module' tag loads a dynamic shared object
            and calls the `gst_initModule' function in it.  Modules
            can register functions so that Smalltalk code can call them,
            and can interact with or manipulate Smalltalk objects. -->
       <module>dbd-sqlite3</module>

       <!-- A separate subpackage can be defined for testing purposes.
            The `SUnit' package is implicitly made a prerequisite of the
            testing subpackage, and the default value of `namespace'
            is the one given for the outer package. -->
       <test>
         <!-- Specifies a testing script that `gst-sunit' (*note SUnit::)
              will run in order to test the package.  If this is specified outside
              the testing subpackage, the package should list `SUnit' among
              the prerequisites. -->
         <sunit>DBI.SQLite.SQLiteTestSuite</sunit>
         <filein>SQLiteTests.st</filein>
       </test>

       <!-- The `filein' tag identifies files that
            compose this package and that should be loaded in the
            image in this order. -->
       <filein>SQLite.st</filein>
       <filein>Connection.st</filein>
       <filein>ResultSet.st</filein>
       <filein>Statement.st</filein>
       <filein>Row.st</filein>
       <filein>ColumnInfo.st</filein>
       <filein>Table.st</filein>
       <filein>TableColumnInfo.st</filein>

       <!-- The `file' tag identifies extra files that
            compose this package's distribution. -->
       <file>SQLiteTests.st</file>
       <file>ChangeLog</file>
     </package>

   Other tags exist:
`url'
     Specifies a URL at which a repository for the package can be found.
     The repository, when checked out, should contain a `package.xml'
     file at its root.  The contents of this tag are not used for local
     packages; they are used when using the `--download' option to
     `gst-package'.

`library'
     Loads a dynamic shared object and registers the functions in it so
     that they can all be called from Smalltalk code.  The `GTK'
     package registers the GTK+ library in this way, so that the
     bindings can use them.

`callout'
     Instructs to load the package only if the C function whose name is
     within the tag is available to be called from Smalltalk code.

`start'
     Specifies a Smalltalk script that `gst-load' and `gst-remote' will
     execute in order to start the execution of the service implemented
     in the package.  Before executing the script, `%1' is replaced
     with either `nil' or a String literal.

`stop'
     Specifies a Smalltalk script that `gst-remote' will execute in
     order to shut down the service implemented in the package.  Before
     executing the script, `%1' is replaced with either `nil' or a
     String literal.

`test'
     Specifies a subpackage that is only loaded by `gst-sunit' in order
     to test the package.  The subpackage may include arbitrary tags
     (including `file', `filein' and `sunit') but not `name'.

`provides'
     In some cases, a single functionality can be provided by multiple
     modules.  For example, GNU Smalltalk includes two browsers but
     only one should be loaded at any time.  To this end, a dummy
     package `Browser' is created pointing to the default browser
     (`VisualGST'), but both browsers use `provides' so that if the old
     BLOX browser is in the image, loading `Browser' will have no
     effect.

   To install your package, you only have to do
         gst-package path/to/package.xml

   `gst-package' is a Smalltalk script which will create a `.star'
archive in the current image directory, with the files specified in the
`file', `filein' and `built-file' tags.  By default the package is
placed in the system-wide package directory; you can use the option
`--target-directory' to create the `.star' file elsewhere.

   Instead of a local `package.xml' file, you can give:
   * a local `.star' file or a `URL' to such a file.  The file will be
     downloaded if necessary, and copied to the target directory;

   * a URL to a `package.xml' file.  The `url' tag in the file will be
     used to find a source code repository (`git' or `svn') or as a
     redirect to another `package.xml' file.

There is also a short form for specifying `package.xml' file on GNU
Smalltalk's web site, so that the following two commands are equivalent:

         gst-package http://smalltalk.gnu.org/project/Iliad/package.xml
         gst-package --download Iliad

   When downloading remote `package.xml' files, `gst-package' also
performs a special check to detect multiple packages in the same
repository.  If the following conditions are met:
   * a package named `PACKAGE' has a prerequisite `PACKAGE-SUBPACKAGE';

   * there is a toplevel subdirectory SUBPACKAGE in the repository;

   * the subdirectory has a `package.xml' file in it

then the `SUBPACKAGE/package.xml' will be installed as well.
`gst-package' does not check if the file actually defines a package
with the correct name, but this may change in future versions.

   Alternatively, `gst-package' can be used to create a skeleton GNU
style source tree.  This includes a `configure.ac' that will find the
installation path of GNU Smalltalk, and a `Makefile.am' to support all
the standard Makefile targets (including `make install' and `make
dist').  To do so, go in the directory that is to become the top of the
source tree and type.

         gst-package --prepare path1/package.xml path2/package.xml

   In this case the generated configure script and Makefile will use
more features of `gst-package', which are yet to be documented.  The
GNU Smalltalk makefile similarly uses `gst-package' to install packages
and to prepare the distribution tarballs.

   The rest of this chapter discusses some of the packages provided
with GNU Smalltalk.

* Menu:

* GTK and VisualGST:                     GUI.
* Parser, STInST, Compiler:              Smalltalk-in-Smalltalk.
* DBI:                                   Database.
* I18N:                                  Locales.
* Seaside:                               Seaside.
* Swazoo:                                Swazoo.
* SUnit:                                 SUnit.
* Sockets, WebServer, NetClients:        Network support.
* XML, XPath, XSL:                       XML.
* Other packages:                        Other packages.


File: gst.info,  Node: GUI,  Next: Smalltalk-in-Smalltalk,  Up: Packages

3.1 GTK and VisualGST
=====================

GNU Smalltalk comes with GTK bindings and with a browser based on it.
The system can be started as `gst-browser' and will allow the
programmer to view the source code for existing classes, to modify
existing classes and methods, to get detailed information about the
classes and methods, and to evaluate code within the browser.  In
addition, simple debugging and unit testing tools are provided.  An
Inspector window allows the programmer to graphically inspect and
modify the representation of an object and a walkback inspector was
designed which will display a backtrace when the program encounters an
error.  SUnit tests (*note SUnit::) can be run from the browser in
order to easily support test driven development.

   The Transcript global object is redirected to print to the
transcript window instead of printing to stdout, and the transcript
window as well as the workspaces, unlike the console read-eval-print
loop, support variables that live across multiple evaluations:

         a := 2   "Do-it"
         a + 2    "Print-it: 4 will be shown"

   To start the browser you can simply type:

         gst-browser

   This will load any requested packages, then, if all goes well, a
_launcher_ window combining all the basic tools will appear on your
display.


File: gst.info,  Node: Smalltalk-in-Smalltalk,  Next: Database,  Prev: GUI,  Up: Packages

3.2 The Smalltalk-in-Smalltalk library
======================================

The Smalltalk-in-Smalltalk library is a set of classes for looking at
Smalltalk code, constructing models of Smalltalk classes that can later
be created for real, analyzing and performing changes to the image,
finding smelly code and automatically doing repetitive changes.  This
package incredibly enhances the reflective capabilities of Smalltalk.

   A fundamental part of the system is the recursive-descent parser
which creates parse nodes in the form of instances of subclasses of
`RBProgramNode'.

   The parser's extreme flexibility can be exploited in three ways, all
of which are demonstrated by source code available in the distribution:

   * First, actions are not hard-coded in the parser itself: the parser
     creates a parse tree, then hands it to methods in `RBParser' that
     can be overridden in different `RBParser' subclasses.  This is done
     by the compiler itself, in which a subclass of `RBParser' (class
     `STFileInParser') hands the parse trees to the `STCompiler' class.

   * Second, an implementation of the "visitor" pattern is provided to
     help in dealing with parse trees created along the way; this
     approach is demonstrated by the Smalltalk code pretty-printer in
     class `RBFormatter', by the syntax highlighting engine included
     with the browser, and by the compiler.

   * The parser is able to perform complex tree searches and rewrites,
     through the ParseTreeSearcher and ParseTreeRewriter classes.

   In addition, two applications were created on top of this library
which are specific to GNU Smalltalk.  The first is a compiler for
Smalltalk methods written in Smalltalk itself, whose source code
provides good insights into the GNU Smalltalk virtual machine.

   The second is the automatic documentation extractor.  `gst-doc' is
able to create documentation even if the library cannot be loaded (for
example, if loading it requires a running X server).  To do so it uses
`STClassLoader' from the `Parser' package to load and interpret
Smalltalk source code, creating objects for the classes and methods
being read in; then, polymorphism allows one to treat these exactly
like usual classes.


File: gst.info,  Node: Database,  Next: Locales,  Prev: Smalltalk-in-Smalltalk,  Up: Packages

3.3 Database connectivity
=========================

GNU Smalltalk includes support for connecting to databases.  Currently
this support is limited to retrieving result sets from SQL selection
queries and executing SQL data manipulation queries; in the future
however a full object model will be available that hides the usage of
SQL.

   Classes that are independent of the database management system that
is in use reside in package `DBI', while the drivers proper reside in
separate packages which have `DBI' as a prerequisite; currently,
drivers are supplied for _MySQL_ and _PostgreSQL_, in packages
`DBD-MySQL' and `DBD-PostgreSQL' respectively.

   Using the library is fairly simple.  To execute a query you need to
create a connection to the database, create a statement on the
connection, and execute your query.  For example, let's say I want to
connect to the `test' database on the localhost.  My user name is `doe'
and my password is `mypass'.

     | connection statement result |

     connection := DBI.Connection
         connect: 'dbi:MySQL:dbname=test:host=localhost'
         user: 'doe'
         password: 'mypass').

   You can see that the DBMS-specific classes live in a sub-namespace
of `DBI', while DBMS-independent classes live in `DBI'.

   Here is how I execute a query.

     statement := connection execute: 'insert into aTable (aField) values (123)'.

   The result that is returned is a `ResultSet'.  For write queries the
object returns the number of rows affected.  For read queries (such as
selection queries) the result set supports standard stream protocol
(`next', `atEnd' to read rows off the result stream) and can also
supply collection of column information.  These are instances of
`ColumnInfo') and describe the type, size, and other characteristics of
the returned column.

   A common usage of a ResultSet would be:

     | resultSet values |
     [resultSet atEnd] whileFalse: [values add: (resultSet next at: 'columnName') ].


File: gst.info,  Node: Locales,  Next: Seaside,  Prev: Database,  Up: Packages

3.4 Internationalization and localization support
=================================================

Different countries and cultures have varying conventions for how to
communicate.  These conventions range from very simple ones, such as the
format for representing dates and times, to very complex ones, such as
the language spoken.  Provided the programs are written to obey the
choice of conventions, they will follow the conventions preferred by the
user.  GNU Smalltalk provides two packages to ease you in doing so.
The `I18N' package covers both "internationalization" and
"multilingualization"; the lighter-weight `Iconv' package covers only
the latter, as it is a prerequisite for correct internationalization.

   "Multilingualizing" software means programming it to be able to
support languages from every part of the world.  In particular, it
includes understanding multi-byte character sets (such as UTF-8) and
Unicode characters whose "code point" (the equivalent of the ASCII
value) is above 127.  To this end, GNU Smalltalk provides the
`UnicodeString' class that stores its data as 32-bit Unicode values.
In addition, `Character' will provide support for all the over one
million available code points in Unicode.

   Loading the `I18N' package improves this support through the
`EncodedStream' class(1), which interprets and transcodes non-ASCII
Unicode characters.  This support is mostly transparent, because the
base classes `Character', `UnicodeCharacter' and `UnicodeString' are
enhanced to use it.  Sending `asString' or `printString' to an instance
of `Character' and `UnicodeString' will convert Unicode characters so
that they are printed correctly in the current locale.  For example,
`$<279> printNl' will print a small Latin letter `e' with a dot above,
when the `I18N' package is loaded.

   Dually, you can convert `String' or `ByteArray' objects to Unicode
with a single method call.  If the current locale's encoding is UTF-8,
`#[196 151] asUnicodeString' will return a Unicode string with the same
character as above, the small Latin letter `e' with a dot above.

   The implementation of multilingualization support is not yet
complete.  For example, methods such as `asLowercase', `asUppercase',
`isLetter' do not yet recognize Unicode characters.

   You need to exercise some care, or your program will be buggy when
Unicode characters are used.  In particular, Characters must *not* be
compared with `=='(2) and should be printed on a Stream with `display:'
rather than `nextPut:'.

   Also, Characters need to be created with the class method
`codePoint:' if you are referring to their Unicode value; `codePoint:'
is also the only method to create characters that is accepted by the
ANSI Standard for Smalltalk.  The method `value:', instead, should be
used if you are referring to a byte in a particular encoding.  This
subtle difference means that, for example, the last two of the
following examples will fail:

         "Correct.  Use #value: with Strings, #codePoint: with UnicodeString."
         String with: (Character value: 65)
         String with: (Character value: 128)
         UnicodeString with: (Character codePoint: 65)
         UnicodeString with: (Character codePoint: 128)

         "Correct.  Only works for characters in the 0-127 range, which may
          be considered as defensive programming."
         String with: (Character codePoint: 65)

         "Dubious, and only works for characters in the 0-127 range.  With
          UnicodeString, probably you always want #codePoint:."
         UnicodeString with: (Character value: 65)

         "Fails, we try to use a high character in a String"
         String with: (Character codePoint: 128)

         "Fails, we try to use an encoding in a Unicode string"
         UnicodeString with: (Character value: 128)

   "Internationalizing" software, instead, means programming it to be
able to adapt to the user's favorite conventions.  These conventions
can get pretty complex; for example, the user might specify the locale
`espana-castellano' for most purposes, but specify the locale
`usa-english' for currency formatting: this might make sense if the user
is a Spanish-speaking American, working in Spanish, but representing
monetary amounts in US dollars.  You can see that this system is simple
but, at the same time, very complete.  This manual, however, is not the
right place for a thorough discussion of how an user would set up his
system for these conventions; for more information, refer to your
operating system's manual or to the GNU C library's manual.

   GNU Smalltalk inherits from ISO C the concept of a "locale", that
is, a collection of conventions, one convention for each purpose, and
maps each of these purposes to a Smalltalk class defined by the `I18N'
package, and these classes form a small hierarchy with class `Locale'
as its roots:

   * `LcNumeric' formats numbers; `LcMonetary' and `LcMonetaryISO'
     format currency amounts.

   * `LcTime' formats dates and times.

   * `LcMessages' translates your program's output.  Of course, the
     package can't automatically translate your program's output
     messages into other languages; the only way you can support output
     in the user's favorite language is to translate these messages by
     hand.  The package does, though, provide methods to easily handle
     translations into multiple languages.

   Basic usage of the `I18N' package involves a single selector, the
question mark (`?'), which is a rarely used yet valid character for a
Smalltalk binary message.  The meaning of the question mark selector is
"How do you say ... under your convention?".  You can send `?' to
either a specific instance of a subclass of `Locale', or to the class
itself; in this case, rules for the default locale (which is specified
via environment variables) apply.  You might say, for example, `LcTime
? Date today' or, for example, `germanMonetaryLocale ? account
balance'.  This syntax can be at first confusing, but turns out to be
convenient because of its consistency and overall simplicity.

   Here is how `?' works for different classes:

 -- Method on LcTime: ? aString
     Format a date, a time or a timestamp (`DateTime' object).

 -- Method on LcNumber: ? aString
     Format a number.

 -- Method on LcMonetary: ? aString
     Format a monetary value together with its currency symbol.

 -- Method on LcMonetaryISO: ? aString
     Format a monetary value together with its ISO currency symbol.

 -- Method on LcMessages: ? aString
     Answer an `LcMessagesDomain' that retrieves translations from the
     specified file.

 -- Method on LcMessagesDomain: ? aString
     Retrieve the translation of the given string.(3)

   These two packages provides much more functionality, including more
advanced formatting options support for Unicode, and conversion to and
from several character sets.  For more information, refer to *note
Multilingual and international support with Iconv and I18N:
(gst-libs)I18N.

   As an aside, the representation of locales that the package uses is
exactly the same as the C library, which has many advantages: the burden
of mantaining locale data is removed from GNU Smalltalk's mantainers;
the need of having two copies of the same data is removed from GNU
Smalltalk's users; and finally, uniformity of the conventions assumed
by different internationalized programs is guaranteed to the end user.

   In addition, the representation of translated strings is the standard
MO file format adopted by the GNU `gettext' library.

   ---------- Footnotes ----------

   (1) All     the classes mentioned in this section reside in the
`I18N' namespace.

   (2) Character equality   with `=' will be as fast as with `=='.

   (3) The `?' method    does not apply to the LcMessagesDomain class
itself, but only to its    instances.  This is because LcMessagesDomain
is not a subclass of    Locale.


File: gst.info,  Node: Seaside,  Next: Swazoo,  Prev: Locales,  Up: Packages

3.5 The Seaside web framework
=============================

Seaside is a framework to build highly interactive web applications
quickly, reusably and maintainably.  Features of Seaside include
callback-based request handling, hierarchical (component-based) page
design, and modal session management to easily implement complex
workflows.

   A simple Seaside component looks like this:

     Seaside.WAComponent subclass: MyCounter [
         | count |
         MyCounter class >> canBeRoot [ ^true ]

         initialize [
             super initialize.
             count := 0.
         ]
         states [ ^{ self } ]
         renderContentOn: html [
             html heading: count.
             html anchor callback: [ count := count + 1 ]; with: '++'.
             html space.
             html anchor callback: [ count := count - 1 ]; with: '--'.
         ]
     ]

     MyCounter registerAsApplication: 'mycounter'

   Most of the time, you will run Seaside in a background virtual
machine.  First of all, you should load the Seaside packages into a new
image like this:

     $ gst-load -iI seaside.im Seaside Seaside-Development Seaside-Examples

Then, you can start Seaside with either of these commands

     $ gst-load -I seaside.im --start Seaside
     $ gst-remote -I seaside.im --daemon --start=Seaside

which will start serving pages at `http://localhost:8080/seaside'.  The
former starts the server in foreground, the latter instead runs a
virtual machine that you can control using further invocations of
`gst-remote'.  For example, you can stop serving Seaside pages, and
bring down the server, respectively with these commands:

     $ gst-remote --stop=Seaside
     $ gst-remote --kill


File: gst.info,  Node: Swazoo,  Next: SUnit,  Prev: Seaside,  Up: Packages

3.6 The Swazoo web server
=========================

Swazoo (Smalltalk Web Application Zoo) is a free Smalltalk HTTP server
supporting both static web serving and a fully-featured web request
resolution framework.

   The server can be started using
     $ gst-load --start[=ARG] Swazoo

or loaded into a background GNU Smalltalk virtual machine with
     $ gst-remote --start=Swazoo[:ARG]

   Usually, the first time you start Swazoo ARG is `swazoodemo' (which
starts a simple "Hello, World!" servlet) or a path to a configuration
file like this one:

     <Site name: 'hello'; port: 8080>
      <CompositeResource uriPattern: ''/''>
       <HelloWorldResource uriPattern: ''hello.html''>
      </CompositeResource>
     </Site>

   After this initial step, ARG can take the following meanings:
   * if omitted altogether, all the sites registered on the server
     are started;

   * if a number, all the sites registered on the server   on that port
     are started;

   * if a configuration file name, the server configuration is
     _replaced_ with the one loaded from that file;

   * if any other string, the site named ARG is started.

   In addition, a background server can be stopped using

     $ gst-remote --stop=Swazoo[:ARG]

where ARG can have the same meanings, except for being a configuration
file.

   In addition, package `WebServer' implements an older web server
engine which is now superseded by Swazoo.  It is based on the GPL'ed
WikiWorks project.  Apart from porting to GNU Smalltalk, a number of
changes were made to the code, including refactoring of classes, better
aesthetics, authentication support, virtual hosting, and HTTP 1.1
compliance.


File: gst.info,  Node: SUnit,  Next: Network support,  Prev: Swazoo,  Up: Packages

3.7 The SUnit testing package
=============================

`SUnit' is a framework to write and perform test cases in Smalltalk,
originarily written by the father of Extreme Programming(1), Kent Beck.
`SUnit' allows one to write the tests and check results in Smalltalk;
while this approach has the disadvantage that testers need to be able
to write simple Smalltalk programs, the resulting tests are very stable.

   What follows is a description of the philosophy of `SUnit' and a
description of its usage, excerpted from Kent Beck's paper in which he
describes `SUnit'.

3.7.1 Where should you start?
-----------------------------

Testing is one of those impossible tasks. You'd like to be absolutely
complete, so you can be sure the software will work.  On the other hand,
the number of possible states of your program is so large that you can't
possibly test all combinations.

   If you start with a vague idea of what you'll be testing, you'll
never get started. Far better to _start with a single configuration
whose behavior is predictable_. As you get more experience with your
software, you will be able to add to the list of configurations.

   Such a configuration is called a "fixture".  Two example fixtures
for testing Floats can be `1.0' and `2.0'; two fixtures for testing
Arrays can be `#()' and `#(1 2 3)'.

   By choosing a fixture you are saying what you will and won't test
for. A complete set of tests for a community of objects will have many
fixtures, each of which will be tested many ways.

   To design a test fixture you have to
   * Subclass TestCase

   * Add an instance variable for each known object in the fixture

   * Override setUp to initialize the variables

3.7.2 How do you represent a single unit of testing?
----------------------------------------------------

You can predict the results of sending a message to a fixture. You need
to represent such a predictable situation somehow.  The simplest way to
represent this is interactively. You open an Inspector on your fixture
and you start sending it messages. There are two drawbacks to this
method. First, you keep sending messages to the same fixture.  If a test
happens to mess that object up, all subsequent tests will fail, even
though the code may be correct.

   More importantly, though, you can't easily communicate interactive
tests to others. If you give someone else your objects, the only way
they have of testing them is to have you come and inspect them.

   By representing each predictable situation as an object, each with
its own fixture, no two tests will ever interfere. Also, you can easily
give tests to others to run. _Represent a predictable reaction of a
fixture as a method._  Add a method to TestCase subclass, and stimulate
the fixture in the method.

3.7.3 How do you test for expected results?
-------------------------------------------

If you're testing interactively, you check for expected results
directly, by printing and inspecting your objects.  Since tests are in
their own objects, you need a way to programmatically look for
problems. One way to accomplish this is to use the standard error
handling mechanism (`#error:') with testing logic to signal errors:

     2 + 3 = 5 ifFalse: [self error: 'Wrong answer']

   When you're testing, you'd like to distinguish between errors you are
checking for, like getting six as the sum of two and three, and errors
you didn't anticipate, like subscripts being out of bounds or messages
not being understood.

   There's not a lot you can do about unanticipated errors (if you did
something about them, they wouldn't be unanticipated any more, would
they?) When a catastrophic error occurs, the framework stops running the
test case, records the error, and runs the next test case. Since each
test case has its own fixture, the error in the previous case will not
affect the next.

   The testing framework makes checking for expected values simple by
providing a method, `#should:', that takes a Block as an argument.  If
the Block evaluates to true, everything is fine. Otherwise, the test
case stops running, the failure is recorded, and the next test case
runs.

   So, you have to _turn checks into a Block evaluating to a Boolean,
and send the Block as the parameter to `#should:'_.

   In the example, after stimulating the fixture by adding an object to
an empty Set, we want to check and make sure it's in there:

     SetTestCase>>#testAdd
         empty add: 5.
         self should: [empty includes: 5]

   There is a variant on `TestCase>>#should:'. `TestCase>>#shouldnt:'
causes the test case to fail if the Block argument evaluates to true.
It is there so you don't have to use `(...) not'.

   Once you have a test case this far, you can run it. Create an
instance of your TestCase subclass, giving it the selector of the
testing method. Send `run' to the resulting object:

     (SetTestCase selector: #testAdd) run

   If it runs to completion, the test worked. If you get a walkback,
something went wrong.

3.7.4 How do you collect and run many different test cases?
-----------------------------------------------------------

As soon as you have two test cases running, you'll want to run them both
one after the other without having to execute two do it's. You could
just string together a bunch of expressions to create and run test
cases. However, when you then wanted to run "this bunch of cases and
that bunch of cases" you'd be stuck.

   The testing framework provides an object to represent "a bunch of
tests", `TestSuite'. A `TestSuite' runs a collection of test cases and
reports their results all at once. Taking advantage of polymorphism,
`TestSuites' can also contain other `TestSuites', so you can put Joe's
tests and Tammy's tests together by creating a higher level suite.
_Combine test cases into a test suite._

     (TestSuite named: 'Money')
         add: (MoneyTestCase selector: #testAdd);
         add: (MoneyTestCase selector: #testSubtract);
         run

   The result of sending `#run' to a `TestSuite' is a `TestResult'
object. It records all the test cases that caused failures or errors,
and the time at which the suite was run.

   All of these objects are suitable for being stored in the image and
retrieved.  You can easily store a suite, then bring it in and run it,
comparing results with previous runs.

3.7.5 Running testsuites from the command line
----------------------------------------------

GNU Smalltalk includes a Smalltalk script to simplify running SUnit
test suites.  It is called `gst-sunit'.  The command-line to `gst-sunit'
specifies the packages, files and classes to test:

`-I'
`--image-file'
     Run tests inside the given image.

`-q'
`--quiet'
     Hide the program's output.  The results are still communicated
     with the program's exit code.

`-v'
`--verbose'
     Be more verbose, in particular this will cause `gst-sunit' to write
     which test is currently being executed.

`-f FILE'
`--file=FILE'
     Load FILE before running the required test cases.

`-p PACKAGE'

`--package=PACKAGE'
     Load PACKAGE and its dependencies, and add PACKAGE's tests to the
     set of test cases to run.

`CLASS'
`CLASS*'
     Add CLASS to the set of test cases to run.  An asterisk after the
     class name adds all the classes in CLASS's hierarchy.  In
     particular, each selector whose name starts with `test'
     constitutes a separate test case.

`VAR=VALUE'
     Associate variable VAR with a value.  Variables allow customization
     of the testing environment.  For example, the username with which
     to access a database can be specified with variables.  From within
     a test, variables are accessible with code like this:

              TestSuitesScripter variableAt: 'mysqluser' ifAbsent: [ 'root' ]

     Note that a `#variableAt:' variant does _not_ exist, because the
     testsuite should pick default values in case the variables are not
     specified by the user.

   ---------- Footnotes ----------

   (1) Extreme    Programming is a software engineering technique that
focuses on    team work (to the point that a programmer looks in
real-time at    what another one is typing), frequent testing of the
program,    and incremental design.


File: gst.info,  Node: Network support,  Next: XML,  Prev: SUnit,  Up: Packages

3.8 Sockets, WebServer, NetClients
==================================

GNU Smalltalk includes an almost complete abstraction of the TCP, UDP
and IP protocols.  Although based on the standard BSD sockets, this
library provides facilities such as buffering and preemptive I/O which
a C programmer usually has to implement manually.

   The distribution includes a few tests (mostly loopback tests that
demonstrate both client and server connection), which are class methods
in `Socket'.  This code should guide you in the process of creating and
using both server and client sockets; after creation, sockets behave
practically the same as standard Smalltalk streams, so you should not
have particular problems.  For more information, refer to *note Network
programming with Sockets: (gst-libs)Sockets.  The library is also used
by many other packages, including Swazoo and the MySQL driver.

   There is also code implementing the most popular Internet protocols:
FTP, HTTP, NNTP, SMTP, POP3 and IMAP.  These classes, loaded by the
`NetClients' package, are derived from multiple public domain and free
software packages available for other Smalltalk dialects and ported to
GNU Smalltalk.  Future version of GNU Smalltalk will include
documentation for these as well.


File: gst.info,  Node: XML,  Next: Other packages,  Prev: Network support,  Up: Packages

3.9 An XML parser and object model for GNU Smalltalk
====================================================

The XML parser library for Smalltalk, loaded as package `XML' includes
a validating XML parser and Document Object Model.  This library is
rapidly becoming a standard in the Smalltalk world and a XSLR
interpreter based on it is bundled with GNU Smalltalk as well (see
packages `XPath' and `XSL').

   Parts of the basic XML package can be loaded independently using
packages `XML-DOM', `XML-SAXParser', `XML-XMLParser', `XML-SAXDriver',
`XML-XMLNodeBuilder'.


File: gst.info,  Node: Other packages,  Prev: XML,  Up: Packages

3.10 Other packages
===================

Various other "minor" packages are provided, typically as examples of
writing modules for GNU Smalltalk (*note Linking your libraries to the
virtual machine: External modules.).  These include:

Complex
     which adds transparent operations with complex numbers

GDBM
     which is an interface to the GNU database manager

Digest
     which provides two easy to use classes to quickly compute
     cryptographically strong hash values using the MD5 and SHA1
     algorithms.

NCurses
     which provides bindings to ncurses

Continuations
     which provides more examples and tests for continuations (an
     advanced feature to support complex control flow).

DebugTools
     which provides a way to attach to another Smalltalk process and
     execute it a bytecode or a method at a time.


File: gst.info,  Node: Emacs,  Next: C and Smalltalk,  Prev: Packages,  Up: Top

4 Smalltalk interface for GNU Emacs
***********************************

GNU Smalltalk comes with its own Emacs mode for hacking Smalltalk code.
It also provides tools for interacting with a running Smalltalk system
in an Emacs subwindow.

   Emacs will automatically go into Smalltalk mode when you edit a
Smalltalk file (one with the extension `.st').

* Menu:

* Editing::           Autoindent and more for GNU Smalltalk.
* Interactor::        Smalltalk interactor mode.


File: gst.info,  Node: Editing,  Next: Interactor,  Up: Emacs

4.1 Smalltalk editing mode
==========================

The GNU Smalltalk editing mode is there to assist you in editing your
Smalltalk code.  It tries to be smart about indentation and provides a
few cooked templates to save you keystrokes.

   Since Smalltalk syntax is highly context sensitive, the Smalltalk
editing mode will occasionally get confused when you are editing
expressions instead of method definitions.  In particular, using local
variables, thus:

     | foo |
         foo := 3.
         ^foo squared !
   will confuse the Smalltalk editing mode, as this might also be a
definition the binary operator `|', with second argument called `foo'.
If you find yourself confused when editing this type of expression, put
a dummy method name before the start of the expression, and take it out
when you're done editing, thus:

     x
     | foo |
         foo := 3.
         ^foo squared !


File: gst.info,  Node: Interactor,  Prev: Editing,  Up: Emacs

4.2 Smalltalk interactor mode
=============================

An interesting feature of Emacs Smalltalk is the Smalltalk interactor,
which basically allows you run in GNU Emacs with Smalltalk files in one
window, and Smalltalk in the other.  You can, with a single command,
edit and change method definitions in the live Smalltalk system,
evaluate expressions, make image snapshots of the system so you can
pick up where you left off, file in an entire Smalltalk file, etc.  It
makes a tremendous difference in the productivity and enjoyment that
you'll have when using GNU Smalltalk.

   To start up the Smalltalk interactor, you must be running GNU Emacs
and in a buffer that's in Smalltalk mode.  Then, if you type `C-c m'.
A second window will appear with GNU Smalltalk running in it.

   This window is in most respects like a Shell mode window.  You can
type Smalltalk expressions to it directly and re-execute previous things
in the window by moving the cursor back to the line that contains the
expression that you wish to re-execute and typing return.

   Notice the status in the mode line (e.g. `starting-up', `idle',
etc).  This status will change when you issue various commands from
Smalltalk mode.

   When you first fire up the Smalltalk interactor, it puts you in the
window in which Smalltalk is running.  You'll want to switch back to
the window with your file in it to explore the rest of the interactor
mode, so do it now.

   To execute a range of code, mark the region around and type `C-c e'.
The expression in the region is sent to Smalltalk and evaluated.  The
status will change to indicate that the expression is executing.  This
will work for any region that you create.  If the region does not end
with an exclamation point (which is syntactically required by
Smalltalk), one will be added for you.

   There is also a shortcut, `C-c d' (also invokeable as `M-x
smalltalk-doit'), which uses a simple heuristic to figure out the start
and end of the expression: it searches forward for a line that begins
with an exclamation point, and backward for a line that does not begin
with space, tab, or the comment character, and sends all the text in
between to Smalltalk.  If you provide a prefix argument (by typing `C-u
C-c d' for instance), it will bypass the heuristic and use the region
instead (just like `C-c e' does).

   `C-c c' will compile a method; it uses a similar heuristic to
determine the bounds of the method definition.  Typically, you'll
change a method definition, type `C-c c' and move on to whatever's
next.  If you want to compile a whole bunch of method definitions,
you'll have to mark the entire set of method definitions (from the
`methodsFor:' line to the `! !') as the region and use `C-c e'.

   After you've compiled and executed some expressions, you may want to
take a snapshot of your work so that you don't have to re-do things
next time you fire up Smalltalk.  To do this, you use the `C-c s'
command, which invokes `ObjectMemory snapshot'.  If you invoke this
command with a prefix argument, you can specify a different name for
the image file, and you can have that image file loaded instead of the
default one by using the `-I' flag on the command line when invoking
Smalltalk.

   You can also evaluate an expression and have the result of the
evaluation printed by using the `C-c p' command.  Mark the region and
use the command.

   To file in an entire file (perhaps the one that you currently have in
the buffer that you are working on), type `C-c f'.  You can type the
name of a file to load at the prompt, or just type return and the file
associated with the current buffer will be loaded into Smalltalk.

   When you're ready to quit using GNU Smalltalk, you can quit cleanly
by using the `C-c q' command.  If you want to fire up Smalltalk again,
or if (heaven forbid) Smalltalk dies on you, you can use the `C-c m'
command, and Smalltalk will be reincarnated.  Even if it's running, but
the Smalltalk window is not visible, `C-c m' will cause it to be
displayed right away.

   You might notice that as you use this mode, the Smalltalk window
will scroll to keep the bottom of the buffer in focus, even when the
Smalltalk window is not the current window.  This was a design choice
that I made to see how it would work.  On the whole, I guess I'm pretty
happy with it, but I am interested in hearing your opinions on the
subject.


File: gst.info,  Node: C and Smalltalk,  Next: Tutorial,  Prev: Emacs,  Up: Top

5 Interoperability between C and GNU Smalltalk
**********************************************

* Menu:

* External modules::        Linking your libraries to the virtual machine
* C callout::               Calls from Smalltalk to C
* C data types::            Manipulating C data from Smalltalk
* Smalltalk types::         Manipulating Smalltalk data from C
* Smalltalk callin::        Calls from C to Smalltalk
* Smalltalk callbacks::     Smalltalk blocks as C function pointers
* Object representation::   Manipulating your own Smalltalk objects
* Incubator::               Protecting newly created objects from garbage
                            collections
* Other C functions::       Handling and creating OOPs
* Using Smalltalk::         The Smalltalk environment as an extension library


File: gst.info,  Node: External modules,  Next: C callout,  Up: C and Smalltalk

5.1 Linking your libraries to the virtual machine
=================================================

A nice thing you can do with GNU Smalltalk is enhancing it with your own
goodies.  If they're written in Smalltalk only, no problem: getting them
to work as packages (*note Packages::), and to fit in with the GNU
Smalltalk packaging system, is likely to be a five-minutes task.

   If your goodie is creating a binding to an external C library and
you do not need particular glue to link it to Smalltalk (for example,
there are no callbacks from C code to Smalltalk code), you can use the
`dynamic library linking' system.  When using this system, you have to
link GNU Smalltalk with the library at run-time using DLD, using either
`DLD class>>#addLibrary:' or a `<library>' tag in a `package.xml' file
(*note Packages::).  The following line:

     DLD addLibrary: 'libc'

is often used to use the standard C library functions from Smalltalk.

   However, if you want to provide a more intimate link between C and
Smalltalk, as is the case with for example the GTK bindings, you should
use the `dynamic module linking' system.  This section explains what to
do, taking the Digest library as a guide.

   A module is distinguished from a standard shared library because it
has a function which Smalltalk calls to initialize the module; the name
of this function must be `gst_initModule'.  Here is the initialization
function used by Digest:

     void
     gst_initModule(proxy)
          VMProxy *proxy;
     {
       vmProxy = proxy;
       vmProxy->defineCFunc ("MD5AllocOOP", MD5AllocOOP);
       vmProxy->defineCFunc ("MD5Update", md5_process_bytes);
       vmProxy->defineCFunc ("MD5Final", md5_finish_ctx);

       vmProxy->defineCFunc ("SHA1AllocOOP", SHA1AllocOOP);
       vmProxy->defineCFunc ("SHA1Update", sha1_process_bytes);
       vmProxy->defineCFunc ("SHA1Final", sha1_finish_ctx);
     }

   Note that the `defineCFunc' function is called through a function
pointer in `gst_initModule', and that the value of its parameter is
saved in order to use it elsewhere in its code.  This is not strictly
necessary on many platforms, namely those where the module is
effectively _linked with the Smalltalk virtual machine_ at run-time;
but since some(1) cannot obtain this, for maximum portability you must
always call the virtual machine through the proxy and never refer to
any symbol which the virtual machine exports.  For uniformity, even
programs that link with `libgst.a' should not call these functions
directly, but through a `VMProxy' exported by `libgst.a' and accessible
through the `gst_interpreter_proxy' variable.

   Modules are shared libraries; the default directory in which modules
are searched for is stored in a `gnu-smalltalk.pc' file that is
installed by GNU Smalltalk so that it can be used with `pkg-config'.
An Autoconf macro `AM_PATH_GST' is also installed that will put the
directory in the `gstmoduledir' Autoconf substitution.  When using GNU
Automake and Libtool, you can then build modules by including something
like this in `Makefile.am':

     gstmodule_LTLIBRARIES = libdigest.la
     libdigest_la_LDFLAGS = -module -no-undefined "... more flags ..."
     libdigest_la_SOURCES = "... your source files ..."

   While you can use `DLD class>>#addModule:' to link a module into the
virtual machine at run time, usually bindings that require a module are
complex enough to be packaged as `.star' files.  In this case, you will
have to add the name of the module in a package file (*note
Packages::).  In this case, the relevant entry in the file will be

     <package>
       <name>Digest</name>
       <filein>digest.st</filein>
       <filein>md5.st</filein>
       <filein>sha1.st</filein>
       <module>digest</module>

       <test>
        <sunit>MD5Test SHA1Test</sunit>
        <filein>mdtests.st</filein>
       </test>
     </package>

   There is also a third case, in which the bindings are a mixture of
code written specially for GNU Smalltalk, and the normal C library.  In
this case, you can use a combination of dynamic shared libraries and
dynamic modules.

   To do this, you can specify both `<library>' and `<module>' tags in
the `package.xml' file; alternatively, the following functions allow
you to call `DLD class>>#addLibrary:' from within a module.

 -- Function: mst_Boolean dlOpen (void *filename, int module)
     Open the library pointed to by with FILENAME (which need not
     include an extension), and invoke gst_initModule if it is found in
     the library.  If MODULE is false, add the file to the list of
     libraries that Smalltalk searches for external symbols.

     Return true if the library was found.

 -- Function: void dlAddSearchDir (const char *dir)
     Add DIR at the beginning of the search path of `dlOpen'.

 -- Function: void dlPushSearchPath (void)
     Save the current value of the search path for `dlOpen'.  This can
     be used to temporarily add the search path for the libraries added
     by a module, without affecting subsequent libraries manually
     opened with the `DLD' class.

 -- Function: void dlPopSearchPath (void)
     Restore the last saved value of the search path.

   ---------- Footnotes ----------

   (1) The most notable are AIX and Windows.


File: gst.info,  Node: C callout,  Next: C data types,  Prev: External modules,  Up: C and Smalltalk

5.2 Using the C callout mechanism
=================================

To use the C callout mechanism, you first need to inform Smalltalk about
the C functions that you wish to call.  You currently need to do this in
two places: 1) you need to establish the mapping between your C
function's address and the name that you wish to refer to it by, and 2)
define that function along with how the argument objects should be
mapped to C data types to the Smalltalk interpreter.  As an example, let
us use the pre-defined (to GNU Smalltalk) functions of `system' and
`getenv'.

   First, the mapping between these functions and string names for the
functions needs to be established in your module.  If you are writing an
external Smalltalk module (which can look at Smalltalk objects and
manipulate them), see *note Linking your libraries to the virtual
machine: External modules.; if you are using function from a dynamically
loaded library, see *note Dynamic loading::.

   Second, we need to define a method that will invoke these C functions
and describe its arguments to the Smalltalk runtime system.  Such a
method is defined with a primitive-like syntax, similar to the
following example (taken from `kernel/CFuncs.st')
     system: aString
         <cCall: 'system' returning: #int args: #(#string)>

     getenv: aString
         <cCall: 'getenv' returning: #string args: #(#string)>

   These methods were defined on class `SystemDictionary', so that we
would invoke it thus:
        Smalltalk system: 'lpr README' !

   However, there is no special significance to which class receives the
method; it could have just as well been Float, but it might look kind of
strange to see:
        1701.0 system: 'mail help-smalltalk@gnu.org' !

   The various keyword arguments are described below.

`cCall: 'system''
     This says that we are defining the C function `system'.  This name
     must be *exactly* the same as the string passed to `defineCFunc'.

     The name of the method does not have to match the name of the C
     function; we could have just as easily defined the selector to be
     `'rambo: fooFoo''; it's just good practice to define the method
     with a similar name and the argument names to reflect the data
     types that should be passed.

`returning: #int'
     This defines the C data type that will be returned.  It is
     converted to the corresponding Smalltalk data type.  The set of
     valid return types is:
    `char'
          Single C character value

    `string'
          A C char *, converted to a Smalltalk string

    `stringOut'
          A C char *, converted to a Smalltalk string and then freed.

    `symbol'
          A C char *, converted to a Smalltalk symbol

    `symbolOut'
          A C char *, converted to a Smalltalk symbol and then freed.

    `int'
          A C int value

    `uInt'
          A C unsigned int value

    `long'
          A C long value

    `uLong'
          A C unsigned long value

    `double'
          A C double, converted to an instance of FloatD

    `longDouble'
          A C long double, converted to an instance of FloatQ

    `void'
          No returned value (`self' returned from Smalltalk)

    `wchar'
          Single C wide character (`wchar_t') value

    `wstring'
          Wide C string (`wchar_t *'), converted to a UnicodeString

    `wstringOut'
          Wide C string (`wchar_t *'), converted to a UnicodeString and
          then freed

    `cObject'
          An anonymous C pointer; useful to pass back to some C
          function later

    `smalltalk'
          An anonymous (to C) Smalltalk object pointer; should have
          been passed to C at some point in the past or created by the
          program by calling other public GNU Smalltalk functions
          (*note Smalltalk types::).

    `CTYPE'
          You can pass an instance of CType or one of its subclasses
          (*note C data types::).  In this case the object will be sent
          `#narrow' before being returned: an example of this feature
          is given in the experimental Gtk+ bindings.

`args: #(#string)'
     This is an array of symbols that describes the types of the
     arguments in order.  For example, to specify a call to open(2),
     the arguments might look something like:
             args: #(#string #int #int)

     The following argument types are supported; see above for details.

    `unknown'
          Smalltalk will make the best conversion that it can guess for
          this object; see the mapping table below

    `boolean'
          passed as `char', which is promoted to `int'

    `char'
          passed as `char', which is promoted to `int'

    `wchar'
          passed as `wchar_t'

    `string'
          passed as `char *'

    `byteArrayOut'
          passed as `char *'.  The contents are expected to be
          overwritten with a new C string, and copied back to the
          object that was passed on return from the C function

    `stringOut'
          passed as `char *', the contents are expected to be
          overwritten with a new C string, and the object that was
          passed becomes the new string on return

    `wstring'
          passed as `wchar_t *'

    `wstringOut'
          passed as `wchar_t *', the contents are expected to be
          overwritten with a new C wide string, and the object that was
          passed becomes the new string on return

    `symbol'
          passed as `char *'

    `byteArray'
          passed as `char *', even though may contain NUL's

    `int'
          passed as `int'

    `uInt'
          passed as `unsigned int'

    `long'
          passed as `long'

    `uLong'
          passed as `unsigned long'

    `double'
          passed as `double'

    `longDouble'
          passed as `long double'

    `cObject'
          C object value passed as `void *'.

          Any class with non-pointer indexed instance variables can be
          passed as a `#cObject', and GNU Smalltalk will pass the
          address of the first indexed instance variable.  This however
          should never be done for functions that allocate objects,
          call back into Smalltalk code or otherwise may cause a
          garbage collection: after a GC, pointers passed as `#cObject'
          may be invalidated.  In this case, it is safer to pass every
          object as `#smalltalk', or to only pass `CObject's that were
          returned by a C function previously.

          In addition, `#cObject' can be used for function pointers.
          These are instances of `CCallable' or one of its subclasses.
          See *note Smalltalk callbacks:: for more information on how
          to create function pointers for Smalltalk blocks.

    `cObjectPtr'
          Pointer to C object value passed as `void **'.  The `CObject'
          is modified on output to reflect the value stored into the
          passed object.

    `smalltalk'
          Pass the object pointer to C.  The C routine should treat the
          value as a pointer to anonymous storage.  This pointer can be
          returned to Smalltalk at some later point in time.

    `variadic'
    `variadicSmalltalk'
          an Array is expected, each of the elements of the array will
          be converted like an `unknown' parameter if `variadic' is
          used, or passed as a raw object pointer for
          `variadicSmalltalk'.

    `self'
    `selfSmalltalk'
          Pass the receiver, converting it to C like an `unknown'
          parameter if `self' is used or passing the raw object pointer
          for `selfSmalltalk'.  Parameters passed this way don't map to
          the message's arguments, instead they map to the message's
          receiver.

   Table of parameter conversions:

Declared param type   Object type             C parameter type used
boolean               Boolean (True, False)   int
byteArray             ByteArray               char *
cObject               CObject                 void *
cObject               ByteArray, etc.         void *
cObjectPtr            CObject                 void **
char                  Boolean (True, False)   int
char                  Character               int (C promotion rule)
char                  Integer                 int
double                Float                   double (C promotion)
longDouble            Float                   long double
int                   Boolean (True, False)   int
int                   Integer                 int
uInt                  Boolean (True, False)   unsigned int
uInt                  Integer                 unsigned int
long                  Boolean (True, False)   long
long                  Integer                 long
uLong                 Boolean (True, False)   unsigned long
uLong                 Integer                 unsigned long
smalltalk,            anything                OOP
selfSmalltalk                                 
string                String                  char *
string                Symbol                  char *
stringOut             String                  char *
symbol                Symbol                  char *
unknown, self         Boolean (True, False)   int
unknown, self         ByteArray               char *
unknown, self         CObject                 void *
unknown, self         Character               int
unknown, self         Float                   double
unknown, self         Integer                 long
unknown, self         String                  char *
unknown, self         Symbol                  char *
unknown, self         anything else           OOP
variadic              Array                   each element is passed
                                              according to "unknown"
variadicSmalltalk     Array                   each element is passed as
                                              an OOP
wchar                 Character               wchar_t
wstring               UnicodeString           wchar_t *
wstringOut            UnicodeString           wchar_t *

   When your call-out returns `#void', depending on your application
you might consider using "asynchronous call-outs".  These are call-outs
that do not suspend the process that initiated them, so the process
might be scheduled again, executing the code that follows the call-out,
during the execution of the call-out itself.  This is particularly
handy when writing event loops (the most common place where you call
back into Smalltalk) because then _you can handle events that arrive
during the handling of an outer event_ before the outer event's
processing has ended.  Depending on your application this might be
correct or not, of course.  In the future, asynchronous call-outs might
be started into a separate thread.

   An asynchronous call-out is defined using an alternate primitive-like
syntax, `asyncCCall:args:'.  Note that the returned value parameter is
missing because an asynchronous call-out always returns `nil'.


File: gst.info,  Node: C data types,  Next: Smalltalk types,  Prev: C callout,  Up: C and Smalltalk

5.3 The C data type manipulation system
=======================================

`CType' is a class used to represent C data types themselves (no
storage, just the type). There are subclasses called things like
`CMUMBLECType'.  The instances can answer their size and alignment.
Their `valueType' is the underlying type of data.  It's either an
integer, which is interpreted by the interpreter as the scalar type, or
the underlying element type, which is another `CType' subclass instance.

   To make life easier, there are global variables which hold onto
instances of `CScalarCType': they are called `CMUMBLEType' (like
`CIntType', not like `CIntCType'), and can be used wherever a C
datatype is used.  If you had an array of strings, the elements would
be CStringType's (a specific instance of CScalarCType).

   `CObject' is the base class of the instances of C data.  It has a
subclass called `CScalar', which has subclasses called `CMUMBLE'.
These subclasses can answer size and alignment information.

   Instances of `CObject' can hold a raw C pointer (for example in
`malloc'ed heap)), or can delegate their storage to a `ByteArray'.  In
the latter case, the storage is automatically garbage collected when
the `CObject' becomes dead, and the VM checks accesses to make sure
they are in bounds.  On the other hand, the storage may move, and for
this reason extra care must be put when using this kind of `CObject'
with C routines that call back into Smalltalk, or that store the passed
pointer somewhere.

   Instances of `CObject' can be created in many ways:
   * creating an instance with `CLASS new' initializes   the pointer to
     `NULL';

   * doing `TYPE new', where TYPE is a `CType'   subclass instance,
     allocates a new instance with `malloc'.

   * doing `TYPE gcNew', where TYPE is a `CType'   subclass instance,
     allocates a new instance backed by garbage-collected   storage.


   `CStruct' and `CUnion' subclasses are special.  First, `new'
allocates a new instance with `malloc' instead of initializing the
pointer to `NULL'. Second, they support `gcNew' which creates a new
instance backed by garbage-collected storage.

   `CObject's created by the C callout mechanism are never backed by
garbage-collected storage.

   `CObject' and its subclasses represent a pointer to a C object and
as such provide the full range of operations supported by C pointers.
For example, `+' `anInteger' which returns a CObject which is higher in
memory by `anInteger' times the size of each item. There is also `-'
which acts like `+' if it is given an integer as its parameter.  If a
CObject is given, it returns the difference between the two pointers.
`incr', `decr', `incrBy:', `decrBy:' adjust the string either forward or
backward, by either 1 or `n' characters.  Only the pointer to the
string is changed; the actual characters in the string remain untouched.

   CObjects can be divided into two families, scalars and non-scalars,
just like C data types.  Scalars fetch a Smalltalk object when sent the
`value' message, and change their value when sent the `value:' message.
Non-scalars do not support these two messages.  Non-scalars include
instances of `CArray' and subclasses of `CStruct' and `CUnion' (but not
`CPtr').

   `CPtr's and `CArray's get their underlying element type through a
`CType' subclass instance which is associated with the `CArray' or
`CPtr' instance.

   `CPtr''s `value' and `value:' method get or change the underlying
value that's pointed to.  `value' returns another `CObject'
corresponding to the pointed value.  That's because, for example, a
`CPtr' to `long' points to a place in memory where a pointer to long is
stored.  It is really a `long **' and must be dereferenced twice with
`cPtr value value' to get the `long'.

   `CString' is a subclass of `CPtr' that answers a Smalltalk `String'
when sent `value', and automatically allocates storage to copy and
null-terminate a Smalltalk `String' when sent `value:'.  `replaceWith:'
replaces the string the instance points to with a new string or
`ByteArray', passed as the argument.  Actually, it copies the bytes
from the Smalltalk `String' instance aString into the same buffer
already pointed to by the `CString', with a null terminator.

   Finally, there are `CStruct' and `CUnion', which are abstract
subclasses of `CObject'(1).  The following will refer to CStruct, but
the same considerations apply to CUnion as well, with the only
difference that CUnions of course implement the semantics of a C union.

   These classes provide direct access to C data structures including
   * `long' (unsigned too)

   * `short' (unsigned too)

   * `char' (unsigned too) & byte type

   * `double', `long double', `float'

   * `string' (NUL terminated char *, with special accessors)

   * arrays of any type

   * pointers to any type

   * other structs containing any fixed size types

   Here is an example struct decl in C:
     struct audio_prinfo {
         unsigned    channels;
         unsigned    precision;
         unsigned    encoding;
         unsigned    gain;
         unsigned    port;
         unsigned    _xxx[4];
         unsigned    samples;
         unsigned    eof;
         unsigned char       pause;
         unsigned char       error;
         unsigned char       waiting;
         unsigned char       _ccc[3];
         unsigned char       open;
         unsigned char       active;
     };

     struct audio_info {
         audio_prinfo_t      play;
         audio_prinfo_t      record;
         unsigned    monitor_gain;
         unsigned    _yyy[4];
     };

   And here is a Smalltalk equivalent decision:
     CStruct subclass: AudioPrinfo [
         <declaration: #( (#sampleRate #uLong)
                          (#channels #uLong)
                          (#precision #uLong)
                          (#encoding #uLong)
                          (#gain #uLong)
                          (#port #uLong)
                          (#xxx (#array #uLong 4))
                          (#samples #uLong)
                          (#eof #uLong)
                          (#pause #uChar)
                          (#error #uChar)
                          (#waiting #uChar)
                          (#ccc (#array #uChar 3))
                          (#open #uChar)
                          (#active #uChar))>

         <category: 'C interface-Audio'>
     ]

     CStruct subclass: AudioInfo [
         <declaration: #( (#play #{AudioPrinfo} )
                          (#record #{AudioPrinfo} )
                          (#monitorGain #uLong)
                          (#yyy (#array #uLong 4)))>

         <category: 'C interface-Audio'>
     ]

   This creates two new subclasses of `CStruct' called `AudioPrinfo'
and `AudioInfo', with the given fields.  The syntax is the same as for
creating standard subclasses, with the additional metadata
`declaration:'.  You can make C functions return `CObject's that are
instances of these classes by passing `AudioPrinfo type' as the
parameter to the `returning:' keyword.

   AudioPrinfo has methods defined on it like:
         #sampleRate
         #channels
         #precision
         #encoding

etc.  These access the various data members.  The array element
accessors (xxx, ccc) just return a pointer to the array itself.

   For simple scalar types, just list the type name after the variable.
Here's the set of scalars names, as defined in `kernel/CStruct.st':
        #long                   CLong
        #uLong                  CULong
        #ulong                  CULong
        #byte                   CByte
        #char                   CChar
        #uChar                  CUChar
        #uchar                  CUChar
        #short                  CShort
        #uShort                 CUShort
        #ushort                 CUShort
        #int                    CInt
        #uInt                   CUInt
        #uint                   CUInt
        #float                  CFloat
        #double                 CDouble
        #longDouble             CLongDouble
        #string                 CString
        #smalltalk              CSmalltalk
        #{...}                  A given subclass of `CObject'

   The `#{...}' syntax is not in the Blue Book, but it is present in
GNU Smalltalk and other Smalltalks; it returns an Association object
corresponding to a global variable.

   To have a pointer to a type, use something like:
             (#example (#ptr #long))

   To have an array pointer of size SIZE, use:
             (#example (#array #string SIZE))

   Note that this maps to `char *example[SIZE]' in C.

   The objects returned by using the fields are CObjects; there is no
implicit value fetching currently.  For example, suppose you somehow got
ahold of an instance of class AudioPrinfo as described above (the
instance is a CObject subclass and points to a real C structure
somewhere).  Let's say you stored this object in variable `audioInfo'.
To get the current gain value, do
         audioInfo gain value
   to change the gain value in the structure, do
         audioInfo gain value: 255

   The structure member message just answers a `CObject' instance, so
you can hang onto it to directly refer to that structure member, or you
can use the `value' or `value:' methods to access or change the value
of the member.

   Note that this is the same kind of access you get if you use the
`addressAt:' method on CStrings or CArrays or CPtrs: they return a
CObject which points to a C object of the right type and you need to use
`value' and `value:' to access and modify the actual C variable.

   ---------- Footnotes ----------

   (1) Actually they have a common superclass named `CCompound'.


File: gst.info,  Node: Smalltalk types,  Next: Smalltalk callin,  Prev: C data types,  Up: C and Smalltalk

5.4 Manipulating Smalltalk data from C
======================================

GNU Smalltalk internally maps every object except Integers to a data
structure named an "OOP" (which is short for "Ordinary Object Pointer").
An OOP is a pointer to an internal data structure; this data structure
basically adds a level of indirection in the representation of objects,
since it contains
   * a pointer to the actual object data

   * a bunch of flags, most of which interest the garbage collection
     process

   This additional level of indirection makes garbage collection very
efficient, since the collector is free to move an object in memory
without updating every reference to that object in the heap, thereby
keeping the heap fully compact and allowing very fast allocation of new
objects.  However, it makes C code that wants to deal with objects even
more messy than it would be without; if you want some examples, look at
the hairy code in GNU Smalltalk that deals with processes.

   To shield you as much as possible from the complications of doing
object-oriented programming in a non-object-oriented environment like C,
GNU Smalltalk provides friendly functions to map between common
Smalltalk objects and C types.  This way you can simply declare OOP
variables and then use these functions to treat their contents like C
data.

   These functions are passed to a module via the `VMProxy' struct, a
pointer to which is passed to the module, as shown in *note Linking
your libraries to the virtual machine: External modules.  They can be
divided in two groups, those that map _from Smalltalk objects to C data
types_ and those that map _from C data types to Smalltalk objects_.

   Here are those in the former group (Smalltalk to C); you can see that
they all begin with `OOPTo':

 -- Function: long OOPToInt (OOP)
     This function assumes that the passed OOP is an Integer and
     returns the C `signed long' for that integer.

 -- Function: long OOPToId (OOP)
     This function returns an unique identifier for the given OOP, valid
     until the OOP is garbage-collected.

 -- Function: double OOPToFloat (OOP)
     This function assumes that the passed OOP is an Integer or Float
     and returns the C `double' for that object.

 -- Function: long double OOPToLongDouble (OOP)
     This function assumes that the passed OOP is an Integer or Float
     and returns the C `long double' for that object.

 -- Function: int OOPToBool (OOP)
     This function returns a C integer which is true (i.e. `!= 0') if
     the given OOP is the `true' object, false (i.e. `== 0') otherwise.

 -- Function: char OOPToChar (OOP)
     This function assumes that the passed OOP is a Character and
     returns the C `char' for that integer.

 -- Function: wchar_t OOPToWChar (OOP)
     This function assumes that the passed OOP is a Character or
     UnicodeCharacter and returns the C `wchar_t' for that integer.

 -- Function: char *OOPToString (OOP)
     This function assumes that the passed OOP is a String or ByteArray
     and returns a C null-terminated `char *' with the same contents.
     It is the caller's responsibility to free the pointer and to
     handle possible `NUL' characters inside the Smalltalk object.

 -- Function: wchar_t *OOPToWString (OOP)
     This function assumes that the passed OOP is a UnicodeString and
     returns a C null-terminated `wchar_t *' with the same contents.
     It is the caller's responsibility to free the pointer and to
     handle possible `NUL' characters inside the Smalltalk object.

 -- Function: char *OOPToByteArray (OOP)
     This function assumes that the passed OOP is a String or ByteArray
     and returns a C `char *' with the same contents, without
     null-terminating it.  It is the caller's responsibility to free the
     pointer.

 -- Function: PTR OOPToCObject (OOP)
     This functions assumes that the passed OOP is a kind of CObject and
     returns a C `PTR' to the C data pointed to by the object. The
     caller should not free the pointer, nor assume anything about its
     size and contents, unless it exactly knows what it's doing.  A
     `PTR' is a `void *' if supported, or otherwise a `char *'.

 -- Function: long OOPToC (OOP)
     This functions assumes that the passed OOP is a String, a
     ByteArray, a CObject, or a built-in object (`nil', `true', `false',
     character, integer).  If the OOP is `nil', it answers 0; else the
     mapping for each object is exactly the same as for the above
     functions.  Note that, even though the function is declared as
     returning a `long', you might need to cast it to either a `char *'
     or `PTR'.

   While special care is needed to use the functions above (you will
probably want to know at least the type of the Smalltalk object you're
converting), the functions below, which convert C data to Smalltalk
objects, are easier to use and also put objects in the incubator so that
they are not swept by a garbage collection (*note Incubator::). These
functions all "end" with `ToOOP', except `cObjectToTypedOOP':

 -- Function: OOP intToOOP (long)
     This object returns a Smalltalk `Integer' which contains the same
     value as the passed C `long'.  Note that Smalltalk integers are
     always signed and have a bit less of precision with respect to C
     longs.  On 32 bit machines, their precision is 30 bits (if
     unsigned) or 31 bits (if signed); on 64 bit machines, their
     precision is 62 bits (if unsigned) or 63 bits (if signed).

 -- Function: OOP idToOOP (OOP)
     This function returns an OOP from a unique identifier returned by
     `OOPToId'.  The OOP will be the same that was passed to `OOPToId'
     only if the original OOP has not been garbage-collected since the
     call to `OOPToId'.

 -- Function: OOP floatToOOP (double)
     This object returns a Smalltalk `FloatD' which contains the same
     value as the passed `double'.  Unlike Integers, FloatDs have
     exactly the same precision as C doubles.

 -- Function: OOP longDoubleToOOP (long double)
     This object returns a Smalltalk `FloatQ' which contains the same
     value as the passed `long double'.  Unlike Integers, FloatQs have
     exactly the same precision as C long doubles.

 -- Function: OOP boolToOOP (int)
     This object returns a Smalltalk `Boolean' which contains the same
     boolean value as the passed C `int'.  That is, the returned OOP is
     the sole instance of either `False' or `True', depending on where
     the parameter is zero or not.

 -- Function: OOP charToOOP (char)
     This object returns a Smalltalk `Character' which represents the
     same char as the passed C `char'.

 -- Function: OOP charToOOP (wchar_t)
     This object returns a Smalltalk `Character' or `UnicodeCharacter'
     which represents the same char as the passed C `wchar_t'.

 -- Function: OOP classNameToOOP (char *)
     This method returns the Smalltalk class (i.e. an instance of a
     subclass of Class) whose name is the given parameter.  Namespaces
     are supported; the parameter must give the complete path to the
     class starting from the `Smalltalk' dictionary.  `NULL' is
     returned if the class is not found.

     This method is slow; you can safely cache its result.

 -- Function: OOP stringToOOP (char *)
     This method returns a String which maps to the given
     null-terminated C string, or the builtin object `nil' if the
     parameter points to address 0 (zero).

 -- Function: OOP wstringToOOP (wchar_t *)
     This method returns a UnicodeString which maps to the given
     null-terminated C wide string, or the builtin object `nil' if the
     parameter points to address 0 (zero).

 -- Function: OOP byteArrayToOOP (char *, int)
     This method returns a ByteArray which maps to the bytes that the
     first parameters points to; the second parameter gives the size of
     the ByteArray.  The builtin object `nil' is returned if the first
     parameter points to address 0 (zero).

 -- Function: OOP symbolToOOP (char *)
     This method returns a String which maps to the given
     null-terminated C string, or the builtin object `nil' if the
     parameter points to address 0 (zero).

 -- Function: OOP cObjectToOOP (PTR)
     This method returns a CObject which maps to the given C pointer,
     or the builtin object `nil' if the parameter points to address 0
     (zero).  The returned value has no precise CType assigned. To
     assign one, use `cObjectToTypedOOP'.

 -- Function: OOP cObjectToTypedOOP (PTR, OOP)
     This method returns a CObject which maps to the given C pointer,
     or the builtin object `nil' if the parameter points to address 0
     (zero).  The returned value has the second parameter as its type;
     to get possible types you can use `typeNameToOOP'.

 -- Function: OOP typeNameToOOP (char *)
     All this method actually does is evaluating its parameter as
     Smalltalk code; so you can, for example, use it in any of these
     ways:
          cIntType = typeNameToOOP("CIntType");
          myOwnCStructType = typeNameToOOP("MyOwnCStruct type");

     This method is primarily used by `msgSendf' (*note Smalltalk
     callin::), but it can be useful if you use lower level call-in
     methods. This method is slow too; you can safely cache its result.

   As said above, the C to Smalltalk layer automatically puts the
objects it creates in the incubator which prevents objects from being
collected as garbage.  A plugin, however, has limited control on the
incubator, and the incubator itself is not at all useful when objects
should be kept registered for a relatively long time, and whose lives
in the registry typically overlap.

   To avoid garbage collection of such object, you can use these
functions, which access a separate registry:

 -- Function: OOP registerOOP (OOP)
     Puts the given OOP in the registry.  If you register an object
     multiple times, you will need to unregister it the same number of
     times.  You may want to register objects returned by Smalltalk
     call-ins.

 -- Function: void unregisterOOP (OOP)
     Removes an occurrence of the given OOP from the registry.

 -- Function: void registerOOPArray (OOP **, OOP **)
     Tells the garbage collector that an array of objects must be made
     part of the root set.  The two parameters point indirectly to the
     base and the top of the array; that is, they are pointers to
     variables holding the base and the top of the array: having
     indirect pointers allows you to dynamically change the size of the
     array and even to relocate it in memory without having to
     unregister and re-register it every time you modify it.  If you
     register an array multiple times, you will need to unregister it
     the same number of times.

 -- Function: void unregisterOOPArray (OOP **)
     Removes the array with the given base from the registry.


File: gst.info,  Node: Smalltalk callin,  Next: Smalltalk callbacks,  Prev: Smalltalk types,  Up: C and Smalltalk

5.5 Calls from C to Smalltalk
=============================

GNU Smalltalk provides seven different function calls that allow you to
call Smalltalk methods in a different execution context than the current
one. The priority in which the method will execute will be the same as
the one of Smalltalk process which is currently active.

   Four of these functions are more low level and are more suited when
the Smalltalk program itself gave a receiver, a selector and maybe some
parameters; the others, instead, are more versatile.  One of them
(`msgSendf') automatically handles most conversions between C data
types and Smalltalk objects, while the others takes care of compiling
full snippets of Smalltalk code.

   All these functions handle properly the case of specifying, say, 5
arguments for a 3-argument selector--see the description of the single
functions for more information).

   In all cases except `msgSendf', passing NULL as the selector will
expect the receiver to be a block and evaluate it.

 -- Function: OOP msgSend (OOP receiver, OOP selector, ...)
     This function sends the given selector (should be a Symbol,
     otherwise `nilOOP' is returned) to the given receiver.  The
     message arguments should also be OOPs (otherwise, an access
     violation exception is pretty likely) and are passed in a
     NULL-terminated list after the selector.  The value returned from
     the method is passed back as an OOP to the C program as the result
     of `msgSend', or `nilOOP' if the number of arguments is wrong.
     Example (same as `1 + 2'):

              OOP shouldBeThreeOOP = vmProxy->msgSend(
                  intToOOP(1),
                  symbolToOOP("+"),
                  intToOOP(2),
                  NULL);

 -- Function: OOP strMsgSend (OOP receiver, char *selector, ...)
     This function is the same as above, but the selector is passed as
     a C string and is automatically converted to a Smalltalk symbol.

     Theoretically, this function is a bit slower than `msgSend' if your
     program has some way to cache the selector and avoiding a call to
     `symbolToOOP' on every call-in.  However, this is not so apparent
     in "real" code because the time spent in the Smalltalk interpreter
     will usually be much higher than the time spent converting the
     selector to a Symbol object.  Example:

              OOP shouldBeThreeOOP = vmProxy->strMsgSend(
                  intToOOP(1),
                  "+",
                  intToOOP(2),
                  NULL);

 -- Function: OOP vmsgSend (OOP receiver, OOP selector, OOP *args)
     This function is the same as msgSend, but accepts a pointer to the
     NULL-terminated list of arguments, instead of being a
     variable-arguments functions.  Example:

              OOP arguments[2], shouldBeThreeOOP;
              arguments[0] = intToOOP(2);
              arguments[1] = NULL;
              /* ... some more code here ... */

              shouldBeThreeOOP = vmProxy->vmsgSend(
                  intToOOP(1),
                  symbolToOOP("+"),
                  arguments);

 -- Function: OOP nvmsgSend (OOP receiver, OOP selector, OOP *args, int
          nargs)
     This function is the same as msgSend, but accepts an additional
     parameter containing the number of arguments to be passed to the
     Smalltalk method, instead of relying on the NULL-termination of
     args.  Example:

              OOP argument, shouldBeThreeOOP;
              argument = intToOOP(2);
              /* ... some more code here ... */

              shouldBeThreeOOP = vmProxy->nvmsgSend(
                  intToOOP(1),
                  symbolToOOP("+"),
                  &argument,
                  1);

 -- Function: OOP perform (OOP, OOP)
     Shortcut function to invoke a unary selector.  The first parameter
     is the receiver, and the second is the selector.

 -- Function: OOP performWith (OOP, OOP, OOP)
     Shortcut function to invoke a one-argument selector.  The first
     parameter is the receiver, the second is the selector, the third
     is the sole argument.

 -- Function: OOP invokeHook (int)
     Calls into Smalltalk to process a `ObjectMemory' hook given by the
     parameter.  In practice, `changed:' is sent to `ObjectMemory' with
     a symbol derived from the parameter.  The parameter can be one of:
        * `GST_BEFORE_EVAL'

        * `GST_AFTER_EVAL'

        * `GST_ABOUT_TO_QUIT'

        * `GST_RETURN_FROM_SNAPSHOT'

        * `GST_ABOUT_TO_SNAPSHOT'

        * `GST_FINISHED_SNAPSHOT'

     All cases where the last three should be used should be covered in
     GNU Smalltalk's source code.  The first three, however, can
     actually be useful in user code.

   The two functions that directly accept Smalltalk code are named
`evalCode' and `evalExpr', and they're basically the same.  They both
accept a single parameter, a pointer to the code to be submitted to the
parser.  The main difference is that `evalCode' discards the result,
while `evalExpr' returns it to the caller as an OOP.

   `msgSendf', instead, has a radically different syntax. Let's first
look at some examples.

         /* 1 + 2 */
         int shouldBeThree;
         vmProxy->msgSendf(&shouldBeThree, "%i %i + %i", 1, 2)

         /* aCollection includes: 'abc' */
         OOP aCollection;
         int aBoolean;
         vmProxy->msgSendf(&aBoolean, "%b %o includes: %s", aCollection, "abc")

         /* 'This is a test' printNl -- in two different ways */
         vmProxy->msgSendf(NULL, "%v %s printNl", "This is a test");
         vmProxy->msgSendf(NULL, "%s %s printNl", "This is a test");

         /* 'This is a test', ' ok?' */
         char *str;
         vmProxy->msgSendf(&str, "%s %s , %s", "This is a test", " ok?");

   As you can see, the parameters to msgSendf are, in order:

   * A pointer to the variable which will contain the record. If this
     pointer is `NULL', it is discarded.

   * A description of the method's interface in this format (the object
     types, after percent signs, will be explained later in this
     section)

          %result_type %receiver_type selector %param1_type %param2_type

   * A C variable or Smalltalk object (depending on the type specifier)
     for the receiver

   * If needed, the C variables and/or Smalltalk object (depending on
     the type specifiers) for the arguments.

   Note that the receiver and parameters are NOT registered in the
object registry (*note Smalltalk types::).  "receiver_type" and
"paramX_type" can be any of these characters, with these meanings:

           Specifier        C data type        equivalent Smalltalk class
               i                long           Integer (see intToOOP)
               f               double          Float (see floatToOOP)
               F            long double        Float (see longDoubleToOOP)
               b                int            True or False (see boolToOOP)
               B                OOP            BlockClosure
               c                char           Character (see charToOOP)
               C               PTR             CObject (see cObjToOOP)
               s               char *          String (see stringToOOP)
               S               char *          Symbol (see symbolToOOP)
               o                OOP            any
               t            char *, PTR        CObject (see below)
               T              OOP, PTR         CObject (see below)
               w              wchar_t          Character (see wcharToOOP)
               W             wchar_t *         UnicodeString (see wstringToOOP)

`%t' and `%T' are particular in the sense that you need to pass "two"
additional arguments to `msgSendf', not one.  The first will be a
description of the type of the CObject to be created, the second
instead will be the CObject's address.  If you specify `%t', the first
of the two arguments will be converted to a Smalltalk `CType' via
`typeNameToOOP' (*note Smalltalk types::); instead, if you specify
`%T', you will have to directly pass an OOP for the new CObject's type.

   For `%B' you should not pass a selector, and the block will be
evaluated.

   The type specifiers you can pass for "result_type" are a bit
different:

                 Result
     Specifier   if nil    C data type      expected result
        i         0L          long          nil or an Integer
        f         0.0        double         nil or a Float
        F         0.0     long double       nil or a Float
        b          0          int           nil or a Boolean
        c        '\0'         char          nil or a Character
        C        NULL        PTR            nil or a CObject
        s        NULL        char *         nil, a String, or a Symbol
        ?         0      char *, PTR        See oopToC
        o       nilOOP        OOP           any (result is not converted)
        w        '\0'       wchar_t         nil or a Character
        W        NULL      wchar_t *        nil or a UnicodeString
        v                      /            any (result is discarded)

   Note that, if resultPtr is `NULL', the "result_type" is always
treated as `%v'.  If an error occurs, the value in the `result if nil'
column is returned.


File: gst.info,  Node: Smalltalk callbacks,  Next: Object representation,  Prev: Smalltalk callin,  Up: C and Smalltalk

5.6 Smalltalk blocks as C function pointers
===========================================

The Smalltalk callin mechanism can be used effectively to construct
bindings to C libraries that require callbacks into Smalltalk.
However, it is a "static" mechanism, as the callback functions passed
to the libraries have to be written in C and their type signatures are
fixed.

   If the signatures of the callbacks are not known in advance, and the
only way to define callbacks is via C function pointers (as opposed to
reflective mechanisms such as the ones in GTK+), then the `VMProxy'
functions for Smalltalk callin are not enough.

   GNU Smalltalk provides a more dynamic way to convert Smalltalk
blocks into C function pointers through the `CCallbackDescriptor' class.
This class has a constructor method that is similar to the `cCall:'
annotation used for callouts.  The method is called
`for:returning:withArgs:' and its parameters are:

   * a block, whose number of arguments is variable

   * a symbol representing the return type

   * an array representing the type of the arguments.

   The array passed as the third parameter represents values that are
passed _from C to Smalltalk_ and, as such, should be filled with the
same rules that are used by the _return   type_ of a C callout.  In
particular, if the C callback accepts an `int *' it is possible (and
indeed useful) to specify the type of the argument as `#{CInt}', so
that the block will receive a `CInt' object.

   Here is an example of creating a callback which is passed to
`glutReshapeFunc'(1).  The desired signature in C is `void (*) (int,
int)'.
     | glut |
     ...
     glut glutReshapeFunc: (CCallbackDescriptor
         for: [ :x :y | self reshape: x@y ]
         returning: #void
         withArgs: #(#int #int))

   It is important to note that this kind of callback does not survive
across an image load (this restriction may be lifted in a future
version).  When the image is loaded, it has to be reset by sending it
the `link' message before it is passed to any C function.  Sending the
`link' message to an already valid callback is harmless and cheap.

   ---------- Footnotes ----------

   (1) The GLUT bindings use a different   scheme for setting up
callbacks.


File: gst.info,  Node: Other C functions,  Next: Using Smalltalk,  Prev: Incubator,  Up: C and Smalltalk

5.7 Other functions available to modules
========================================

In addition to the functions described so far, the `VMProxy' that is
available to modules contains entry-points for many functions that aid
in developing GNU Smalltalk extensions in C.  This node documents these
functions and the macros that are defined by `libgst/gstpub.h'.

 -- Function: void asyncCall (void (*) (OOP), OOP)
     This functions accepts a function pointer and an OOP (or `NULL',
     but not an arbitrary pointer) and sets up the interpreter to call
     the function as soon as the next message send is executed.

     _Caution:_ This and the next two are the only functions in the
     `intepreterProxy' that are thread-safe.

 -- Function: void asyncSignal (OOP)
     This functions accepts an OOP for a `Semaphore' object and signals
     that object so that one of the processes waiting on that semaphore
     is waken up.  Since a Smalltalk call-in is not an atomic
     operation, the correct way to signal a semaphore is not to send
     the `signal' method to the object but, rather, to use:

              asyncSignal(semaphoreOOP)

     The signal request will be processed as soon as the next message
     send is executed.

 -- Function: void asyncSignalAndUnregister (OOP)
     This functions accepts an OOP for a `Semaphore' object and signals
     that object so that one of the processes waiting on that semaphore
     is waken up; the signal request will be processed as soon as the
     next message send is executed.  The object is then removed from
     the registry.

 -- Function: void wakeUp (void)
     When no Smalltalk process is running, GNU Smalltalk tries to limit
     CPU usage by pausing until it gets a signal from the OS.  `wakeUp'
     is an alternative way to wake up the main Smalltalk loop.  This
     should rarely be necessary, since the above functions already call
     it automatically.

 -- Function: void syncSignal (OOP, mst_Boolean)
     This functions accepts an OOP for a `Semaphore' object and signals
     that object so that one of the processes waiting on that semaphore
     is waken up.  If the semaphore has no process waiting in the queue
     and the second argument is true, an excess signal is added to the
     semaphore.  Since a Smalltalk call-in is not an atomic operation,
     the correct way to signal a semaphore is not to send the `signal'
     or `notify' methods to the object but, rather, to use:

              syncSignal(semaphoreOOP, true)

     The `sync' in the name of this function distinguishes it from
     `asyncSignal', in that it can only be called from a procedure
     already scheduled with `asyncCall'.  It cannot be called from a
     call-in, or from other threads than the interpreter thread.

 -- Function: void syncWait (OOP)
     This function is present for backwards-compatibility only and
     should not be used.

 -- Function: void showBacktrace (FILE *)
     This functions show a backtrace on the given file.

 -- Function: OOP objectAlloc (OOP, int)
     The `objectAlloc' function allocates an OOP for a newly created
     instance of the class whose OOP is passed as the first parameter;
     if that parameter is not a class the results are undefined (for
     now, read as "the program will most likely core dump", but that
     could change in a future version).

     The second parameter is used only if the class is an indexable one,
     otherwise it is discarded: it contains the number of indexed
     instance variables in the object that is going to be created.
     Simple uses of `objectAlloc' include:

          OOP myClassOOP;
          OOP myNewObject;
          myNewObjectData obj;
          ...
          myNewObject = objectAlloc(myClassOOP, 0);
          obj = (myNewObjectData) OOP_TO_OBJ (myNewObject);
          obj->arguments = objectAlloc(classNameToOOP("Array"), 10);
          ...

 -- Function: size_t OOPSize (OOP)
     Return the number of indexed instance variables in the given
     object.

 -- Function: OOP OOPAt (OOP, size_t)
     Return an indexed instance variable of the given object.  The
     index is in the second parameter and is zero-based.  The function
     aborts if the index is out of range.

 -- Function: OOP OOPAtPut (OOP, size_t, OOP)
     Put the object given as the third parameter into an indexed
     instance variable of the object given as the first parameter.  The
     index in the second parameter and is zero-based.  The function
     aborts if the index is out of range.

     The function returns the old value of the indexed instance
     variable.

 -- Function: enum gst_indexed_kind OOPIndexedKind (OOP)
     Return the kind of indexed instance variables that the given
     object has.

 -- Function: void * OOPIndexedBase (OOP)
     Return a pointer to the first indexed instance variable of the
     given object.  The program should first retrieve the kind of data
     using OOPIndexedKind.

 -- Function: OOP getObjectClass (OOP)
     Return the class of the Smalltalk object passed as a parameter.

 -- Function: OOP getSuperclass (OOP)
     Return the superclass of the class given by the Smalltalk object,
     that is passed as a parameter.

 -- Function: mst_Boolean classIsKindOf (OOP, OOP)
     Return true if the class given as the first parameter, is the same
     or a superclass of the class given as the second parameter.

 -- Function: mst_Boolean objectIsKindOf (OOP, OOP)
     Return true if the object given as the first parameter is an
     instance of the class given as the second parameter, or of any of
     its subclasses.

 -- Function: mst_Boolean classImplementsSelector (OOP, OOP)
     Return true if the class given as the first parameter implements
     or overrides the method whose selector is given as the second
     parameter.

 -- Function: mst_Boolean classCanUnderstand (OOP, OOP)
     Return true if instances of the class given as the first parameter
     respond to the message whose selector is given as the second
     parameter.

 -- Function: mst_Boolean respondsTo (OOP, OOP)
     Return true if the object given as the first parameter responds to
     the message whose selector is given as the second parameter.

   Finally, several slots of the interpreter proxy provide access to the
system objects and to the most important classes.  These are:
   * `nilOOP', `trueOOP', `falseOOP', `processorOOP'

   * `objectClass', `arrayClass', `stringClass',     `characterClass',
     `smallIntegerClass', `floatDClass',     `floatEClass',
     `byteArrayClass', `objectMemoryClass',     `classClass',
     `behaviorClass', `blockClosureClass',     `contextPartClass',
     `blockContextClass',     `methodContextClass',
     `compiledMethodClass',     `compiledBlockClass',
     `fileDescriptorClass',     `fileStreamClass', `processClass',
     `semaphoreClass',     `cObjectClass'

   More may be added in the future

   The macros are(1):

 -- Macro: gst_object OOP_TO_OBJ (OOP)
     Dereference a pointer to an OOP into a pointer to the actual
     object data (*note Object representation::).  The result of
     `OOP_TO_OBJ' is not valid anymore if a garbage-collection happens;
     for this reason, you should assume that a pointer to object data
     is not valid after doing a call-in, calling `objectAlloc', and
     caling any of the "C to Smalltalk" functions (*note Smalltalk
     types::).

 -- Macro: OOP OOP_CLASS (OOP)
     Return the OOP for the class of the given object.  For example,
     `OOP_CLASS(proxy->stringToOOP("Wonderful GNU Smalltalk"))' is the
     `String' class, as returned by `classNameToOOP("String")'.

 -- Macro: mst_Boolean IS_INT (OOP)
     Return a Boolean indicating whether or not the OOP is an Integer
     object; the value of SmallInteger objects is encoded directly in
     the OOP, not separately in a `gst_object' structure.  It is not
     safe to use `OOP_TO_OBJ' and `OOP_CLASS' if `isInt' returns false.

 -- Macro: mst_Boolean IS_OOP (OOP)
     Return a Boolean indicating whether or not the OOP is a `real'
     object (and not a SmallInteger).  It is safe to use `OOP_TO_OBJ'
     and `OOP_CLASS' only if `IS_OOP' returns true.

 -- Macro: mst_Boolean ARRAY_OOP_AT (gst_object, int)
     Access the character given in the second parameter of the given
     Array object.  Note that this is necessary because of the way
     `gst_object' is defined, which prevents `indexedOOP' from working.

 -- Macro: mst_Boolean STRING_OOP_AT (gst_object, int)
     Access the character given in the second parameter of the given
     String or ByteArray object.  Note that this is necessary because
     of the way `gst_object' is defined, which prevents `indexedByte'
     from working.

 -- Macro: mst_Boolean INDEXED_WORD (SOME-OBJECT-TYPE, int)
     Access the given indexed instance variable in a
     `variableWordSubclass'.  The first parameter must be a structure
     declared as described in *note Object representation::).

 -- Macro: mst_Boolean INDEXED_BYTE (SOME-OBJECT-TYPE, int)
     Access the given indexed instance variable in a
     `variableByteSubclass'.  The first parameter must be a structure
     declared as described in *note Object representation::).

 -- Macro: mst_Boolean INDEXED_OOP (SOME-OBJECT-TYPE, int)
     Access the given indexed instance variable in a `variableSubclass'.
     The first parameter must be a structure declared as described in
     *note Object representation::).

   ---------- Footnotes ----------

   (1) IS_NIL and IS_CLASS have been removed because   they are
problematic in shared libraries (modules), where they   caused
undefined symbols to be present in the shared library.  These   are now
private to `libgst.a'.  You should use the `nilOOP'   field of the
interpreter proxy, or `getObjectClass'.


File: gst.info,  Node: Object representation,  Next: Incubator,  Prev: Smalltalk callbacks,  Up: C and Smalltalk

5.8 Manipulating instances of your own Smalltalk classes from C
===============================================================

Although GNU Smalltalk's library exposes functions to deal with
instances of the most common base class, it's likely that, sooner or
later, you'll want your C code to directly deal with instances of
classes defined by your program.  There are three steps in doing so:

   * Defining the Smalltalk class

   * Defining a C `struct' that maps the representation of the class

   * Actually using the C struct

   In this chapter you will be taken through these steps considering the
hypotetical task of defining a Smalltalk interface to an SQL server.

   The first part is also the simplest, since defining the Smalltalk
class can be done in a single way which is also easy and very
practical; just evaluate the standard Smalltalk code that does that:

         Object subclass: SQLAction [
              | database request |
              <category: 'SQL-C interface'>
         ]

         SQLAction subclass: SQLRequest [
              | returnedRows |
              <category: 'SQL-C interface'>
         ]

   To define the C `struct' for a class derived from Object, GNU
Smalltalk's `gstpub.h' include file defines an `OBJ_HEADER' macro which
defines the fields that constitute the header of every object.  Defining
a `struct' for SQLAction results then in the following code:

         struct st_SQLAction {
             OBJ_HEADER;
             OOP database;
             OOP request;
         }

   The representation of SQLRequest in memory is this:

         .------------------------------.
         |     common object header     |    2 longs
         |------------------------------|
         | SQLAction instance variables |
         |           database           |    2 longs
         |           request            |
         |------------------------------|
         | SQLRequest instance variable |
         |         returnedRows         |    1 long
         '------------------------------'

   A first way to define the struct would then be:
         typedef struct st_SQLAction {
             OBJ_HEADER;
             OOP database;
             OOP request;
             OOP returnedRows;
         } *SQLAction;

but this results in a lot of duplicated code.  Think of what would
happen if you had other subclasses of `SQLAction' such as
`SQLObjectCreation', `SQLUpdateQuery', and so on! The solution, which
is also the one used in GNU Smalltalk's source code is to define a
macro for each superclass, in this way:

         /* SQLAction
             |-- SQLRequest
             |     `-- SQLUpdateQuery
             `-- SQLObjectCreation       */

         #define ST_SQLACTION_HEADER         \
             OBJ_HEADER;                     \
             OOP database;                   \
             OOP request                     /* no semicolon */

         #define ST_SQLREQUEST_HEADER        \
             ST_SQLACTION_HEADER;            \
             OOP returnedRows                /* no semicolon */

         typedef struct st_SQLAction {
             ST_SQLACTION_HEADER;
         } *SQLAction;

         typedef struct st_SQLRequest {
             ST_SQLREQUEST_HEADER;
         } *SQLRequest;

         typedef struct st_SQLObjectCreation {
             ST_SQLACTION_HEADER;
             OOP newDBObject;
         } *SQLObjectCreation;

         typedef struct st_SQLUpdateQuery {
             ST_SQLREQUEST_HEADER;
             OOP numUpdatedRows;
         } *SQLUpdateQuery;

   Note that the macro you declare is used instead of `OBJ_HEADER' in
the declaration of both the superclass and the subclasses.

   Although this example does not show that, please note that you should
not declare anything if the class has indexed instance variables.

   The first step in actually using your structs is obtaining a pointer
to an OOP which is an instance of your class.  Ways to do so include
doing a call-in, receiving the object from a call-out (using
`#smalltalk', `#self' or `#selfSmalltalk' as the type specifier).

   Let's assume that the `oop' variable contains such an object.  Then,
you have to dereference the OOP (which, as you might recall from *note
Smalltalk types::, point to the actual object only indirectly) and get
a pointer to the actual data.  You do that with the `OOP_TO_OBJ' macro
(note the type casting):

         SQLAction action = (SQLAction) OOP_TO_OBJ(oop);

   Now you can use the fields in the object like in this pseudo-code:

         /* These are retrieved via classNameToOOP and then cached in global
            variables */
         OOP sqlUpdateQueryClass, sqlActionClass, sqlObjectCreationClass;
         ...
         invoke_sql_query(
             vmProxy->oopToCObject(action->database),
             vmProxy->oopToString(action->request),
             query_completed_callback,       /* Callback function */
             oop);                           /* Passed to the callback */

         ...

         /* Imagine that invoke_sql_query runs asynchronously and calls this
            when the job is done. */
         void
         query_completed_callback(result, database, request, clientData)
              struct query_result *result;
              struct db *database;
              char *request;
              OOP clientData;
         {
           SQLUpdateQuery query;
           OOP rows;
           OOP cObject;

           /* Free the memory allocated by oopToString */
           free(request);

           if (OOP_CLASS (oop) == sqlActionClass)
             return;

           if (OOP_CLASS (oop) == sqlObjectCreationClass)
             {
               SQLObjectCreation oc;
               oc = (SQLObjectCreation) OOP_TO_OBJ (clientData);
               cObject = vmProxy->cObjectToOOP (result->dbObject)
               oc->newDBObject = cObject;
             }
           else
             {
               /* SQLRequest or SQLUpdateQuery */
               cObject = vmProxy->cObjectToOOP (result->rows);
               query = (SQLUpdateQuery) OOP_TO_OBJ (clientData);
               query->returnedRows = cObject;
               if (OOP_CLASS (oop) == sqlUpdateQueryClass)
                 query->numReturnedRows = vmProxy->intToOOP (result->count);
             }

         }

   Note that the result of `OOP_TO_OBJ' is not valid anymore if a
garbage-collection happens; for this reason, you should assume that a
pointer to object data is not valid after doing a call-in, calling
`objectAlloc', and using any of the "C to Smalltalk" functions except
`intToOOP' (*note Smalltalk types::).  That's why I passed the OOP to
the callback, not the object pointer itself.

   If your class has indexed instance variables, you can use the
`INDEXED_WORD', `INDEXED_OOP' and `INDEXED_BYTE' macros declared in
`gstpub.h', which return an lvalue for the given indexed instance
variable--for more information, *note Other C functions::.


File: gst.info,  Node: Using Smalltalk,  Prev: Other C functions,  Up: C and Smalltalk

5.9 Using the Smalltalk environment as an extension library
===========================================================

If you are reading this chapter because you are going to write
extensions to GNU Smalltalk, this section won't probably interest you.
But if you intend to use GNU Smalltalk as a scripting language or an
extension language for your future marvellous software projects, you
might be interest.

   How to initialize GNU Smalltalk is most briefly and easily explained
by looking at GNU Smalltalk's own source code.  For this reason, here
is a simplified snippet from `gst-tool.c'.

     int main(argc, argv)
     int     argc;
     char    **argv;
     {
       gst_set_var (GST_VERBOSITY, 1);
       gst_smalltalk_args (argc - 1, argv + 1);
       gst_set_executable_path (argv[0]);
       result = gst_initialize ("KERNEL-DIR", "IMAGE-FILE", GST_NO_TTY);
       if (result != 0)
         exit (result < 0 ? 1 : result);

       if (!gst_process_file ("SOURCE-FILE", GST_DIR_KERNEL_SYSTEM))
         perror ("gst: couldn't load `SOURCE-FILE'");

       gst_invoke_hook (GST_ABOUT_TO_QUIT);
       exit (0);
     }

   Your initialization code will be almost the same as that in GNU
Smalltalk's `main()', with the exception of the call to
`gst_process_file'.  All you'll have to do is to pass some arguments to
the GNU Smalltalk library via `gst_smalltalk_args', possibly modify
some defaults using `gst_get_var' and `gst_set_var', and then call
`gst_initialize'.

   Variable indices that can be passed to  `gst_get_var' and
`gst_set_var' include:

`GST_DECLARE_TRACING'

`GST_EXECUTION_TRACING'

`GST_EXECUTION_TRACING_VERBOSE'

`GST_GC_MESSAGE'

`GST_VERBOSITY'

`GST_MAKE_CORE_FILE'

`GST_REGRESSION_TESTING'

   While the flags that can be passed as the last parameter to
`gst_initialize' are any combination of these:

`GST_REBUILD_IMAGE'

`GST_MAYBE_REBUILD_IMAGE'

`GST_IGNORE_USER_FILES'

`GST_IGNORE_BAD_IMAGE_NAME'

`GST_IGNORE_BAD_IMAGE_PATH'

`GST_IGNORE_BAD_KERNEL_PATH'

`GST_NO_TTY'

   Note that `gst_initialize' will likely take some time (from a tenth
of a second to 3-4 seconds), because it has to check if the image file
must be be rebuilt and, if so, it reloads and recompiles the over 50,000
lines of Smalltalk code that form a basic image.  To avoid this check,
pass a valid image file as the second argument to `gst_initialize'.

   The result of `gst_init_smalltalk' is `0' for success, while
anything else is an error code.

   If you're using GNU Smalltalk as an extension library, you might
also want to disable the two `ObjectMemory' class methods, `quit' and
`quit:' method.  I advice you not to change the Smalltalk kernel code.
Instead, in the script that loads your extension classes add these two
lines:

     ObjectMemory class compile: 'quit        self shouldNotImplement'!
     ObjectMemory class compile: 'quit: n     self shouldNotImplement'!
   which will effectively disable the two offending methods.  Other
possibilities include using `atexit' (from the C library) to exit your
program in a less traumatic way, or redefining these two methods to
exit through a call out to a C routine in your program.

   Also, note that it is not a problem if you develop the class
libraries for your programs within GNU Smalltalk's environment (which
will not call `defineCFunc' for your own C call-outs), since the
addresses of the C call-outs are looked up again when an image is
restored.


File: gst.info,  Node: Incubator,  Next: Other C functions,  Prev: Object representation,  Up: C and Smalltalk

5.10 Incubator support
======================

The incubator concept provides a mechanism to protect newly created
objects from being accidentally garbage collected before they can be
attached to some object which is reachable from the root set.

   If you are creating some set of objects which will not be
immediately (that means, before the next object is allocated from the
Smalltalk memory system) be attached to an object which is still "live"
(reachable from the root set of objects), you'll need to use this
interface.

   If you are writing a C call-out from Smalltalk (for example, inside a
module), you will not have direct access to the incubator; instead the
functions described in *note Smalltalk types:: automatically put the
objects that they create in the incubator, and the virtual machine
takes care of wrapping C call-outs so that the incubator state is
restored at the end of the call.

   This section describes its usage from the point of view of a program
that is linking with `libgst.a'.  Such a program has much finer control
to the incubator.  The interface provides the following operations:

 -- Macro: void INC_ADD_OOP (OOP anOOP)
     Adds a new object to the protected set.

 -- Macro: inc_ptr INC_SAVE_POINTER ()
     Retrieves the current incubator pointer.  Think of the incubator
     as a stack, and this operation returns the current stack pointer
     for later use (restoration) with the incRestorePointer function.

 -- Macro: void INC_RESTORE_POINTER (inc_ptr ptr)
     Sets (restores) the incubator pointer to the given pointer value.

   Typically, when you are within a function which allocates more than
one object at a time, either directly or indirectly, you'd want to use
the incubator mechanism.  First you'd save a copy of the current
pointer in a local variable.  Then, for each object you allocate
(except the last, if you want to be optimal), after you create the
object you add it to the incubator's list.  When you return, you need
to restore the incubator's pointer to the value you got with
`INC_SAVE_POINTER' using the `INC_RESTORE_POINTER' macro.

   Here's an example from cint.c:

   The old code was (the comments are added for this example):

       desc = (_gst_cfunc_descriptor)
          new_instance_with (cFuncDescriptorClass, numArgs);
       desc->cFunction = _gst_cobject_new (funcAddr);    // 1
       desc->cFunctionName = _gst_string_new (funcName); // 2
       desc->numFixedArgs = FROM_INT (numArgs);
       desc->returnType = _gst_classify_type_symbol (returnTypeOOP, true);
       for (i = 1; i <= numArgs; i++) {
         desc->argTypes[i - 1] =
          _gst_classify_type_symbol(ARRAY_AT(argsOOP, i), false);
       }

       return (_gst_alloc_oop(desc));

   `desc' is originally allocated via `newInstanceWith' and `allocOOP',
two private routines which are encapsulated by the public routine
`objectAlloc'.  At "1", more storage is allocated, and the garbage
collector has the potential to run and free (since no live object is
referring to it) desc's storage.  At "2" another object is allocated,
and again the potential for losing both `desc' and `desc->cFunction' is
there if the GC runs (this actually happened!).

   To fix this code to use the incubator, modify it like this:

       OOP     descOOP;
       IncPtr  ptr;

       incPtr = INC_SAVE_POINTER();
       desc = (_gst_cfunc_descriptor)
          new_instance_with (cFuncDescriptorClass, numArgs);
       descOOP = _gst_alloc_oop(desc);
       INC_ADD_OOP (descOOP);

       desc->cFunction = _gst_cobject_new (funcAddr);    // 1
       INC_ADD_OOP (desc->cFunction);

       desc->cFunctionName = _gst_string_new (funcName); // 2
       /* since none of the rest of the function (or the functions it calls)
        * allocates any storage, we don't have to add desc->cFunctionName
        * to the incubator's set of objects, although we could if we wanted
        * to be completely safe against changes to the implementations of
        * the functions called from this function.
        */

       desc->numFixedArgs = FROM_INT (numArgs);
       desc->returnType = _gst_classify_type_symbol (returnTypeOOP, true);
       for (i = 1; i <= numArgs; i++) {
         desc->argTypes[i - 1] =
          _gst_classify_type_symbol(ARRAY_AT(argsOOP, i), false);
       }

       return (_gst_alloc_oop(desc));

   Note that it is permissible for two or more functions to cooperate
with their use of the incubator.  For example, say function A allocates
some objects, then calls function B which allocates some more objects,
and then control returns to A where it does some more execution with the
allocated objects.  If B is only called by A, B can leave the management
of the incubator pointer up to A, and just register the objects it
allocates with the incubator.  When A does a `INC_RESTORE_POINTER', it
automatically clears out the objects that B has registered from the
incubator's set of objects as well; the incubator doesn't know about
functions A & B, so as far as it is concerned, all of the registered
objects were registered from the same function.


File: gst.info,  Node: Tutorial,  Prev: C and Smalltalk,  Up: Top

6 Tutorial
**********

What this manual presents
     This document provides a tutorial introduction to the Smalltalk
     language in general, and the GNU Smalltalk implementation in
     particular.  It does not provide exhaustive coverage of every
     feature of the language and its libraries; instead, it attempts to
     introduce a critical mass of ideas and techniques to get the
     Smalltalk novice moving in the right direction.

Who this manual is written for
     This manual assumes that the reader is acquainted with the basics
     of computer science, and has reasonable proficiency with a
     procedural language such as C.  It also assumes that the reader is
     already familiar with the usual janitorial tasks associated with
     programming: editing, moving files, and so forth.

* Menu:

* Getting started::              Starting to explore GNU Smalltalk
* Some classes::                 Using some of the Smalltalk classes
* The hierarchy::                The Smalltalk class hierarchy
* Creating classes::             Creating a new class of objects
* Creating subclasses::          Adding subclasses to another class
* Code blocks (I)::              Control structures in Smalltalk
* Code blocks (II)::             Guess what?  More control structures
* Debugging::                    Things go bad in Smalltalk too!
* More subclassing::             Coexisting in the class hierarchy
* Streams::                      A powerful abstraction useful in scripts
* Exception handling::           More sophisticated error handling
* Behind the scenes::            Some nice stuff from the Smalltalk innards
* And now::                      Some final words
* The syntax::                   For the most die-hard computer scientists


File: gst.info,  Node: Getting started,  Next: Some classes,  Up: Tutorial

6.1 Getting started
===================

* Menu:

* Starting Smalltalk::           Starting up Smalltalk
* Saying hello::                 Saying hello
* What happened::                But how does it say hello?
* Doing math::                   Smalltalk too can do it!
* Math in Smalltalk::            But in a peculiar way of course...


File: gst.info,  Node: Starting Smalltalk,  Next: Saying hello,  Up: Getting started

6.1.1 Starting up Smalltalk
---------------------------

Assuming that GNU Smalltalk has been installed on your system, starting
it is as simple as:
        $ gst
   the system loads in Smalltalk, and displays a startup banner like:
          GNU Smalltalk ready

          st>

   You are now ready to try your hand at Smalltalk!  By the way, when
you're ready to quit, you exit Smalltalk by typing `control-D' on an
empty line.


File: gst.info,  Node: Saying hello,  Next: What happened,  Prev: Starting Smalltalk,  Up: Getting started

6.1.2 Saying hello
------------------

An initial exercise is to make Smalltalk say "hello" to you.  Type in
the following line (`printNl' is a upper case N and a lower case L):
        'Hello, world' printNl
   The system then prints back 'Hello, world' to you.  It prints it
twice, the first time because you asked to print and the second time
because the snipped evaluated to the 'Hello, world' string.(1)

   ---------- Footnotes ----------

   (1)     You can also have the system print out a lot of statistics
which     provide information on the performance of the underlying
Smalltalk     engine.  You can enable them by starting Smalltalk as:
        $ gst -V


File: gst.info,  Node: What happened,  Next: Doing math,  Prev: Saying hello,  Up: Getting started

6.1.3 What actually happened
----------------------------

The front-line Smalltalk interpreter gathers all text until a '!'
character and executes it.  So the actual Smalltalk code executed was:
        'Hello, world' printNl

   This code does two things.  First, it creates an object of type
`String' which contains the characters "Hello, world".  Second, it
sends the message named `printNl' to the object.  When the object is
done processing the message, the code is done and we get our prompt
back.  You'll notice that we didn't say anything about printing ing the
string, even though that's in fact what happened.  This was very much
on purpose: the code we typed in doesn't know anything about printing
strings.  It knew how to get a string object, and it knew how to send a
message to that object.  That's the end of the story for the code we
wrote.

   But for fun, let's take a look at what happened when the string
object received the `printNl' message.  The string object then went to
a table (1) which lists the messages which strings can receive, and
what code to execute.  It found that there is indeed an entry for
`printNl' in that table and ran this code.  This code then walked
through its characters, printing each of them out to the terminal.  (2)

   The central point is that an object is entirely self-contained; only
the object knew how to print itself out.  When we want an object to
print out, we ask the object itself to do the printing.

   ---------- Footnotes ----------

   (1) Which table?  This is determined by the type of the object.  An
object has a type, known as the class to which it belongs.  Each class
has a table of methods.  For the object we created, it is known as a
member of the `String' class.  So we go to the table associated with
the String class.

   (2) Actually, the message `printNl' was inherited from Object.  It
sent a `print' message, also inherited by Object, which then sent
`printOn:' to the object, specifying that it print to the `Transcript'
object.  The String class then prints its characters to the standard
output.


File: gst.info,  Node: Doing math,  Next: Math in Smalltalk,  Prev: What happened,  Up: Getting started

6.1.4 Doing math
----------------

A similar piece of code prints numbers:
       1234 printNl

   Notice how we used the same message, but have sent it to a new type
of object--an integer (from class `Integer').  The way in which an
integer is printed is much different from the way a string is printed
on the inside, but because we are just sending a message, we do not
have to be aware of this.  We tell it to `printNl', and it prints
itself out.

   As a user of an object, we can thus usually send a particular
message and expect basically the same kind of behavior, regardless of
object's internal structure (for instance, we have seen that sending
`printNl' to an object makes the object print itself).  In later
chapters we will see a wide range of types of objects.  Yet all of them
can be printed out the same way--with `printNl'.

   White space is ignored, except as it separates words.  This example
could also have looked like:
                   1234 printNl

   However, GNU Smalltalk tries to execute each line by itself if
possible.  If you wanted to write the code on two lines, you might have
written something like:
     (1234
         printNl)

   From now on, we'll omit `printNl' since GNU Smalltalk does the
service of printing the answer for us.

   An integer can be sent a number of messages in addition to just
printing itself.  An important set of messages for integers are the
ones which do math:

        9 + 7

   Answers (correctly!) the value 16.  The way that it does this,
however, is a significant departure from a procedural language.


File: gst.info,  Node: Math in Smalltalk,  Prev: Doing math,  Up: Getting started

6.1.5 Math in Smalltalk
-----------------------

In this case, what happened was that the object `9' (an Integer),
received a `+' message with an argument of `7' (also an Integer).  The
`+' message for integers then caused Smalltalk to create a new object
`16' and return it as the resultant object.  This `16' object was then
given the `printNl' message, and printed `16' on the terminal.

   Thus, math is not a special case in Smalltalk; it is done, exactly
like everything else, by creating objects, and sending them messages.
This may seem odd to the Smalltalk novice, but this regularity turns
out to be quite a boon: once you've mastered just a few paradigms, all
of the language "falls into place". Before you go on to the next
chapter, make sure you try math involving `*' (multiplication), `-'
(subtraction), and `/' (division) also.  These examples should get you
started:

        8 * (4 / 2)
        8 - (4 + 1)
        5 + 4
        2/3 + 7
        2 + 3 * 4
        2 + (3 * 4)


File: gst.info,  Node: Some classes,  Next: The hierarchy,  Prev: Getting started,  Up: Tutorial

6.2 Using some of the Smalltalk classes
=======================================

This chapter has examples which need a place to hold the objects they
create.  Such place is created automatically as necessary; when you
want to discard all the objects you stored, write an exclamation mark
at the end of the statement.

   Now let's create some new objects.

* Menu:

* Arrays::                   An array in Smalltalk
* Sets::                     A set in Smalltalk
* Dictionaries::             Getting more sophisticated, eh?
* Closing thoughts::         There always ought to be some closing thoughts


File: gst.info,  Node: Arrays,  Next: Sets,  Up: Some classes

6.2.1 An array in Smalltalk
---------------------------

An array in Smalltalk is similar to an array in any other language,
although the syntax may seem peculiar at first.  To create an array
with room for 20 elements, do(1):
        x := Array new: 20

   The `Array new: 20' creates the array; the `x :=' part connects the
name `x' with the object.  Until you assign something else to `x', you
can refer to this array by the name `x'.  Changing elements of the
array is not done using the `:=' operator; this operator is used only
to bind names to objects.  In fact, you never modify data structures;
instead, you send a message to the object, and it will modify itself.

   For instance:
        x at: 1
   which prints:
        nil

   The slots of an array are initially set to "nothing" (which
Smalltalk calls `nil').  Let's set the first slot to the number 99:
        x at: 1 put: 99
   and now make sure the 99 is actually there:
        x at: 1
   which then prints out:
        99

   These examples show how to manipulate an array.  They also show the
standard way in which messages are passed arguments ments.  In most
cases, if a message takes an argument, its name will end with `:'.(2)

   So when we said `x at: 1' we were sending a message to whatever
object was currently bound to `x' with an argument of 1.  For an array,
this results in the first slot of the array being returned.

   The second operation, `x at: 1 put: 99' is a message with two
arguments.  It tells the array to place the second argument (99) in the
slot specified by the first (1).  Thus, when we re-examine the first
slot, it does indeed now contain 99.

   There is a shorthand for describing the messages you send to
objects.  You just run the message names together.  So we would say
that our array accepts both the `at:' and `at:put:' messages.

   There is quite a bit of sanity checking built into an array.  The
request
        6 at: 1
   fails with an error; 6 is an integer, and can't be indexed.  Further,
        x at: 21
   fails with an error, because the array we created only has room for
20 objects.

   Finally, note that the object stored in an array is just like any
other object, so we can do things like:
        (x at: 1) + 1
   which (assuming you've been typing in the examples) will print 100.

   ---------- Footnotes ----------

   (1)     GNU Smalltalk supports completion in the same way as Bash or
GDB.      To enter the following line, you can for example type     `x
:= Arr<TAB> new: 20'.  This can come in handy     when you have to type
long names such as `IdentityDictionary',     which becomes
`Ide<TAB>D<TAB>'.  Everything     starting with a capital letter or
ending with a colon can     be completed.

   (2) Alert readers will remember that the math examples of the
previous chapter deviated from this.


File: gst.info,  Node: Sets,  Next: Dictionaries,  Prev: Arrays,  Up: Some classes

6.2.2 A set in Smalltalk
------------------------

We're done with the array we've been using, so we'll assign something
new to our `x' variable.  Note that we don't need to do anything
special about the old array: the fact that nobody is using it any more
will be automatically detected, and the memory reclaimed.  This is
known as garbage collection and it is generally done when Smalltalk
finds that it is running low on memory.  So, to get our new object,
simply do:
        x := Set new
   which creates an empty set.  To view its contents, do:
        x

   The kind of object is printed out (i.e., `Set'), and then the
members are listed within parenthesis.  Since it's empty, we see:
        Set ()

   Now let's toss some stuff into it.  We'll add the numbers 5 and 7,
plus the string 'foo'.  This is also the first example where we're
using more than one statement, and thus a good place to present the
statement separator--the `.' period:

        x add: 5. x add: 7. x add: 'foo'

   Like Pascal, and unlike C, statements are separated rather than
terminated.  Thus you need only use a `.' when you have finished one
statement and are starting another.  This is why our last statement,
`^r', does not have a `.' following.  Once again like Pascal, however,
Smalltalk won't complain if your enter a spurious statement separator
after the last statement.

   However, we can save a little typing by using a Smalltalk shorthand:

        x add: 5; add: 7; add: 'foo'

   This line does exactly what the previous one did.  The trick is that
the semicolon operator causes the message to be sent to the same object
as the last message sent.  So saying `; add: 7' is the same as saying
`x add: 7', because `x' was the last thing a message was sent to.

   This may not seem like such a big savings, but compare the ease when
your variable is named `aVeryLongVariableName' instead of just `x'!
We'll revisit some other occasions where `;' saves you trouble, but for
now let's continue with our set.  Type either version of the example,
and make sure that we've added 5, 7, and "foo":
        x
   we'll see that it now contains our data:
        Set ('foo' 5 7)

   What if we add something twice?  No problem--it just stays in the
set.  So a set is like a big checklist--either it's in there, or it
isn't.  To wit:
        x add:5; add: 5; add: 5; add: 5; yourself

   We've added 5 several times, but when we printed our set back out,
we just see:
        Set ('foo' 5 7)

   `yourself' is commonly sent at the end of the cascade, if what you
are interested in is the object itself--in this case, we were not
interested in the return value of `add: 5', which happens to be `5'
simply.  There's nothing magic in `yourself'; it is a unary message
like `printNl', which does nothing but returning the object itself.  So
you can do this too:

        x yourself

   What you put into a set with `add:', you can take out with
`remove:'.  Try:

        x remove: 5
        x printNl

   The set now prints as:
        Set ('foo' 7)

   The "5" is indeed gone from the set.

   We'll finish up with one more of the many things you can do with a
set--checking for membership.  Try:
        x includes: 7
        x includes: 5

   From which we see that x does indeed contain 7, but not 5.  Notice
that the answer is printed as `true' or `false'.  Once again, the thing
returned is an object--in this case, an object known as a boolean.
We'll look at the use of booleans later, but for now we'll just say
that booleans are nothing more than objects which can only either be
true or false--nothing else.  So they're very useful for answers to yes
or no questions, like the ones we just posed.  Let's take a look at
just one more kind of data structure:


File: gst.info,  Node: Dictionaries,  Next: Closing thoughts,  Prev: Sets,  Up: Some classes

6.2.3 Dictionaries
------------------

A dictionary is a special kind of collection.  With a regular array,
you must index it with integers.  With dictionaries, you can index it
with any object at all.  Dictionaries thus provide a very powerful way
of correlating one piece of information to another.  Their only
downside is that they are somewhat less efficient than simple arrays.
Try the following:
        y := Dictionary new
        y at: 'One' put: 1
        y at: 'Two' put: 2
        y at: 1 put: 'One'
        y at: 2 put: 'Two'

   This fills our dictionary in with some data.  The data is actually
stored in pairs of key and value (the key is what you give to `at:'--it
specifies a slot; the value is what is actually stored at that slot).
Notice how we were able to specify not only integers but also strings
as both the key and the value.  In fact, we can use any kind of object
we want as either--the dictionary doesn't care.

   Now we can map each key to a value:
        y at: 1
        y at: 'Two'

   which prints respectively:
        'One'
        2

   We can also ask a dictionary to print itself:
        y
   which prints:
        Dictionary (1->'One' 2->'Two' 'One'->1 'Two'->2 )
   where the first member of each pair is the key, and the second the
value.  It is now time to take a final look at the objects we have
created, and send them to oblivion:

        y
        x!

   The exclamation mark deleted GNU Smalltalk's knowledge of both
variables.  Asking for them again will return just `nil'.


File: gst.info,  Node: Closing thoughts,  Prev: Dictionaries,  Up: Some classes

6.2.4 Closing thoughts
----------------------

You've seen how Smalltalk provides you with some very powerful data
structures.  You've also seen how Smalltalk itself uses these same
facilities to implement the language.  But this is only the tip of the
iceberg--Smalltalk is much more than a collection of "neat" facilities
to use.  The objects and methods which are automatically available are
only the beginning of the foundation on which you build your
programs--Smalltalk allows you to add your own objects and methods into
the system, and then use them along with everything else.  The art of
programming in Smalltalk is the art of looking at your problems in
terms of objects, using the existing object types to good effect, and
enhancing Smalltalk with new types of objects.  Now that you've been
exposed to the basics of Smalltalk manipulation, we can begin to look
at this object-oriented technique of programming.


File: gst.info,  Node: The hierarchy,  Next: Creating classes,  Prev: Some classes,  Up: Tutorial

6.3 The Smalltalk class hierarchy
=================================

When programming in Smalltalk, you sometimes need to create new kinds
of objects, and define what various messages will do to these objects.
In the next chapter we will create some new classes, but first we need
to understand how Smalltalk organizes the types and objects it contains.
Because this is a pure "concept" chapter, without any actual Smalltalk
code to run, we will keep it short and to the point.

* Menu:

* Class Object::                The grandfather of every class
* Animals::                     A classic in learning OOP!
* But why::                     The bottom line of the class hierarchy


File: gst.info,  Node: Class Object,  Next: Animals,  Up: The hierarchy

6.3.1 Class `Object'
--------------------

Smalltalk organizes all of its classes as a tree hierarchy.  At the
very top of this hierarchy is class Object.  Following somewhere below
it are more specific classes, such as the ones we've worked
with--strings, integers, arrays, and so forth.  They are grouped
together based on their similarities; for instance, types of objects
which may be compared as greater or less than each other fall under a
class known as Magnitude.

   One of the first tasks when creating a new object is to figure out
where within this hierarchy your object falls.  Coming up with an
answer to this problem is at least as much art as science, and there
are no hard-and-fast rules to nail it down.  We'll take a look at three
kinds of objects to give you a feel for how this organization matters.


File: gst.info,  Node: Animals,  Next: But why,  Prev: Class Object,  Up: The hierarchy

6.3.2 Animals
-------------

Imagine that we have three kinds of objects, representing Animals,
Parrots, and Pigs.  Our messages will be eat, sing, and snort.  Our
first pass at inserting these objects into the Smalltalk hierarchy would
organize them like:
        Object
            Animals
            Parrots
            Pigs

   This means that Animals, Parrots, and Pigs are all direct
descendants of Object, and are not descendants of each other.

   Now we must define how each animal responds to each kind of message.

            Animals
                eat -> Say "I have now eaten"
                sing -> Error
                snort -> Error
            Parrots
                eat -> Say "I have now eaten"
                sing -> Say "Tweet"
                snort -> Error
            Pigs
                eat -> Say "I have now eaten""
                sing -> Error
                snort -> Say "Oink"

   Notice how we kept having to indicate an action for eat.  An
experienced object designer would immediately recognize this as a clue
that we haven't set up our hierarchy correctly.  Let's try a different
organization:
        Object
            Animals
                Parrots
                       Pigs

   That is, Parrots inherit from Animals, and Pigs from Parrots.  Now
Parrots inherit all of the actions from Animals, and Pigs from both
Parrots and Animals.  Because of this inheritance, we may now define a
new set of actions which spares us the redundancy of the previous set:
            Animals
                eat -> Say "I have now eaten"
                sing -> Error
                snort -> Error
            Parrots
                sing -> Say "Tweet"
            Pigs
                snort -> Say "Oink"

   Because Parrots and Pigs both inherit from Animals, we have only had
to define the eat action once.  However, we have made one mistake in
our class setup--what happens when we tell a Pig to sing?  It says
"Tweet", because we have put Pigs as an inheritor of Parrots.  Let's
try one final organization:
        Object
            Animals
                Parrots
                Pigs

   Now Parrots and Pigs inherit from Animals, but not from each other.
Let's also define one final pithy set of actions:
            Animals
                eat -> Say "I have eaten"
            Parrots
                sing -> Say "Tweet"
            Pigs
                snort -> Say "Oink"

   The change is just to leave out messages which are inappropriate.
If Smalltalk detects that a message is not known by an object or any of
its ancestors, it will automatically give an error--so you don't have
to do this sort of thing yourself.  Notice that now sending sing to a
Pig does indeed not say "Tweet"--it will cause a Smalltalk error
instead.


File: gst.info,  Node: But why,  Prev: Animals,  Up: The hierarchy

6.3.3 The bottom line of the class hierarchy
--------------------------------------------

The goal of the class hierarchy is to allow you to organize objects
into a relationship which allows a particular object to inherit the
code of its ancestors.  Once you have identified an effective
organization of types, you should find that a particular technique need
only be implemented once, then inherited by the children below.  This
keeps your code smaller, and allows you to fix a bug in a particular
algorithm in only once place--then have all users of it just inherit
the fix.

   You will find your decisions for adding objects change as you gain
experience.  As you become more familiar with the existing set of
objects and messages, your selections will increasingly "fit in" with
the existing ones.  But even a Smalltalk pro stops and thinks carefully
at this stage, so don't be daunted if your first choices seem difficult
and error-prone.


File: gst.info,  Node: Creating classes,  Next: Creating subclasses,  Prev: The hierarchy,  Up: Tutorial

6.4 Creating a new class of objects
===================================

With the basic techniques presented in the preceding chapters, we're
ready do our first real Smalltalk program.  In this chapter we will
construct three new types of objects (known as classes), using the
Smalltalk technique of inheritance to tie the classes together, create
new objects belonging to these classes (known as creating instances of
the class), and send messages to these objects.

   We'll exercise all this by implementing a toy home-finance
accounting system.  We will keep track of our overall cash, and will
have special handling for our checking and savings accounts.  From this
point on, we will be defining classes which will be used in future
chapters.  Since you will probably not be running this whole tutorial
in one Smalltalk session, it would be nice to save off the state of
Smalltalk and resume it without having to retype all the previous
examples.  To save the current state of GNU Smalltalk, type:

        ObjectMemory snapshot: 'myimage.im'
   and from your shell, to later restart Smalltalk from this "snapshot":
        $ gst -I myimage.im

   Such a snapshot currently takes a little more than a megabyte, and
contains all variables, classes, and definitions you have added.

* Menu:

* A new class::                  Creating a new class
* Documenting the class::        So anybody will know what it's about
* Defining methods::             So it will be useful
* Instance methods::             One of two kind of methods (the others,
                                 class methods, are above)
* A look at our object::         which will sorely show that something
                                 is still missing.
* Moving money around::          Let's make it more fun!
* Next coming::                  Yeah, what's next?!?


File: gst.info,  Node: A new class,  Next: Documenting the class,  Up: Creating classes

6.4.1 Creating a new class
--------------------------

Guess how you create a new class?  This should be getting monotonous by
now--by sending a message to an object.  The way we create our first
"custom" class is by sending the following message:

        Object subclass: #Account.
        Account instanceVariableNames: 'balance'.

   Quite a mouthful, isn't it?  GNU Smalltalk provides a simpler way to
write this, but for now let's stick with this.  Conceptually, it isn't
really that bad.  The Smalltalk variable Object is bound to the
grand-daddy of all classes on the system.  What we're doing here is
telling the Object class that we want to add to it a subclass known as
Account.  Then, `instanceVariableNames: 'balance'' tells the new class
that each of its objects ("instances") will have a hidden variable
named `balance'.


File: gst.info,  Node: Documenting the class,  Next: Defining methods,  Prev: A new class,  Up: Creating classes

6.4.2 Documenting the class
---------------------------

The next step is to associate a description with the class.  You do
this by sending a message to the new class:
        Account comment:
        'I represent a place to deposit and withdraw money'

   A description is associated with every Smalltalk class, and it's
considered good form to add a description to each new class you define.
To get the description for a given class:
        Account comment

   And your string is printed back to you.  Try this with class
Integer, too:
        Integer comment

   However, there is another way to define classes.  This still
translates to sending objects, but looks more like a traditional
programming language or scripting language:

     Object subclass: Account [
         | balance |
         <comment:
             'I represent a place to deposit and withdraw money'>
     ]

   This has created a class.  If we want to access it again, for
example to modify the comment, we can do so like this:

     Account extend [
         <comment:
             'I represent a place to withdraw money that has been deposited'>
     ]

   This instructs Smalltalk to pick an existing class, rather than
trying to create a subclass.


File: gst.info,  Node: Defining methods,  Next: Instance methods,  Prev: Documenting the class,  Up: Creating classes

6.4.3 Defining a method for the class
-------------------------------------

We have created a class, but it isn't ready to do any work for us--we
have to define some messages which the class can process first.  We'll
start at the beginning by defining methods for instance creation:
         Account class extend [
            new [
                | r |
                <category: 'instance creation'>
                r := super new.
                r init.
                ^r
           ]
        ]

   The important points about this are:

   * `Account class' means that we are defining messages which are to
     be sent to the Account class itself.

   * `<category: 'instance creation'>' is more documentation support;
     it says that the methods we are defining supports creating objects
     of type Account.

   * The text starting with `new [' and ending with `]' defined what
     action to take for the message `new'.  When you enter this
     definition, GNU Smalltalk will simply give you another prompt, but
     your method has been compiled in and is ready for use.  GNU
     Smalltalk is pretty quiet on successful method definitions--but
     you'll get plenty of error messages if there's a problem!

     If you're familiar with other Smalltalks, note that the body of
     the method is always in brackets.

   The best way to describe how this method works is to step through
it.  Imagine we sent a message to the new class Account with the
command line:
        Account new

   `Account' receives the message `new' and looks up how to process
this message.  It finds our new definition, and starts running it.  The
first line, `| r |', creates a local variable named `r' which can be
used as a placeholder for the objects we create.  `r' will go away as
soon as the message is done being processed; note the parallel with
`balance', which goes away as soon as the object is not used anymore.
And note that here you have to declare local variables explicitly,
unlike what you did in previous examples.

   The first real step is to actually create the object.  The line `r
:= super new' does this using a fancy trick.  The word `super' stands
for the same object that the message `new' was originally sent to
(remember?  it's `Account'), except that when Smalltalk goes to search
for the methods, it starts one level higher up in the hierarchy than
the current level.  So for a method in the Account class, this is the
Object class (because the class Account inherits from is Object--go
back and look at how we created the Account class), and the Object
class' methods then execute some code in response to the `#new'
message.  As it turns out, Object will do the actual creation of the
object when sent a `#new' message.

   One more time in slow motion: the Account method `#new' wants to do
some fiddling about when new objects are created, but he also wants to
let his parent do some work with a method of the same name.  By saying
`r := super new' he is letting his parent create the object, and then
he is attaching it to the variable `r'.  So after this line of code
executes, we have a brand new object of type Account, and `r' is bound
to it.  You will understand this better as time goes on, but for now
scratch your head once, accept it as a recipe, and keep going.

   We have the new object, but we haven't set it up correctly.
Remember the hidden variable `balance' which we saw in the beginning of
this chapter?  `super new' gives us the object with the `balance' field
containing nothing, but we want our balance field to start at 0.  (1)

   So what we need to do is ask the object to set itself up.  By saying
`r init', we are sending the `init' message to our new Account.  We'll
define this method in the next section--for now just assume that
sending the `init' message will get our Account set up.

   Finally, we say `^r'.  In English, this is return what r is attached
to.  This means that whoever sent to Account the `new' message will get
back this brand new account.  At the same time, our temporary variable
`r' ceases to exist.

   ---------- Footnotes ----------

   (1) And unlike C, Smalltalk draws a distinction between `0' and
`nil'.  `nil' is the nothing object, and you will receive an error if
you try to do, say, math on it.  It really does matter that we
initialize our instance variable to the number 0 if we wish to do math
on it in the future.


File: gst.info,  Node: Instance methods,  Next: A look at our object,  Prev: Defining methods,  Up: Creating classes

6.4.4 Defining an instance method
---------------------------------

We need to define the `init' method for our Account objects, so that
our `new' method defined above will work.  Here's the Smalltalk code:
     Account extend [
         init [
             <category: 'initialization'>
             balance := 0
         ]
     ]

   It looks quite a bit like the previous method definition, except
that the first one said `Account class extend', and ours says `Account
extend'.

   The difference is that the first one defined a method for messages
sent directly to `Account', but the second one is for messages which
are sent to Account objects once they are created.

   The method named `init' has only one line, `balance := 0'.  This
initializes the hidden variable `balance' (actually called an instance
variable) to zero, which makes sense for an account balance.  Notice
that the method doesn't end with `^r' or anything like it: this method
doesn't return a value to the message sender.  When you do not specify
a return value, Smalltalk defaults the return value to the object
currently executing.  For clarity of programming, you might consider
explicitly returning `self' in cases where you intend the return value
to be used.(1)

   ---------- Footnotes ----------

   (1) And why didn't the designers default the return value to nil?
Perhaps they didn't appreciate the value of void functions.  After all,
at the time Smalltalk was being designed, C didn't even have a void
data type.


File: gst.info,  Node: A look at our object,  Next: Moving money around,  Prev: Instance methods,  Up: Creating classes

6.4.5 Looking at our Account
----------------------------

Let's create an instance of class Account:
        a := Account new

   Can you guess what this does?  The `Smalltalk at: #a put:
<something>' creates a Smalltalk variable.  And the `Account new'
creates a new Account, and returns it.  So this line creates a Smalltalk
variable named `a', and attaches it to a new Account--all in one line.
It also prints the Account object we just created:
        an Account

   Hmmm...  not very informative.  The problem is that we didn't tell
our Account how to print itself, so we're just getting the default
system `printNl' method--which tells what the object is, but not what
it contains.  So clearly we must add such a method:
         Account extend [
             printOn: stream [
                 <category: 'printing'>
                 super printOn: stream.
                 stream nextPutAll: ' with balance: '.
                 balance printOn: stream
             ]
         ]

   Now give it a try again:
        a

which prints:
        an Account with balance: 0

   This may seem a little strange.  We added a new method, printOn:,
and our printNl message starts behaving differently.  It turns out that
the printOn: message is the central printing function--once you've
defined it, all of the other printing methods end up calling it.  Its
argument is a place to print to--quite often it is the variable
`Transcript'.  This variable is usually hooked to your terminal, and
thus you get the printout to your screen.

   The `super printOn: stream' lets our parent do what it did
before--print out what our type is.  The `an Account' part of the
printout came from this.  `stream nextPutAll: ' with balance: ''
creates the string ` with balance: ', and prints it out to the stream,
too; note that we don't use `printOn:' here because that would enclose
our string within quotes.  Finally, `balance printOn: stream' asks
whatever object is hooked to the `balance' variable to print itself to
the stream.  We set `balance' to 0, so the 0 gets printed out.


File: gst.info,  Node: Moving money around,  Next: Next coming,  Prev: A look at our object,  Up: Creating classes

6.4.6 Moving money around
-------------------------

We can now create accounts, and look at them.  As it stands, though,
our balance will always be 0--what a tragedy!  Our final methods will
let us deposit and spend money.  They're very simple:

        Account extend [
            spend: amount [
                <category: 'moving money'>
                balance := balance - amount
            ]
            deposit: amount [
                <category: 'moving money'>
                balance := balance + amount
            ]
        ]

   With these methods you can now deposit and spend amounts of money.
Try these operations:
        a deposit: 125
        a deposit: 20
        a spend: 10


File: gst.info,  Node: Next coming,  Prev: Moving money around,  Up: Creating classes

6.4.7 What's next?
------------------

We now have a generic concept, an "Account".  We can create them, check
their balance, and move money in and out of them.  They provide a good
foundation, but leave out important information that particular types
of accounts might want.  In the next chapter, we'll take a look at
fixing this problem using subclasses.


File: gst.info,  Node: Creating subclasses,  Next: Code blocks (I),  Prev: Creating classes,  Up: Tutorial

6.5 Two Subclasses for the Account Class
========================================

This chapter continues from the previous chapter in demonstrating how
one creates classes and subclasses in Smalltalk.  In this chapter we
will create two special subclasses of Account, known as Checking and
Savings.  We will continue to inherit the capabilities of Account, but
will tailor the two kinds of objects to better manage particular kinds
of accounts.

* Menu:

* The Savings class::           One of the two subclasses we'll put together
* The Checking class::          And here is the other
* Writing checks::              Only in Smalltalk, of course


File: gst.info,  Node: The Savings class,  Next: The Checking class,  Up: Creating subclasses

6.5.1 The Savings class
-----------------------

We create the Savings class as a subclass of Account.  It holds money,
just like an Account, but has an additional property that we will
model: it is paid interest based on its balance.  We create the class
Savings as a subclass of Account.
        Account subclass: Savings [
            | interest |

   This is already telling something: the instance variable `interest'
will accumulate interest paid.  Thus, in addition to the `spend:' and
`deposit:' messages which we inherit from our parent, Account, we will
need to define a method to add in interest deposits, and a way to clear
the interest variable (which we would do yearly, after we have paid
taxes).  We first define a method for allocating a new account--we need
to make sure that the interest field starts at 0.

   We can do so within the `Account subclass: Savings' scope, which we
have not closed above.
        init [
            <category: 'initialization'>
            interest := 0.
            ^super init
        ]

   Recall that the parent took care of the `new' message, and created a
new object of the appropriate size.  After creation, the parent also
sent an `init' message to the new object.  As a subclass of Account,
the new object will receive the `init' message first; it sets up its own
instance variable, and then passes the `init' message up the chain to
let its parent take care of its part of the initialization.

   With our new `Savings' account created, we can define two methods
for dealing specially with such an account:

        interest: amount [
            interest := interest + amount.
            self deposit: amount
        ]
        clearInterest [
            | oldinterest |
            oldinterest := interest.
            interest := 0.
            ^oldinterest
        ]

   We are now finished, and close the class scope:

     ]

   The first method says that we add the `amount' to our running total
of interest.  The line `self deposit: amount' tells Smalltalk to send
ourselves a message, in this case `deposit: amount'.  This then causes
Smalltalk to look up the method for `deposit:', which it finds in our
parent, Account.  Executing this method then updates our overall
balance.(1)

   One may wonder why we don't just replace this with the simpler
`balance := balance + amount'.  The answer lies in one of the
philosophies of object-oriented languages in general, and Smalltalk in
particular.  Our goal is to encode a technique for doing something once
only, and then re-using that technique when needed.  If we had directly
encoded `balance := balance + amount' here, there would have been two
places that knew how to update the balance from a deposit.  This may
seem like a useless difference.  But consider if later we decided to
start counting the number of deposits made.  If we had encoded `balance
:= balance + amount' in each place that needed to update the balance,
we would have to hunt each of them down in order to update the count of
deposits.  By sending `self' the message `deposit:', we need only
update this method once; each sender of this message would then
automatically get the correct up-to-date technique for updating the
balance.

   The second method, `clearInterest', is simpler.  We create a
temporary variable `oldinterest' to hold the current amount of
interest.  We then zero out our interest to start the year afresh.
Finally, we return the old interest as our result, so that our year-end
accountant can see how much we made.(2)

   ---------- Footnotes ----------

   (1) `self' is much like `super', except that `self' will start
looking for a method at the bottom of the type hierarchy for the
object, while `super' starts looking one level up from the current
level.  Thus, using `super' forces inheritance, but `self' will find
the first definition of the message which it can.

   (2) Of course, in a real accounting system we would never discard
such information--we'd probably throw it into a Dictionary object,
indexed by the year that we're finishing.  The ambitious might want to
try their hand at implementing such an enhancement.


File: gst.info,  Node: The Checking class,  Next: Writing checks,  Prev: The Savings class,  Up: Creating subclasses

6.5.2 The Checking class
------------------------

Our second subclass of Account represents a checking account.  We will
keep track of two facets:

   * What check number we are on

   * How many checks we have left in our checkbook

   We will define this as another subclass of Account:
     Account subclass: Checking [
         | checknum checksleft |

   We have two instance variables, but we really only need to
initialize one of them--if there are no checks left, the current check
number can't matter.  Remember, our parent class Account will send us
the `init' message.  We don't need our own class-specific `new'
function, since our parent's will provide everything we need.
         init [
            <category: 'initialization'>
            checksleft := 0.
            ^super init
        ]

   As in Savings, we inherit most of abilities from our superclass,
Account.  For initialization, we leave `checknum' alone, but set the
number of checks in our checkbook to zero.  We finish by letting our
parent class do its own initialization.


File: gst.info,  Node: Writing checks,  Prev: The Checking class,  Up: Creating subclasses

6.5.3 Writing checks
--------------------

We will finish this chapter by adding a method for spending money
through our checkbook.  The mechanics of taking a message and updating
variables should be familiar:
        newChecks: number count: checkcount [
            <category: 'spending'>
            checknum := number.
            checksleft := checkcount
        ]

        writeCheck: amount [
            <category: 'spending'>
            | num |
            num := checknum.
            checknum := checknum + 1.
            checksleft := checksleft - 1.
            self spend: amount.
            ^ num
        ]
     ]

   `newChecks:' fills our checkbook with checks.  We record what check
number we're starting with, and update the count of the number of
checks in the checkbook.

   `writeCheck:' merely notes the next check number, then bumps up the
check number, and down the check count.  The message `self spend:
amount' resends the message `spend:' to our own object.  This causes
its method to be looked up by Smalltalk.  The method is then found in
our parent class, Account, and our balance is then updated to reflect
our spending.

   You can try the following examples:
        c := Checking new
        c deposit: 250
        c newChecks: 100 count: 50
        c writeCheck: 32
        c

   For amusement, you might want to add a printOn: message to the
checking class so you can see the checking-specific information.

   In this chapter, you have seen how to create subclasses of your own
classes.  You have added new methods, and inherited methods from the
parent classes.  These techniques provide the majority of the structure
for building solutions to problems.  In the following chapters we will
be filling in details on further language mechanisms and types, and
providing details on how to debug software written in Smalltalk.


File: gst.info,  Node: Code blocks (I),  Next: Code blocks (II),  Prev: Creating subclasses,  Up: Tutorial

6.6 Code blocks
===============

The Account/Saving/Checking example from the last chapter has several
deficiencies.  It has no record of the checks and their values.  Worse,
it allows you to write a check when there are no more checks--the
Integer value for the number of checks will just calmly go negative!
To fix these problems we will need to introduce more sophisticated
control structures.

* Menu:

* Conditions::               Making some decisions
* Iteration::                Making some loops


File: gst.info,  Node: Conditions,  Next: Iteration,  Up: Code blocks (I)

6.6.1 Conditions and decision making
------------------------------------

Let's first add some code to keep you from writing too many checks.  We
will simply update our current method for the Checking class; if you
have entered the methods from the previous chapters, the old definition
will be overridden by this new one.
     Checking extend [
         writeCheck: amount [
            | num |

            (checksleft < 1)
                ifTrue: [ ^self error: 'Out of checks' ].
            num := checknum.
            checknum := checknum + 1.
            checksleft := checksleft - 1.
            self spend: amount
            ^ num
        ]
     ]

   The two new lines are:
        (checksleft < 1)
            ifTrue: [ ^self error: 'Out of checks' ].

   At first glance, this appears to be a completely new structure.
But, look again!  The only new construct is the square brackets, which
appear within a method and not only surround it.

   The first line is a simple boolean expression.  `checksleft' is our
integer, as initialized by our Checking class.  It is sent the message
`<', and the argument 1.  The current number bound to `checksleft'
compares itself against 1, and returns a boolean object telling whether
it is less than 1.

   Now this boolean, which is either true or false, is sent the message
`ifTrue:', with an argument which is called a code block.  A code block
is an object, just like any other.  But instead of holding a number, or
a Set, it holds executable statements.  So what does a boolean do with
a code block which is an argument to a `ifTrue:' message?  It depends
on which boolean!  If the object is the `true' object, it executes the
code block it has been handed.  If it is the `false' object, it returns
without executing the code block.  So the traditional conditional
construct has been replaced in Smalltalk with boolean objects which
execute the indicated code block or not, depending on their truth-value.
(1)

   In the case of our example, the actual code within the block sends
an error message to the current object.  `error:' is handled by the
parent class Object, and will pop up an appropriate complaint when the
user tries to write too many checks.  In general, the way you handle a
fatal error in Smalltalk is to send an error message to yourself
(through the `self' pseudo-variable), and let the error handling
mechanisms inherited from the Object class take over.

   As you might guess, there is also an `ifFalse:' message which
booleans accept.  It works exactly like `ifTrue:', except that the
logic has been reversed; a boolean `false' will execute the code block,
and a boolean `true' will not.

   You should take a little time to play with this method of
representing conditionals.  You can run your checkbook, but can also
invoke the conditional functions directly:
        true ifTrue: [ 'Hello, world!' printNl ]
        false ifTrue: [ 'Hello, world!' printNl ]
        true ifFalse: [ 'Hello, world!' printNl ]
        false ifFalse: [ 'Hello, world!' printNl ]

   ---------- Footnotes ----------

   (1) It is interesting to note that because of the way conditionals
are done, conditional constructs are not part of the Smalltalk
language, instead they are merely a defined behavior for the Boolean
class of objects.


File: gst.info,  Node: Iteration,  Prev: Conditions,  Up: Code blocks (I)

6.6.2 Iteration and collections
-------------------------------

Now that we have some sanity checking in place, it remains for us to
keep a log of the checks we write.  We will do so by adding a
Dictionary object to our Checking class, logging checks into it, and
providing some messages for querying our check-writing history.  But
this enhancement brings up a very interesting question--when we change
the "shape" of an object (in this case, by adding our dictionary as a
new instance variable to the Checking class), what happens to the
existing class, and its objects?  The answer is that the old objects
are mutated to keep their new shape, and all methods are recompiled so
that they work with the new shape.  New objects will have exactly the
same shape as old ones, but old objects might happen to be initialized
incorrectly (since the newly added variables will be simply put to
nil).  As this can lead to very puzzling behavior, it is usually best
to eradicate all of the old objects, and then implement your changes.

   If this were more than a toy object accounting system, this would
probably entail saving the objects off, converting to the new class,
and reading the objects back into the new format.  For now, we'll just
ignore what's currently there, and define our latest Checking class.

     Checking extend [
         | history |

   This is the same syntax as the last time we defined a checking
account, except that we start with `extend' (since the class is already
there).  Then, the two instance variables we had defined remain, and we
add a new `history' variable; the old methods will be recompiled
without errors.  We must now feed in our definitions for each of the
messages our object can handle, since we are basically defining a new
class under an old name.

   With our new Checking instance variable, we are all set to start
recording our checking history.  Our first change will be in the
handling of the `init' message:
            init [
                <category: 'initialization'>
                checksleft := 0.
                history := Dictionary new.
                ^ super init
            ]

   This provides us with a Dictionary, and hooks it to our new
`history' variable.

   Our next method records each check as it's written.  The method is a
little more involved, as we've added some more sanity checks to the
writing of checks.

        writeCheck: amount [
            <category: 'spending'>
            | num |

            "Sanity check that we have checks left in our checkbook"
            (checksleft < 1)
                ifTrue: [ ^self error: 'Out of checks' ].

            "Make sure we've never used this check number before"
            num := checknum.
            (history includesKey: num)
                ifTrue: [ ^self error: 'Duplicate check number' ].

            "Record the check number and amount"
            history at: num put: amount.

            "Update our next checknumber, checks left, and balance"
            checknum := checknum + 1.
            checksleft := checksleft - 1.
            self spend: amount.
            ^ num
        ]

   We have added three things to our latest version of `writeCheck:'.
First, since our routine has become somewhat involved, we have added
comments.  In Smalltalk, single quotes are used for strings; double
quotes enclose comments.  We have added comments before each section of
code.

   Second, we have added a sanity check on the check number we propose
to use.  Dictionary objects respond to the `includesKey:' message with
a boolean, depending on whether something is currently stored under the
given key in the dictionary.  If the check number is already used, the
`error:' message is sent to our object, aborting the operation.

   Finally, we add a new entry to the dictionary.  We have already seen
the `at:put:' message (often found written as `#at:put:', with a sharp
in front of it) at the start of this tutorial.  Our use here simply
associates a check number with an amount of money spent.(1) With this,
we now have a working Checking class, with reasonable sanity checks and
per-check information.

   Let us finish the chapter by enhancing our ability to get access to
all this information.  We will start with some simple print-out
functions.

        printOn: stream [
            super printOn: stream.
            ', checks left: ' printOn: stream.
            checksleft printOn: stream.
            ', checks written: ' printOn: stream.
            (history size) printOn: stream.
        ]
        check: num [
            | c |
            c := history
                at: num
                ifAbsent: [ ^self error: 'No such check #' ].
            ^c
        ]

   There should be very few surprises here.  We format and print our
information, while letting our parent classes handle their own share of
the work.  When looking up a check number, we once again take advantage
of the fact that blocks of executable statements are an object; in this
case, we are using the `at:ifAbsent:' message supported by the
Dictionary class.  As you can probably anticipate, if the requested key
value is not found in the dictionary, the code block is executed.  This
allows us to customize our error handling, as the generic error would
only tell the user "key not found".

   While we can look up a check if we know its number, we have not yet
written a way to "riffle through" our collection of checks.  The
following function loops over the checks, printing them out one per
line.  Because there is currently only a single numeric value under
each key, this might seem wasteful.  But we have already considered
storing multiple values under each check number, so it is best to leave
some room for each item.  And, of course, because we are simply sending
a printing message to an object, we will not have to come back and
re-write this code so long as the object in the dictionary honors our
`printNl'/`printOn:' messages sages.

         printChecks [
             history keysAndValuesDo: [ :key :value |
                 key print.
                 ' - ' print.
                 value printNl.
             ]
         ]
     ]

   We still see a code block object being passed to the dictionary, but
`:key :value |' is something new.  A code block can optionally receive
arguments.  In this case, the two arguments represent a key/value pair.
If you only wanted the value portion, you could call history with a
`do:' message instead; if you only wanted the key portion, you could
call history with a `keysDo:' message instead.

   We then invoke our printing interface upon them.  We don't want a
newline until the end, so the `print' message is used instead.  It is
pretty much the same as `printNl', since both implicitly use
`Transcript', except it doesn't add a newline.

   It is important that you be clear that in principle there is no
relationship between the code block and the dictionary you passed it
to.  The dictionary just invokes the passed code block with a key/value
pair when processing a keysAndValuesDo: message.  But the same
two-parameter code block can be passed to any message that wishes to
evaluate it (and passes the exact number of parameters to it).  In the
next chapter we'll see more on how code blocks are used; we'll also
look at how you can invoke code blocks in your own code.

   ---------- Footnotes ----------

   (1) You might start to wonder what one would do if you wished to
associate two pieces of information under one key.  Say, the value and
who the check was written to.  There are several ways; the best would
probably be to create a new, custom object which contained this
information, and then store this object under the check number key in
the dictionary.  It would also be valid (though probably overkill) to
store a dictionary as the value--and then store as many pieces of
information as you'd like under each slot!


File: gst.info,  Node: Code blocks (II),  Next: Debugging,  Prev: Code blocks (I),  Up: Tutorial

6.7 Code blocks, part two
=========================

In the last chapter, we looked at how code blocks could be used to
build conditional expressions, and how you could iterate across all
entries in a collection.(1) We built our own code blocks, and handed
them off for use by system objects.  But there is nothing magic about
invoking code blocks; your own code will often need to do so.  This
chapter will shows some examples of loop construction in Smalltalk, and
then demonstrate how you invoke code blocks for yourself.

* Menu:

* Integer loops::            Well, Smalltalk too has them
* Intervals::                And of course here's a peculiar way to use them
* Invoking code blocks::     You can do it, too

   ---------- Footnotes ----------

   (1) The   `do:' message is understood by most types   of Smalltalk
collections.  It works for the   Dictionary class, as well as sets,
arrays, strings,   intervals, linked lists, bags, and streams.  The
`keysDo:' message, for example, works only with dictionaries.


File: gst.info,  Node: Integer loops,  Next: Intervals,  Up: Code blocks (II)

6.7.1 Integer loops
-------------------

Integer loops are constructed by telling a number to drive the loop.
Try this example to count from 1 to 20:
        1 to: 20 do: [:x | x printNl ]

   There's also a way to count up by more than one:
        1 to: 20 by: 2 do: [:x | x printNl ]

   Finally, counting down is done with a negative step:
        20 to: 1 by: -1 do: [:x | x printNl ]

   Note that the `x' variable is local to the block.
        x
   just prints `nil'.


File: gst.info,  Node: Intervals,  Next: Invoking code blocks,  Prev: Integer loops,  Up: Code blocks (II)

6.7.2 Intervals
---------------

It is also possible to represent a range of numbers as a standalone
object.  This allows you to represent a range of numbers as a single
object, which can be passed around the system.
        i := Interval from: 5 to: 10
        i do: [:x | x printNl]

   As with the integer loops, the Interval class can also represent
steps greater than 1.  It is done much like it was for our numeric loop
above:
        i := (Interval from: 5 to: 10 by: 2)
        i do: [:x| x printNl]


File: gst.info,  Node: Invoking code blocks,  Prev: Intervals,  Up: Code blocks (II)

6.7.3 Invoking code blocks
--------------------------

Let us revisit the checking example and add a method for scanning only
checks over a certain amount.  This would allow our user to find "big"
checks, by passing in a value below which we will not invoke their
function.  We will invoke their code block with the check number as an
argument ment; they can use our existing check: message to get the
amount.

        Checking extend [
            checksOver: amount do: aBlock
                history keysAndValuesDo: [:key :value |
                    (value > amount)
                           ifTrue: [aBlock value: key]
                ]
        ]

   The structure of this loop is much like our printChecks message sage
from chapter 6.  However, in this case we consider each entry, and only
invoke the supplied block if the check's value is greater than the
specified amount.  The line:

        ifTrue: [aBlock value: key]

invokes the user-supplied block, passing as an argument the key, which
is the check number.  The `value:' message, when received by a code
block, causes the code block to execute.  Code blocks take `value',
`value:', `value:value:', and `value:value:value:' messages, so you can
pass from 0 to 3 arguments to a code block.(1)

   You might find it puzzling that an association takes a `value'
message, and so does a code block.  Remember, each object can do its
own thing with a message.  A code block gets run when it receives a
`value' message.  An association merely returns the value part of its
key/value pair.  The fact that both take the same message is, in this
case, coincidence.

   Let's quickly set up a new checking account with $250 (wouldn't this
be nice in real life?) and write a couple checks.  Then we'll see if
our new method does the job correctly:
        mycheck := Checking new.
        mycheck deposit: 250
        mycheck newChecks: 100 count: 40
        mycheck writeCheck: 10
        mycheck writeCheck: 52
        mycheck writeCheck: 15
        mycheck checksOver: 1 do: [:x | x printNl]
        mycheck checksOver: 17 do: [:x | x printNl]
        mycheck checksOver: 200 do: [:x | x printNl]

   We will finish this chapter with an alternative way of writing our
`checksOver:' code.  In this example, we will use the message `select:'
to pick the checks which exceed our value, instead of doing the
comparison ourselves.  We can then invoke the new resulting collection
against the user's code block.

        Checking extend [
            checksOver: amount do: aBlock [
                | chosen |
                chosen := history select: [:amt| amt > amount].
                chosen keysDo: aBlock
            ]
        ]

   Note that `extend' will also overwrite methods.  Try the same tests
as above, they should yield the same result!

   ---------- Footnotes ----------

   (1) There is also a `valueWithArguments:' message which accepts an
array holding as many arguments as you would like.


File: gst.info,  Node: Debugging,  Next: More subclassing,  Prev: Code blocks (II),  Up: Tutorial

6.8 When Things Go Bad
======================

So far we've been working with examples which work the first time.  If
you didn't type them in correctly, you probably received a flood of
unintelligible complaints.  You probably ignored the complaints, and
typed the example again.

   When developing your own Smalltalk code, however, these messages are
the way you find out what went wrong.  Because your objects, their
methods, the error printout, and your interactive environment are all
contained within the same Smalltalk session, you can use these error
messages to debug your code using very powerful techniques.

* Menu:

* Simple errors::                   Those that only happen in examples
* Nested calls::                    Those that actually happen in real life
* Looking at objects::              Trying to figure it out


File: gst.info,  Node: Simple errors,  Next: Nested calls,  Up: Debugging

6.8.1 A Simple Error
--------------------

First, let's take a look at a typical error.  Type:
        7 plus: 1

   This will print out:
        7 did not understand selector 'plus:'
        <blah blah>
        UndefinedObject>>#executeStatements

   The first line is pretty simple; we sent a message to the `7' object
which was not understood; not surprising since the `plus:' operation
should have been `+'.  Then there are a few lines of gobbledegook: just
ignore them, they reflect the fact that the error passed throgh GNU
Smalltalk's exception handling system.  The remaining line reflect the
way the GNU Smalltalk invokes code which we type to our command prompt;
it generates a block of code which is invoked via an internal method
`executeStatements' defined in class Object and evaluated like `nil
executeStatements' (nil is an instance of UndefinedObject).  Thus, this
output tells you that you directly typed a line which sent an invalid
message to the `7' object.

   All the error output but the first line is actually a stack
backtrace.  The most recent call is the one nearer the top of the
screen.  In the next example, we will cause an error which happens
deeper within an object.


File: gst.info,  Node: Nested calls,  Next: Looking at objects,  Prev: Simple errors,  Up: Debugging

6.8.2 Nested Calls
------------------

Type the following lines:
        x := Dictionary new
        x at: 1

   The error you receive will look like:
        Dictionary new: 31 "<0x33788>" error: key not found
        ...blah blah...
        Dictionary>>#error:
        [] in Dictionary>>#at:
        [] in Dictionary>>#at:ifAbsent:
        Dictionary(HashedCollection)>>#findIndex:ifAbsent:
        Dictionary>>#at:ifAbsent:
        Dictionary>>#at:
        UndefinedObject(Object)>>#executeStatements

   The error itself is pretty clear; we asked for something within the
Dictionary which wasn't there.  The object which had the error is
identified as `Dictionary new: 31'.  A Dictionary's default size is 31;
thus, this is the object we created with `Dictionary new'.

   The stack backtrace shows us the inner structure of how a Dictionary
responds to the `#at:' message.  Our hand-entered command causes the
usual entry for `UndefinedObject(Object)'.  Then we see a Dictionary
object responding to an `#at:' message (the "Dictionary>>#at:" line).
This code called the object with an `#at:ifAbsent:' message.  All of a
sudden, Dictionary calls that strange method `#findIndex:ifAbsent:',
which evaluates two blocks, and then the error happens.

   To understand this better, it is necessary to know that a very
common way to handle errors in Smalltalk is to hand down a block of
code which will be called when an error occurs.  For the Dictionary
code, the `at:' message passes in a block of code to the at:ifAbsent:
code to be called when `at:ifAbsent:' can't find the given key, and
`at:ifAbsent:' does the same with `findIndex:ifAbsent:'.  Thus, without
even looking at the code for Dictionary itself, we can guess something
of the code for Dictionary's implementation:

        findIndex: key ifAbsent: errCodeBlock [
            ...look for key...
            (keyNotFound) ifTrue: [ ^(errCodeBlock value) ]
            ...
        ]

        at: key [
            ^self at: key ifAbsent: [^self error: 'key not found']
        ]

   Actually, `findIndex:ifAbsent:' lies in class `HashedCollection', as
that `Dictionary(HashedCollection)' in the backtrace says.

   It would be nice if each entry on the stack backtrace included
source line numbers.  Unfortunately, at this point GNU Smalltalk doesn't
provide this feature.  Of course, you have the source code available...


File: gst.info,  Node: Looking at objects,  Prev: Nested calls,  Up: Debugging

6.8.3 Looking at Objects
------------------------

When you are chasing an error, it is often helpful to examine the
instance variables of your objects.  While strategic calls to `printNl'
will no doubt help, you can look at an object without having to write
all the code yourself.  The `inspect' message works on any object, and
dumps out the values of each instance variable within the object.(1)

   Thus:
        x := Interval from: 1 to: 5.
        x inspect

   displays:
        An instance of Interval
        start: 1
        stop: 5
        step: 1
        contents: [
            [1]: 1
            [2]: 2
            [3]: 3
            [4]: 4
            [5]: 5
        ]

   We'll finish this chapter by emphasizing a technique which has
already been covered: the use of the `error:' message in your own
objects.  As you saw in the case of Dictionary, an object can send
itself an `error:' message with a descriptive string to abort execution
and dump a stack backtrace.  You should plan on using this technique in
your own objects.  It can be used both for explicit user-caused errors,
as well as in internal sanity checks.

   ---------- Footnotes ----------

   (1) When using the Blox GUI, it actually pops up a so-called
"Inspector window".


File: gst.info,  Node: More subclassing,  Next: Streams,  Prev: Debugging,  Up: Tutorial

6.9 Coexisting in the Class Hierarchy
=====================================

The early chapters of this tutorial discussed classes in one of two
ways.  The "toy" classes we developed were rooted at Object; the
system-provided classes were treated as immutable entities.  While one
shouldn't modify the behavior of the standard classes lightly,
"plugging in" your own classes in the right place among their
system-provided brethren can provide you powerful new classes with very
little effort.

   This chapter will create two complete classes which enhance the
existing Smalltalk hierarchy.  The discussion will start with the issue
of where to connect our new classes, and then continue onto
implementation.  Like most programming efforts, the result will leave
many possibilities for improvements.  The framework, however, should
begin to give you an intuition of how to develop your own Smalltalk
classes.

* Menu:

* The existing hierarchy::           We've been talking about it for a while,
                                     so here it is at last
* Playing with Arrays::              Again.
* New kinds of Numbers::             Sounds interesting, doesn't it?
* Inheritance and Polymorphism::     Sounds daunting, doesn't it?


File: gst.info,  Node: The existing hierarchy,  Next: Playing with Arrays,  Up: More subclassing

6.9.1 The Existing Class Hierarchy
----------------------------------

To discuss where a new class might go, it is helpful to have a map of
the current classes.  The following is the basic class hierarchy of GNU
Smalltalk.  Indentation means that the line inherits from the earlier
line with one less level of indentation.(1).

       Object
         Behavior
           ClassDescription
             Class
             Metaclass
         BlockClosure
         Boolean
           False
           True
         Browser
         CFunctionDescriptor
         CObject
           CAggregate
             CArray
             CPtr
           CCompound
             CStruct
             CUnion
           CScalar
             CChar
             CDouble
             CFloat
             CInt
             CLong
             CShort
             CSmalltalk
             CString
             CUChar
               CByte
                 CBoolean
             CUInt
             CULong
             CUShort
         Collection
           Bag
           MappedCollection
           SequenceableCollection
             ArrayedCollection
               Array
               ByteArray
               WordArray
               LargeArrayedCollection
                 LargeArray
                 LargeByteArray
                 LargeWordArray
               CompiledCode
                 CompiledMethod
                 CompiledBlock
               Interval
               CharacterArray
                 String
                   Symbol
             LinkedList
               Semaphore
             OrderedCollection
               RunArray
               SortedCollection
           HashedCollection
             Dictionary
               IdentityDictionary
                 MethodDictionary
               RootNamespace
                 Namespace
                 SystemDictionary
             Set
               IdentitySet
         ContextPart
           BlockContext
           MethodContext
         CType
           CArrayCType
           CPtrCType
           CScalarCType
         Delay
         DLD
         DumperProxy
           AlternativeObjectProxy
             NullProxy
               VersionableObjectProxy
             PluggableProxy
         File
           Directory
         FileSegment
         Link
           Process
           SymLink
         Magnitude
           Association
           Character
           Date
           LargeArraySubpart
           Number
             Float
             Fraction
             Integer
               LargeInteger
                 LargeNegativeInteger
                 LargePositiveInteger
                   LargeZeroInteger
               SmallInteger
           Time
         Memory
         Message
           DirectedMessage
         MethodInfo
         NullProxy
         PackageLoader
         Point
         ProcessorScheduler
         Rectangle
         SharedQueue
         Signal
           Exception
             Error
               Halt
                 ArithmeticError
                   ZeroDivide
                 MessageNotUnderstood
               UserBreak
             Notification
               Warning
         Stream
           ObjectDumper
           PositionableStream
             ReadStream
             WriteStream
               ReadWriteStream
                 ByteStream
                   FileStream
           Random
           TextCollector
           TokenStream
         TrappableEvent
           CoreException
           ExceptionCollection
         UndefinedObject
         ValueAdaptor
           NullValueHolder
           PluggableAdaptor
             DelayedAdaptor
           ValueHolder

   While initially a daunting list, you should take the time to hunt
down the classes we've examined in this tutorial so far.  Notice, for
instance, how an Array is a subclass below the SequenceableCollection
class.  This makes sense; you can walk an Array from one end to the
other.  By contrast, notice how this is not true for Sets: it doesn't
make sense to walk a Set from one end to the other.

   A little puzzling is the relationship of a Bag to a Set, since a Bag
is actually a Set supporting multiple occurrences of its elements.  The
answer lies in the purpose of both a Set and a Bag.  Both hold an
unordered collection of objects; but a Bag needs to be optimized for
the case when an object has possibly thousands of occurrences, while a
Set is optimized for checking object uniqueness.  That's why Set being
a subclass or Bag, or the other way round, would be a source of
problems in the actual implementation of the class.  Currently a Bag
holds a Dictionary associating each object to each count; it would be
feasible however to have Bag as a subclass of HashedCollection and a
sibling of Set.

   Look at the treatment of numbers--starting with the class Magnitude.
While numbers can indeed be ordered by _less than_, _greater than_, and
so forth, so can a number of other objects.  Each subclass of Magnitude
is such an object.  So we can compare characters with other characters,
dates with other dates, and times with other times, as well as numbers
with numbers.

   Finally, you will have probably noted some pretty strange classes,
representing language entities that you might have never thought of as
objects themselves: Namespace, Class and even CompiledMethod.  They are
the base of Smalltalk's "reflection" mechanism which will be discussed
later, in *note The truth on metaclasses: Why is #new there?!?.

   ---------- Footnotes ----------

   (1) This listing is courtesy of the printHierarchy method supplied
by GNU Smalltalk author Steve Byrne.  It's in the `kernel/Browser.st'
file.


File: gst.info,  Node: Playing with Arrays,  Next: New kinds of Numbers,  Prev: The existing hierarchy,  Up: More subclassing

6.9.2 Playing with Arrays
-------------------------

Imagine that you need an array, but alas you need that if an index is
out of bounds, it returns nil.  You could modify the Smalltalk
implementation, but that might break some code in the image, so it is
not practical.  Why not add a subclass?

        "We could subclass from Array, but that class is specifically
         optimized by the VM (which assumes, among other things, that
         it does not have any instance variables).  So we use its
         abstract superclass instead.  The discussion below holds
         equally well."
        ArrayedCollection subclass: NiledArray [
            <shape: #pointer>

            boundsCheck: index [
                ^(index < 1) | (index > (self basicSize))
            ]

            at: index [
                ^(self boundsCheck: index)
                    ifTrue: [ nil ]
                    ifFalse: [ super at: index ]
            ]

            at: index put: val [
                ^(self boundsCheck: index)
                    ifTrue: [ val ]
                    ifFalse: [ super at: index put: val ]
            ]
        ]

   Much of the machinery of adding a class should be familiar.  We see
another declaration like `comment:', that is `shape:' message.  This
sets up `NiledArray' to have the same underlying structure of an
`Array' object; we'll delay discussing this until the chapter on the
nuts and bolts of arrays.  In any case, we inherit all of the actual
knowledge of how to create arrays, reference them, and so forth.  All
that we do is intercept `at:' and `at:put:' messages, call our common
function to validate the array index, and do something special if the
index is not valid.  The way that we coded the bounds check bears a
little examination.

   Making a first cut at coding the bounds check, you might have coded
the bounds check in NiledArray's methods twice (once for `at:', and
again for `at:put:'.  As always, it's preferable to code things once,
and then re-use them.  So we instead add a method for bounds checking
`boundsCheck:', and use it for both cases.  If we ever wanted to
enhance the bounds checking (perhaps emit an error if the index is < 1
and answer nil only for indices greater than the array size?), we only
have to change it in one place.

   The actual math for calculating whether the bounds have been
violated is a little interesting.  The first part of the expression
returned by the method:
        (index < 1) | (index > (self basicSize))

is true if the index is less than 1, otherwise it's false.  This part
of the expression thus becomes the boolean object true or false.  The
boolean object then receives the message `|', and the argument `(index
> (self basicSize))'.  `|' means "or"--we want to OR together the two
possible out-of-range checks.  What is the second part of the
expression?  (1)

   `index' is our argument, an integer; it receives the message `>',
and thus will compare itself to the value `self basicSize' returns.
While we haven't covered the underlying structures Smalltalk uses to
build arrays, we can briefly say that the `#basicSize' message returns
the number of elements the Array object can contain.  So the index is
checked to see if it's less than 1 (the lowest legal Array index) or
greater than the highest allocated slot in the Array.  If it is either
(the `|' operator!), the expression is true, otherwise false.

   From there it's downhill; our boolean object, returned by
`boundsCheck:', receives the `ifTrue:ifFalse:' message, and a code
block which will do the appropriate thing.  Why do we have `at:put:'
return val?  Well, because that's what it's supposed to do: look at
every implementor of `at:put' or `at:' and you'll find that it returns
its second parameter.  In general, the result is discarded; but one
could write a program which uses it, so we'll write it this way anyway.

   ---------- Footnotes ----------

   (1) Smalltalk also offers an `or:' message, which is different in a
subtle way from `|'.  or: takes a code block, and only invokes the code
block if it's necessary to determine the value of the expression.  This
is analogous to the guaranteed C semantic that `||' evaluates
left-to-right only as far as needed.  We could have written the
expressions as `((index < 1) or: [index > (self basicSize)])'.  Since
we expect both sides of or: to be false most of the time, there isn't
much reason to delay evaluation of either side in this case.


File: gst.info,  Node: New kinds of Numbers,  Next: Inheritance and Polymorphism,  Prev: Playing with Arrays,  Up: More subclassing

6.9.3 Adding a New Kind of Number
---------------------------------

If we were programming an application which did a large amount of
complex math, we could probably manage it with a number of two-element
arrays.  But we'd forever be writing in-line code for the math and
comparisons; it would be much easier to just implement an object class
to support the complex numeric type.  Where in the class hierarchy
would it be placed?

   You've probably already guessed--but let's step down the hierarchy
anyway.  Everything inherits from Object, so that's a safe starting
point.  Complex numbers can not be compared with `<' and `>', and yet
we strongly suspect that, since they are numbers, we should place them
under the Number class.  But Number inherits from Magnitude--how do we
resolve this conflict?  A subclass can place itself under a superclass
which allows some operations the subclass doesn't wish to allow.  All
that you must do is make sure you intercept these messages and return
an error.  So we will place our new Complex class under Number, and
make sure to disallow comparisons.

   One can reasonably ask whether the real and imaginary parts of our
complex number will be integer or floating point.  In the grand
Smalltalk tradition, we'll just leave them as objects, and hope that
they respond to numeric messages reasonably.  If they don't, the user
will doubtless receive errors and be able to track back their mistake
with little fuss.

   We'll define the four basic math operators, as well as the (illegal)
relationals.  We'll add `printOn:' so that the printing methods work,
and that should give us our Complex class.  The class as presented
suffers some limitations, which we'll cover later in the chapter.

        Number subclass: Complex [
            | realpart imagpart |

            "This is a quick way to define class-side methods."
            Complex class >> new [
                <category: 'instance creation'>
                ^self error: 'use real:imaginary:'
            ]
            Complex class >> new: ignore [
                <category: 'instance creation'>
                ^self new
            ]
            Complex class >> real: r imaginary: i [
                <category: 'instance creation'>
                ^(super new) setReal: r setImag: i
            ]

            setReal: r setImag: i [
                <category: 'basic'>
                realpart := r.
                imagpart := i.
                ^self
            ]

            real [
                <category: 'basic'>
                ^realpart
            ]
            imaginary [
                <category: 'basic'>
                ^imagpart
            ]

            + val [
                <category: 'math'>
                ^Complex real: (realpart + val real)
                    imaginary: (imagpart + val imaginary)
            ]
            - val [
                <category: 'math'>
                ^Complex real: (realpart - val real)
                    imaginary: (imagpart - val imaginary)
            ]
            * val [
                <category: 'math'>
                ^Complex real: (realpart * val real) - (imagpart * val imaginary)
                    imaginary: (imagpart * val real) + (realpart * val imaginary)
            ]
            / val [
                <category: 'math'>
                | d r i |
                d := (val real * val real) + (val imaginary * val imaginary).
                r := ((realpart * val real) + (imagpart * val imaginary)).
                i := ((imagpart * val real) - (realpart * val imaginary)).
                ^Complex real: r / d imaginary: i / d
            ]

            = val [
                <category: 'comparison'>
                ^(realpart = val real) & (imagpart = val imaginary)
            ]

            "All other comparison methods are based on <"
            < val [
                <category: 'comparison'>
                ^self shouldNotImplement
            ]

            printOn: aStream [
                <category: 'printing'>
                realpart printOn: aStream.
                aStream nextPut: $+.
                imagpart printOn: aStream.
                aStream nextPut: $i
            ]
        ]

   There should be surprisingly little which is actually new in this
example.  The printing method uses both `printOn:' as well as
`nextPut:' to do its printing.  While we haven't covered it, it's
pretty clear that `$+' generates the ASCII character `+' as an
object(1), and `nextPut:' puts its argument as the next thing on the
stream.

   The math operations all generate a new object, calculating the real
and imaginary parts, and invoking the Complex class to create the new
object.  Our creation code is a little more compact than earlier
examples; instead of using a local variable to name the newly-created
object, we just use the return value and send a message directly to the
new object.  Our initialization code explicitly returns self; what
would happen if we left this off?

   ---------- Footnotes ----------

   (1) A GNU Smalltalk extension   allows you to type characters by
ASCII code too, as in   `$<43>'.


File: gst.info,  Node: Inheritance and Polymorphism,  Prev: New kinds of Numbers,  Up: More subclassing

6.9.4 Inheritance and Polymorphism
----------------------------------

This is a good time to look at what we've done with the two previous
examples at a higher level.  With the NiledArray class, we inherited
almost all of the functionality ality of arrays, with only a little bit
of code added to address our specific needs.  While you may have not
thought to try it, all the existing methods for an Array continue to
work without further effort-you might find it interesting to ponder why
the following still works:
        a := NiledArray new: 10
        a at: 5 put: 1234
        a do: [:i| i printNl ]

   The strength of inheritance is that you focus on the incremental
changes you make; the things you don't change will generally continue
to work.

   In the Complex class, the value of polymorphism was exercised.  A
Complex number responds to exactly the same set of messages as any
other number.  If you had handed this code to someone, they would know
how to do math with Complex numbers without further instruction.
Compare this with C, where a complex number package would require the
user to first find out if the complex-add function was complex_plus(),
or perhaps complex_add(), or add_complex(), or...

   However, one glaring deficiency is present in the Complex class:
what happens if you mix normal numbers with Complex numbers?
Currently, the Complex class assumes that it will only interact with
other Complex numbers.  But this is unrealistic: mathematically, a
"normal" number is simply one with an imaginary part of 0.  Smalltalk
was designed to allow numbers to coerce themselves into a form which
will work with other numbers.

   The system is clever and requires very little additional code.
Unfortunately, it would have tripled the amount of explanation
required.  If you're interested in how coercion works in GNU Smalltalk,
you should find the Smalltalk library source, and trace back the
execution of the `retry:coercing:' messages.  You want to consider the
value which the `generality' message returns for each type of number.
Finally, you need to examine the `coerce:' handling in each numeric
class.


File: gst.info,  Node: Streams,  Next: Exception handling,  Prev: More subclassing,  Up: Tutorial

6.10 Smalltalk Streams
======================

Our examples have used a mechanism extensively, even though we haven't
discussed it yet.  The Stream class provides a framework for a number
of data structures, including input and output functionality, queues,
and endless sources of dynamically-generated data.  A Smalltalk stream
is quite similar to the UNIX streams you've used from C.  A stream
provides a sequential view to an underlying resource; as you read or
write elements, the stream position advances until you finally reach
the end of the underlying medium.  Most streams also allow you to set
the current position, providing random access to the medium.

* Menu:

* The output stream::                Which, even though you maybe didn't know
                                     it, we've used all the time
* Your own stream::                  Which, instead, is something new
* Files::                            Which are streams too
* Dynamic Strings::                  A useful application of Streams


File: gst.info,  Node: The output stream,  Next: Your own stream,  Up: Streams

6.10.1 The Output Stream
------------------------

The examples in this book all work because they write their output to
the `Transcript' stream.  Each class implements the `printOn:' method,
and writes its output to the supplied stream.  The `printNl' method all
objects use is simply to send the current object a `printOn:' message
whose argument is `Transcript' (by default attached to the standard
output stream found in the `stdout' global).  You can invoke the
standard output stream directly:
        'Hello, world' printOn: stdout
        stdout inspect

or you can do the same for the Transcript, which is yet another stream:
        'Hello, world' printOn: stdout
        Transcript inspect

the last `inspect' statement will show you how the `Transcript' is
linked to `stdout'(1).

   ---------- Footnotes ----------

   (1) Try executing it under Blox, where the Transcript is linked to
the omonymous window!


File: gst.info,  Node: Your own stream,  Next: Files,  Prev: The output stream,  Up: Streams

6.10.2 Your Own Stream
----------------------

Unlike a pipe you might create in C, the underlying storage of a Stream
is under your control.  Thus, a Stream can provide an anonymous buffer
of data, but it can also provide a stream-like interpretation to an
existing array of data.  Consider this example:
        a := Array new: 10
        a at: 4 put: 1234
        a at: 9 put: 5678
        s := ReadWriteStream on: a.
        s inspect
        s position: 1
        s inspect
        s nextPut: 11; nextPut: 22
        (a at: 1) printNl
        a do: [:x| x printNl]
        s position: 2
        s do: [:x| x printNl]
        s position: 5
        s do: [:x| x printNl]
        s inspect

   The key is the `on:' message; it tells a stream class to create
itself in terms of the existing storage.  Because of polymorphism, the
object specified by on: does not have to be an Array; any object which
responds to numeric at: messages can be used.  If you happen to have
the NiledArray class still loaded from the previous chapter, you might
try streaming over that kind of array instead.

   You're wondering if you're stuck with having to know how much data
will be queued in a Stream at the time you create the stream.  If you
use the right class of stream, the answer is no.  A ReadStream provides
read-only access to an existing collection.  You will receive an error
if you try to write to it.  If you try to read off the end of the
stream, you will also get an error.

   By contrast, WriteStream and ReadWriteStream (used in our example)
will tell the underlying collection to grow when you write off the end
of the existing collection.  Thus, if you want to write several
strings, and don't want to add up their lengths yourself:

        s := ReadWriteStream on: String new
        s inspect
        s nextPutAll: 'Hello, '
        s inspect
        s nextPutAll: 'world'
        s inspect
        s position: 1
        s inspect
        s do: [:c | stdout nextPut: c ]
        s contents

   In this case, we have used a String as the collection for the
Stream.  The `printOn:' messages add bytes to the initially empty
string.  Once we've added the data, you can continue to treat the data
as a stream.  Alternatively, you can ask the stream to return to you
the underlying object.  After that, you can use the object (a String,
in this example) using its own access methods.

   There are many amenities available on a stream object.  You can ask
if there's more to read with `atEnd'.  You can query the position with
`position', and set it with `position:'.  You can see what will be read
next with `peek', and you can read the next element with `next'.

   In the writing direction, you can write an element with `nextPut:'.
You don't need to worry about objects doing a `printOn:' with your
stream as a destination; this operation ends up as a sequence of
`nextPut:' operations to your stream.  If you have a collection of
things to write, you can use `nextPutAll:' with the collection as an
argument; each member of the collection will be written onto the
stream.  If you want to write an object to the stream several times, you
can use `next:put:', like this:

        s := ReadWriteStream on: (Array new: 0)
        s next: 4 put: 'Hi!'
        s position: 1
        s do: [:x | x printNl]


File: gst.info,  Node: Files,  Next: Dynamic Strings,  Prev: Your own stream,  Up: Streams

6.10.3 Files
------------

Streams can also operate on files.  If you wanted to dump the file
`/etc/passwd' to your terminal, you could create a stream on the file,
and then stream over its contents:
        f := FileStream open: '/etc/passwd' mode: FileStream read
        f linesDo: [ :c | Transcript nextPutAll: c; nl ]
        f position: 30
        25 timesRepeat: [ Transcript nextPut: (f next) ]
        f close

   and, of course, you can load Smalltalk source code into your image:
        FileStream fileIn: '/Users/myself/src/source.st'


File: gst.info,  Node: Dynamic Strings,  Prev: Files,  Up: Streams

6.10.4 Dynamic Strings
----------------------

Streams provide a powerful abstraction for a number of data structures.
Concepts like current position, writing the next position, and changing
the way you view a data structure when convenient combine to let you
write compact, powerful code.  The last example is taken from the actual
Smalltalk source code--it shows a general method for making an object
print itself onto a string.

        printString [
            | stream |
            stream := WriteStream on: (String new).
            self printOn: stream.
            ^stream contents
        ]

   This method, residing in Object, is inherited by every class in
Smalltalk.  The first line creates a WriteStream which stores on a
String whose length is currently 0 (`String new' simply creates an
empty string.  It then invokes the current object with `printOn:'.  As
the object prints itself to "stream", the String grows to accommodate
new characters.  When the object is done printing, the method simply
returns the underlying string.

   As we've written code, the assumption has been that printOn: would
go to the terminal.  But replacing a stream to a file like `/dev/tty'
with a stream to a data structure (`String new') works just as well.
The last line tells the Stream to return its underlying collection,
which will be the string which has had all the printing added to it.
The result is that the `printString' message returns an object of the
String class whose contents are the printed representation of the very
object receiving the message.


File: gst.info,  Node: Exception handling,  Next: Behind the scenes,  Prev: Streams,  Up: Tutorial

6.11 Exception handling in Smalltalk
====================================

Up to this point of the tutorial, you used the original Smalltalk-80
error signalling mechanism:

        check: num [
            | c |
            c := history
                at: num
                ifAbsent: [ ^self error: 'No such check #' ].
            ^c
        ]

   In the above code, if a matching check number is found, the method
will answer the object associated to it.  If no prefix is found,
Smalltalk will unwind the stack and print an error message including
the message you gave and stack information.

     CheckingAccount new: 31 "<0x33788>" error: No such check #
     ...blah blah...
     CheckingAccount>>#error:
     [] in Dictionary>>#at:ifAbsent:
     Dictionary(HashedCollection)>>#findIndex:ifAbsent:
     Dictionary>>#at:ifAbsent:
     [] in CheckingAccount>>#check:
     CheckingAccount>>#check:
     UndefinedObject(Object)>>#executeStatements

   Above we see the object that received the #error: message, the
message text itself, and the frames (innermost-first) running when the
error was captured by the system.  In addition, the rest of the code in
methods like `CheckingAccount>>#check:' was not executed.

   So simple error reporting gives us most of the features we want:

   * Execution stops immediately, preventing programs from continuing
     as if nothing is wrong.

   * The failing code provides a more-or-less useful error message.

   * Basic system state information is provided for diagnosis.

   * A debugger can drill further into the state, providing information
     like details of the receivers and arguments on the stack.

   However, there is a more powerful and complex error handling
mechanism, that is "exception".  They are like "exceptions" in other
programming languages, but are more powerful and do not always indicate
error conditions.  Even though we use the term "signal" often with
regard to them, do not confuse them with the signals like `SIGTERM' and
`SIGINT' provided by some operating systems; they are a different
concept altogether.

   Deciding to use exceptions instead of `#error:' is a matter of
aesthetics, but you can use a simple rule: use exceptions only if you
want to provide callers with a way to recover sensibly from certain
errors, and then only for signalling those particular errors.

   For example, if you are writing a word processor, you might provide
the user with a way to make regions of text read-only.  Then, if the
user tries to edit the text, the objects that model the read-only text
can signal a `ReadOnlyText' or other kind of exception, whereupon the
user interface code can stop the exception from unwinding and report
the error to the user.

   When in doubt about whether exceptions would be useful, err on the
side of simplicity; use `#error:' instead.  It is much easier to
convert an #error: to an explicit exception than to do the opposite.

* Menu:

* Creating exceptions::                Starting to use the mechanism
* Raising exceptions::                 What to do when exceptional events happen
* Handling exceptions::                The other side
* When an exception isn't handled::    Default actions
* Creating new exception classes::     Your own exceptions
* Hooking into the stack unwinding::   An alternative exception handling system
* Handler stack unwinding caveat::     Differences with other languages


File: gst.info,  Node: Creating exceptions,  Next: Raising exceptions,  Up: Exception handling

6.11.1 Creating exceptions
--------------------------

GNU Smalltalk provides a few exceptions, all of which are subclasses of
`Exception'.  Most of the ones you might want to create yourself are in
the `SystemExceptions' namespace.  You can browse the builtin
exceptions in the base library reference, and look at their names with
`Exception printHierarchy'.

   Some useful examples from the system exceptions are
`SystemExceptions.InvalidValue', whose meaning should be obvious, and
`SystemExceptions.WrongMessageSent', which we will demonstrate below.

   Let's say that you change one of your classes to no longer support
#new for creating new instances.  However, because you use the
first-class classes feature of Smalltalk, it is not so easy to find and
change all sends.  Now, you can do something like this:

     Object subclass: Toaster [
         Toaster class >> new [
             ^SystemExceptions.WrongMessageSent
                 signalOn: #new useInstead: #toast:
         ]

         Toaster class >> toast: reason [
             ^super new reason: reason; yourself
         ]

         ...
     ]

   Admittedly, this doesn't quite fit the conditions for using
exceptions.  However, since the exception type is already provided, it
is probably easier to use it than #error: when you are doing defensive
programming of this sort.

