# STklos Reference Manual 3. Program structure

Contents
| *Top*
1 | Introduction
2 | Expressions
3 | Program structure
4 | Standard Procedures
5 | Regular Expressions
6 | Pattern Matching
7 | Exceptions and Conditions
8 | STklos Object System
9 | Threads, Mutexes and Condition Variables
10 | Customizations
11 | The ScmPkg Package System
12 | Foreign Function Interface
13 | Using the SLIB package
14 | SRFIs
15 | Index
| Bibliography
| R5RS discusses how to structure programs. Everything which is defined in Section 5 of R5RS applies also to STklos. To make things shorter, this aspects will not be described here (see R5RS for complete information). STklos modules can be used to organize a program into separate environments (or name spaces). Modules provide a clean way to organize and enforce the barriers between the components of a program. STklos provides a simple module system which is largely inspired from the one of Tung and Dybvig exposed in [14]. As their modules system, STklos modules are defined to be easily used in an interactive environment.
`(define-module <name> <expr1> <expr2> ...)` | STklos syntax
> `Define-module` evaluates the expressions `<expr1>`, `<expr2>` ... which constitute the body of the module `<name>` in the environment of that module. `Name` must be a valid symbol. If this symbol has not already been used to define a module, a new module, named `name`, is created. Otherwise, the expressions `<expr1>`, `<expr2>` ... are evaluated in the environment of the (old) module `<name>` 1. Definitions done in a module are local to the module and do not interact with the definitions in other modules. Consider the following definitions,
>     (define-module M1
>        (define a 1))

>     (define-module M2
>       (define a 2)
>       (define b (* 2 x)))
Here, two modules are defined and they both bind the symbol `a` to a value. However, since `a` has been defined in two distinct modules they denote two different locations. The `STklos` module, which is predefined, is a special module which contains all the global variables of a R5RS program. A symbol defined in the `STklos` module, if not hidden by a local definition, is always visible from inside a module. So, in the previous exemple, the `x` symbol refers the `x` symbol defined in the `STklos` module. The result of `define-module` is void.
`(current-module)` | STklos procedure
> Returns the current module.
>     (define-module M
>       (display
>           (cons (eq? (current-module) (find-module 'M))
>                 (eq? (current-module) (find-module 'STklos)))))
>        -| (#t . #f)
`(find-module name)` | STklos procedure
`(find-module name default)` STklos modules are first class objects and `find-module` returns the module associated to `name` if it exists. If there is no module associated to `name`, an error is signaled if no `default` is provided, otherwise `find-module` returns `default`.
`(module? object)` | STklos procedure
> Returns `#t` if `object` is a module and `#f` otherwise.
>     (module? (find-module 'STklos))  ⇒ #t
>     (module? 'STklos)                ⇒ #f
>     (module? 123 'no)                ⇒ no
`(export <symbol1> <symbol2> ...)` | STklos syntax
Specifies the symbols which are exported (i.e. visible) outside the current module. By default, symbols defined in a module are not visible outside this module, excepted if they appear in an `export` clause. If several `export` clauses appear in a module, the set of exported symbols is determined by ``unionizing'' symbols exported in all the `export` clauses. The result of `export` is void.
`(import <module1> <module2> ...)` | STklos syntax
> Specifies the modules which are imported by the current module. Importing a module makes the symbols it exports visible to the importer, if not hidden by local definitions. When a symbol is exported by several of the imported modules, the location denoted by this symbol in the importer module correspond to the one of the first module in the list
>     (<module1> <module2> ...)
> which exports it. If several `import` clauses appear in a module, the set of imported modules is determined by appending the various list of modules in their apparition order.
>     (define-module M1
>       (export a b)
>       (define a 'M1-a)
>       (define b 'M1-b))

>     (define-module M2
>       (export b c d)
>       (define b 'M2-b)
>       (define c 'M2-c)
>       (define d 'M2-d))

>     (define-module M3
>       (import M1 M2)
>       (display (list a b c d)))  -| (M1-a M1-b M2-c M2-d)
(define-module M4 (import M2 M1) (display (list a b c d))) -| (M1-a M2-b M2-c M2-d)
`(select-module <name>)` | STklos syntax
> Changes the value of the current module to the module with the given `name`. The expressions evaluated after `select-module` will take place in module `name` environment. Module `name` must have been created previously by a `define-module`. The result of `select-module` is void. `Select-module` is particularly useful when debugging since it allows to place toplevel evaluation in a particular module. The following transcript shows an usage of `select-module`. 2:
>     stklos> (define foo 1)
>     stklos> (define-module bar
>               (define foo 2))
>     stklos> foo
>     1
>     stklos> (select-module bar)
>     bar> foo
>     2
>     bar> (select-module stklos)
>     stklos>
`(symbol-value symbol module)` | STklos procedure
`(symbol-value symbol module default)` Returns the value bound to `symbol` in `module`. If `symbol` is not bound, an error is signaled if no `default` is provided, otherwise `symbol-value` returns `default`.
`(symbol-value* symbol module)` | STklos procedure
`(symbol-value* symbol module default)` Returns the value bound to `symbol` in `module`. If `symbol` is not bound, an error is signaled if no `default` is provided, otherwise `symbol-value` returns `default`. Note that this function searches the value of `symbol` in `module` and all the modules it imports whereas `symbol-value` searches only in `module`.
`(module-name module)` | STklos procedure
Returns the name (a symbol) associated to a `module`.
`(module-imports module)` | STklos procedure
Returns the list of modules that `module` (fully) imports.
`(module-exports module)` | STklos procedure
Returns the list of symbols exported by `module`. Note that this function returns the list of symbols given in the module `export` clause and that some of these symbols can be not yet defined.
`(module-symbols module)` | STklos procedure
Returns the list of symbols already defined in `module`.
`(all-modules)` | STklos procedure
Returns the list of all the living modules.
`(in-module mod s)` | STklos syntax
> `(in-module mod s default)` This form returns the value of symbol with name `s` in the module with name `mod`. If this symbol is not bound, an error is signaled if no `default` is provided, otherwise `in-module` returns `default`. Note that the value of `s` is searched in `mod` and all the modules it imports. This form is in fact a shortcut. In effect,
>     (in-module my-module foo)
> is equivalent to
>     (symbol-value* 'foo (find-module 'my-module))
1: In fact `define-module` on a given name defines a new module only the first time it is invoked on this name. By this way, interactively reloading a module does not define a new entity, and the other modules which use it are not altered. 2: This transcript uses the default toplevel loop which displays the name of the current module in the evaluator prompt.

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Last update Sat Dec 31 15:49:30 2011