# pt::peg::to::tclparam(n) 1 pt "Parser Tools"

## Name

pt::peg::to::tclparam - PEG Conversion. Write TCLPARAM format

## Table Of Contents

## Synopsis

- package require Tcl 8.5
- package require pt::peg::to::tclparam ?1?

- pt::peg::to::tclparam reset
- pt::peg::to::tclparam configure
- pt::peg::to::tclparam configure option
- pt::peg::to::tclparam configure option value...
- pt::peg::to::tclparam convert serial

## Description

Are you lost ? Do you have trouble understanding this document ? In that case please read the overview provided by the Introduction to Parser Tools. This document is the entrypoint to the whole system the current package is a part of.

This package implements the converter from parsing expression grammars to TCLPARAM markup.

It resides in the Export section of the Core Layer of Parser Tools, and can be used either directly with the other packages of this layer, or indirectly through the export manager provided by pt::peg::export. The latter is intented for use in untrusted environments and done through the corresponding export plugin pt::peg::export::tclparam sitting between converter and export manager.

Image: arch_core_eplugins

## API

The API provided by this package satisfies the specification of the Converter API found in the Parser Tools Export API specification.

pt::peg::to::tclparam reset

  This command resets the configuration of the package to its default settings.

pt::peg::to::tclparam configure

  This command returns a dictionary containing the current configuration of the package.

pt::peg::to::tclparam configure option

  This command returns the current value of the specified configuration option of the package. For the set of legal options, please read the section Options.

pt::peg::to::tclparam configure option value...

  This command sets the given configuration options of the package, to the specified values. For the set of legal options, please read the section Options.

pt::peg::to::tclparam convert serial

  This command takes the canonical serialization of a parsing expression grammar, as specified in section PEG serialization format, and contained in serial, and generates TCLPARAM markup encoding the grammar, per the current package configuration. The created string is then returned as the result of the command.

## Options

The converter to Tcl/PARAM markup recognizes the following configuration variables and changes its behaviour as they specify.

-template string

  The value of this configuration variable is a string into which to put the generated text and the other configuration settings. The various locations for user-data are expected to be specified with the placeholders listed below. The default value is "@code@".

  @user@

    To be replaced with the value of the configuration variable -user.

  @format@

    To be replaced with the the constant Tcl/PARAM.

  @file@

    To be replaced with the value of the configuration variable -file.

  @name@

    To be replaced with the value of the configuration variable -name.

  @code@

    To be replaced with the generated Tcl code.

  The following configuration variables are special, in that they will occur within the generated code, and are replaced there as well.

  @runtime@

    To be replaced with the value of the configuration variable runtime-command.

  @self@

    To be replaced with the value of the configuration variable self-command.

  @def@

    To be replaced with the value of the configuration variable proc-command.

  @ns@

    To be replaced with the value of the configuration variable namespace.

  @main@

    To be replaced with the value of the configuration variable main.

  @prelude@

    To be replaced with the value of the configuration variable prelude.

-name string

  The value of this configuration variable is the name of the grammar for which the conversion is run. The default value is a_pe_grammar.

-user string

  The value of this configuration variable is the name of the user for which the conversion is run. The default value is unknown.

-file string

  The value of this configuration variable is the name of the file or other entity from which the grammar came, for which the conversion is run. The default value is unknown.

-runtime-command string

  A Tcl string representing the Tcl command or reference to it used to call PARAM instruction from parser procedures, per the chosen framework (template). The default value is the empty string.

-self-command string

  A Tcl string representing the Tcl command or reference to it used to call the parser procedures (methods ...) from another parser procedure, per the chosen framework (template). The default value is the empty string.

-proc-command string

  The name of the Tcl command used to define procedures (methods ...), per the chosen framework (template). The default value is proc.

-namespace string

  The name of the namespace the parser procedures (methods, ...) shall reside in, including the trailing '::' needed to separate it from the actual procedure name. The default value is ::.

-main string

  The name of the main procedure (method, ...) to be called by the chosen framework (template) to start parsing input. The default value is __main.

-prelude string

  A snippet of code to be insert at the head of each generated parsing command. The default value is the empty string.

-indent integer

  The number of characters to indent each line of the generated code by. The default value is 0.

While the high parameterizability of this converter, as shown by the multitude of options it supports, is an advantage to the advanced user, allowing her to customize the output of the converter as needed, a novice user will likely not see the forest for the trees.

To help these latter users two adjunct packages are provided, each containing a canned configuration which will generate immediately useful full parsers. These are

pt::tclparam::configuration::snit

  Generated parsers are classes based on the snit package, i.e. snit::type's.

pt::tclparam::configuration::tcloo

  Generated parsers are classes based on the OO package.

## Tcl/PARAM code representation of parsing expression grammars

The Tcl/PARAM representation of parsing expression grammars is Tcl code whose execution will parse input per the grammar. The code is based on the virtual machine documented in the PackRat Machine Specification, using its instructions and a few more to handle control flow.

Note that the generated code by itself is not functional. It expects to be embedded into a framework which provides services like the PARAM state, implementations for the PARAM instructions, etc. The bulk of such a framework has to be specified through the option -template. The additional options

-indent integer
-main string
-namespace string
-prelude string
-proc-command string
-runtime-command string
-self-command string

provide code snippets which help to glue framework and generated code together. Their placeholders are in the generated code.

## PEG serialization format

Here we specify the format used by the Parser Tools to serialize Parsing Expression Grammars as immutable values for transport, comparison, etc.

We distinguish between regular and canonical serializations. While a PEG may have more than one regular serialization only exactly one of them will be canonical.

regular serialization

  1. The serialization of any PEG is a nested Tcl dictionary.

  2. This dictionary holds a single key, pt::grammar::peg, and its value. This value holds the contents of the grammar.

  3. The contents of the grammar are a Tcl dictionary holding the set of nonterminal symbols and the starting expression. The relevant keys and their values are

     rules

       The value is a Tcl dictionary whose keys are the names of the nonterminal symbols known to the grammar.

       1. Each nonterminal symbol may occur only once.

       2. The empty string is not a legal nonterminal symbol.

       3. The value for each symbol is a Tcl dictionary itself. The relevant keys and their values in this dictionary are

          is

            The value is the serialization of the parsing expression describing the symbols sentennial structure, as specified in the section PE serialization format.

          mode

            The value can be one of three values specifying how a parser should handle the semantic value produced by the symbol.

            value

              The semantic value of the nonterminal symbol is an abstract syntax tree consisting of a single node node for the nonterminal itself, which has the ASTs of the symbol's right hand side as its children.

            leaf

              The semantic value of the nonterminal symbol is an abstract syntax tree consisting of a single node node for the nonterminal, without any children. Any ASTs generated by the symbol's right hand side are discarded.

            void

              The nonterminal has no semantic value. Any ASTs generated by the symbol's right hand side are discarded (as well).

     start

       The value is the serialization of the start parsing expression of the grammar, as specified in the section PE serialization format.

  4. The terminal symbols of the grammar are specified implicitly as the set of all terminal symbols used in the start expression and on the RHS of the grammar rules.

canonical serialization

  The canonical serialization of a grammar has the format as specified in the previous item, and then additionally satisfies the constraints below, which make it unique among all the possible serializations of this grammar.

  1. The keys found in all the nested Tcl dictionaries are sorted in ascending dictionary order, as generated by Tcl's builtin command lsort -increasing -dict.

  2. The string representation of the value is the canonical representation of a Tcl dictionary. I.e. it does not contain superfluous whitespace.

### Example

Assuming the following PEG for simple mathematical expressions

    PEG calculator (Expression)
        Digit      <- '0'/'1'/'2'/'3'/'4'/'5'/'6'/'7'/'8'/'9'       ;
        Sign       <- '-' / '+'                                     ;
        Number     <- Sign? Digit+                                  ;
        Expression <- Term (AddOp Term)*                            ;
        MulOp      <- '*' / '/'                                     ;
        Term       <- Factor (MulOp Factor)*                        ;
        AddOp      <- '+'/'-'                                       ;
        Factor     <- '(' Expression ')' / Number                   ;
    END;

then its canonical serialization (except for whitespace) is

    pt::grammar::peg {
        rules {
            AddOp      {is {/ {t -} {t +}}                                                                mode value}
            Digit      {is {/ {t 0} {t 1} {t 2} {t 3} {t 4} {t 5} {t 6} {t 7} {t 8} {t 9}}                mode value}
            Expression {is {x {n Term} {* {x {n AddOp} {n Term}}}}                                        mode value}
            Factor     {is {/ {x {t (} {n Expression} {t )}} {n Number}}                                  mode value}
            MulOp      {is {/ {t *} {t /}}                                                                mode value}
            Number     {is {x {? {n Sign}} {+ {n Digit}}}                                                 mode value}
            Sign       {is {/ {t -} {t +}}                                                                mode value}
            Term       {is {x {n Factor} {* {x {n MulOp} {n Factor}}}}                                    mode value}
        }
        start {n Expression}
    }

## PE serialization format

Here we specify the format used by the Parser Tools to serialize Parsing Expressions as immutable values for transport, comparison, etc.

We distinguish between regular and canonical serializations. While a parsing expression may have more than one regular serialization only exactly one of them will be canonical.

Regular serialization
  Atomic Parsing Expressions

    1. The string epsilon is an atomic parsing expression. It matches the empty string.

    2. The string dot is an atomic parsing expression. It matches any character.

    3. The string alnum is an atomic parsing expression. It matches any Unicode alphabet or digit character. This is a custom extension of PEs based on Tcl's builtin command string is.

    4. The string alpha is an atomic parsing expression. It matches any Unicode alphabet character. This is a custom extension of PEs based on Tcl's builtin command string is.

    5. The string ascii is an atomic parsing expression. It matches any Unicode character below U0080. This is a custom extension of PEs based on Tcl's builtin command string is.

    6. The string control is an atomic parsing expression. It matches any Unicode control character. This is a custom extension of PEs based on Tcl's builtin command string is.

    7. The string digit is an atomic parsing expression. It matches any Unicode digit character. Note that this includes characters outside of the [0..9] range. This is a custom extension of PEs based on Tcl's builtin command string is.

    8. The string graph is an atomic parsing expression. It matches any Unicode printing character, except for space. This is a custom extension of PEs based on Tcl's builtin command string is.

    9. The string lower is an atomic parsing expression. It matches any Unicode lower-case alphabet character. This is a custom extension of PEs based on Tcl's builtin command string is.

    10. The string print is an atomic parsing expression. It matches any Unicode printing character, including space. This is a custom extension of PEs based on Tcl's builtin command string is.

    11. The string punct is an atomic parsing expression. It matches any Unicode punctuation character. This is a custom extension of PEs based on Tcl's builtin command string is.

    12. The string space is an atomic parsing expression. It matches any Unicode space character. This is a custom extension of PEs based on Tcl's builtin command string is.

    13. The string upper is an atomic parsing expression. It matches any Unicode upper-case alphabet character. This is a custom extension of PEs based on Tcl's builtin command string is.

    14. The string wordchar is an atomic parsing expression. It matches any Unicode word character. This is any alphanumeric character (see alnum), and any connector punctuation characters (e.g. underscore). This is a custom extension of PEs based on Tcl's builtin command string is.

    15. The string xdigit is an atomic parsing expression. It matches any hexadecimal digit character. This is a custom extension of PEs based on Tcl's builtin command string is.

    16. The string ddigit is an atomic parsing expression. It matches any decimal digit character. This is a custom extension of PEs based on Tcl's builtin command regexp.

    17. The expression [list t x] is an atomic parsing expression. It matches the terminal string x.

    18. The expression [list n A] is an atomic parsing expression. It matches the nonterminal A.

  Combined Parsing Expressions

    1. For parsing expressions e1, e2, ... the result of [list / e1 e2 ... ] is a parsing expression as well. This is the ordered choice, aka prioritized choice.

    2. For parsing expressions e1, e2, ... the result of [list x e1 e2 ... ] is a parsing expression as well. This is the sequence.

    3. For a parsing expression e the result of [list * e] is a parsing expression as well. This is the kleene closure, describing zero or more repetitions.

    4. For a parsing expression e the result of [list + e] is a parsing expression as well. This is the positive kleene closure, describing one or more repetitions.

    5. For a parsing expression e the result of [list & e] is a parsing expression as well. This is the and lookahead predicate.

    6. For a parsing expression e the result of [list ! e] is a parsing expression as well. This is the not lookahead predicate.

    7. For a parsing expression e the result of [list ? e] is a parsing expression as well. This is the optional input.

Canonical serialization

  The canonical serialization of a parsing expression has the format as specified in the previous item, and then additionally satisfies the constraints below, which make it unique among all the possible serializations of this parsing expression.

  1. The string representation of the value is the canonical representation of a pure Tcl list. I.e. it does not contain superfluous whitespace.

  2. Terminals are not encoded as ranges (where start and end of the range are identical).

### Example

Assuming the parsing expression shown on the right-hand side of the rule

        Expression <- Term (AddOp Term)*

then its canonical serialization (except for whitespace) is

        {x {n Term} {* {x {n AddOp} {n Term}}}}

## Bugs, Ideas, Feedback

This document, and the package it describes, will undoubtedly contain bugs and other problems. Please report such in the category pt of the Tcllib SF Trackers. Please also report any ideas for enhancements you may have for either package and/or documentation.

## Keywords

EBNF, LL(k), PEG, TCLPARAM, TDPL, context-free languages, conversion, expression, format conversion, grammar, matching, parser, parsing expression, parsing expression grammar, push down automaton, recursive descent, serialization, state, top-down parsing languages, transducer

## Category

Parsing and Grammars

## Copyright

Copyright © 2009 Andreas Kupries <andreas_kupries@users.sourceforge.net>