# pt::peg::to::container(3tcl) 1 pt "Parser Tools"

## Name

pt::peg::to::container - PEG Conversion. Write CONTAINER format

## Table Of Contents

## Synopsis

- package require Tcl 8.5
- package require pt::peg::to::container ?1?
- package require pt::peg
- package require text::write
- package require char

- pt::peg::to::container reset
- pt::peg::to::container configure
- pt::peg::to::container configure option
- pt::peg::to::container configure option value...
- pt::peg::to::container 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 CONTAINER 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::container 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::container reset

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

pt::peg::to::container configure

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

pt::peg::to::container 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::container 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::container 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 CONTAINER 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 the CONTAINER format recognizes the following options and changes its behaviour as they specify.

-file string

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

-name string

  The value of this option is the name of the grammar we are processing. The default value is a_pe_grammar.

-user string

  The value of this option is the name of the user for which the command is run. The default value is unknown.

-mode bulk|incremental

  The value of this option controls which methods of pt::peg::container instances are used to specify the grammar, i.e. preload it into the container. There are two legal values, as listed below. The default is bulk.

  bulk

    In this mode the methods start, add, modes, and rules are used to specify the grammar in a bulk manner, i.e. as a set of nonterminal symbols, and two dictionaries mapping from the symbols to their semantic modes and parsing expressions.

    This mode is the default.

  incremental

    In this mode the methods start, add, mode, and rule are used to specify the grammar piecemal, with each nonterminal having its own block of defining commands.

-template string

  The value of this option is a string into which to put the generated code 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 option -user.

  @format@

    To be replaced with the the constant CONTAINER.

  @file@

    To be replaced with the value of the option -file.

  @name@

    To be replaced with the value of the option -name.

  @mode@

    To be replaced with the value of the option -mode.

  @code@

    To be replaced with the generated code.

## Grammar Container

The container format is another form of describing parsing expression grammars. While data in this format is executable it does not constitute a parser for the grammar. It always has to be used in conjunction with the package pt::peg::interp, a grammar interpreter.

The format represents grammars by a snit::type, i.e. class, whose instances are API-compatible to the instances of the pt::peg::container package, and which are preloaded with the grammar in question.

It has no direct formal specification beyond what was said above.

### 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;

one possible CONTAINER serialization for it is

    snit::type a_pe_grammar {
        constructor {} {
            install myg using pt::peg::container ${selfns}::G
            $myg start {n Expression}
            $myg add   AddOp Digit Expression Factor MulOp Number Sign Term
            $myg modes {
                AddOp      value
                Digit      value
                Expression value
                Factor     value
                MulOp      value
                Number     value
                Sign       value
                Term       value
            }
            $myg rules {
                AddOp      {/ {t -} {t +}}
                Digit      {/ {t 0} {t 1} {t 2} {t 3} {t 4} {t 5} {t 6} {t 7} {t 8} {t 9}}
                Expression {/ {x {t \50} {n Expression} {t \51}} {x {n Factor} {* {x {n MulOp} {n Factor}}}}}
                Factor     {x {n Term} {* {x {n AddOp} {n Term}}}}
                MulOp      {/ {t *} {t /}}
                Number     {x {? {n Sign}} {+ {n Digit}}}
                Sign       {/ {t -} {t +}}
                Term       {n Number}
            }
            return
        }
        component myg
        delegate method * to myg
    }

## 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 Trackers. Please also report any ideas for enhancements you may have for either package and/or documentation.

## Keywords

CONTAINER, EBNF, LL(k), PEG, 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>