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NAME

pt::peg::export::peg - PEG Export Plugin. Write PEG format

Table Of Contents

SYNOPSIS

package require Tcl 8.5 9
package require pt::peg::export::peg ?1?
package require pt::peg::to::peg

export serial configuration

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 parsing expression grammar export plugin for the generation of PEG markup.

It resides in the Export section of the Core Layer of Parser Tools and is intended to be used by pt::peg::export, the export manager, sitting between it and the corresponding core conversion functionality provided by pt::peg::to::peg.

While the direct use of this package with a regular interpreter is possible, this is strongly disrecommended and requires a number of contortions to provide the expected environment. The proper way to use this functionality depends on the situation:

  1. In an untrusted environment the proper access is through the package pt::peg::export and the export manager objects it provides.

  2. In a trusted environment however simply use the package pt::peg::to::peg and access the core conversion functionality directly.

API

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

Configuration

The PEG export plugin recognizes the following configuration variables and changes its behaviour as they specify.

Note that this plugin may ignore the standard configuration variables user, format, file, and their values, depending on the chosen template.

The content of the standard configuration variable name, if set, is used as name of the grammar in the output. Otherwise the plugin falls back to the default name a_pe_grammar.

PEG Specification Language

peg, a language for the specification of parsing expression grammars is meant to be human readable, and writable as well, yet strict enough to allow its processing by machine. Like any computer language. It was defined to make writing the specification of a grammar easy, something the other formats found in the Parser Tools do not lend themselves too.

It is formally specified by the grammar shown below, written in itself. For a tutorial / introduction to the language please go and read the PEG Language Tutorial.

PEG pe_grammar_for_peg (Grammar)

	# --------------------------------------------------------------------
        # Syntactical constructs

        Grammar         <- WHITESPACE Header Definition* Final EOF ;

        Header          <- PEG Identifier StartExpr ;
        Definition      <- Attribute? Identifier IS Expression SEMICOLON ;
        Attribute       <- (VOID / LEAF) COLON ;
        Expression      <- Sequence (SLASH Sequence)* ;
        Sequence        <- Prefix+ ;
        Prefix          <- (AND / NOT)? Suffix ;
        Suffix          <- Primary (QUESTION / STAR / PLUS)? ;
        Primary         <- ALNUM / ALPHA / ASCII / CONTROL / DDIGIT / DIGIT
                        /  GRAPH / LOWER / PRINTABLE / PUNCT / SPACE / UPPER
                        /  WORDCHAR / XDIGIT
                        / Identifier
                        /  OPEN Expression CLOSE
                        /  Literal
                        /  Class
                        /  DOT
                        ;
        Literal         <- APOSTROPH  (!APOSTROPH  Char)* APOSTROPH  WHITESPACE
                        /  DAPOSTROPH (!DAPOSTROPH Char)* DAPOSTROPH WHITESPACE ;
        Class           <- OPENB (!CLOSEB Range)* CLOSEB WHITESPACE ;
        Range           <- Char TO Char / Char ;

        StartExpr       <- OPEN Expression CLOSE ;
void:   Final           <- "END" WHITESPACE SEMICOLON WHITESPACE ;

        # --------------------------------------------------------------------
        # Lexing constructs

        Identifier      <- Ident WHITESPACE ;
leaf:   Ident           <- ([_:] / <alpha>) ([_:] / <alnum>)* ;
        Char            <- CharSpecial / CharOctalFull / CharOctalPart
                        /  CharUnicode / CharUnescaped
                        ;

leaf:   CharSpecial     <- "\\" [nrt'"\[\]\\] ;
leaf:   CharOctalFull   <- "\\" [0-3][0-7][0-7] ;
leaf:   CharOctalPart   <- "\\" [0-7][0-7]? ;
leaf:   CharUnicode     <- "\\" 'u' HexDigit (HexDigit (HexDigit HexDigit?)?)? ;
leaf:   CharUnescaped   <- !"\\" . ;

void:   HexDigit        <- [0-9a-fA-F] ;

void:   TO              <- '-'           ;
void:   OPENB           <- "["           ;
void:   CLOSEB          <- "]"           ;
void:   APOSTROPH       <- "'"           ;
void:   DAPOSTROPH      <- '"'           ;
void:   PEG             <- "PEG" !([_:] / <alnum>) WHITESPACE ;
void:   IS              <- "<-"    WHITESPACE ;
leaf:   VOID            <- "void"  WHITESPACE ; # Implies that definition has no semantic value.
leaf:   LEAF            <- "leaf"  WHITESPACE ; # Implies that definition has no terminals.
void:   SEMICOLON       <- ";"     WHITESPACE ;
void:   COLON           <- ":"     WHITESPACE ;
void:   SLASH           <- "/"     WHITESPACE ;
leaf:   AND             <- "&"     WHITESPACE ;
leaf:   NOT             <- "!"     WHITESPACE ;
leaf:   QUESTION        <- "?"     WHITESPACE ;
leaf:   STAR            <- "*"     WHITESPACE ;
leaf:   PLUS            <- "+"     WHITESPACE ;
void:   OPEN            <- "("     WHITESPACE ;
void:   CLOSE           <- ")"     WHITESPACE ;
leaf:   DOT             <- "."     WHITESPACE ;

leaf:   ALNUM           <- "<alnum>"    WHITESPACE ;
leaf:   ALPHA           <- "<alpha>"    WHITESPACE ;
leaf:   ASCII           <- "<ascii>"    WHITESPACE ;
leaf:   CONTROL         <- "<control>"  WHITESPACE ;
leaf:   DDIGIT          <- "<ddigit>"   WHITESPACE ;
leaf:   DIGIT           <- "<digit>"    WHITESPACE ;
leaf:   GRAPH           <- "<graph>"    WHITESPACE ;
leaf:   LOWER           <- "<lower>"    WHITESPACE ;
leaf:   PRINTABLE       <- "<print>"    WHITESPACE ;
leaf:   PUNCT           <- "<punct>"    WHITESPACE ;
leaf:   SPACE           <- "<space>"    WHITESPACE ;
leaf:   UPPER           <- "<upper>"    WHITESPACE ;
leaf:   WORDCHAR        <- "<wordchar>" WHITESPACE ;
leaf:   XDIGIT          <- "<xdigit>"   WHITESPACE ;

void:   WHITESPACE      <- (" " / "\t" / EOL / COMMENT)* ;
void:   COMMENT         <- '#' (!EOL .)* EOL ;
void:   EOL             <- "\n\r" / "\n" / "\r" ;
void:   EOF             <- !. ;

        # --------------------------------------------------------------------
END;

Example

Our example specifies the grammar for a basic 4-operation calculator.

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;

Using higher-level features of the notation, i.e. the character classes (predefined and custom), this example can be rewritten as

PEG calculator (Expression)
    Sign       <- [-+]                                          ;
    Number     <- Sign? <ddigit>+                               ;
    Expression <- Term (AddOp Term)*                            ;
    MulOp      <- [*/]						;
    Term       <- Factor (MulOp Factor)*                        ;
    AddOp      <- [-+]						;
    Factor     <- '(' Expression ')' / Number                   ;
END;

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.

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.

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.

When proposing code changes, please provide unified diffs, i.e the output of diff -u.

Note further that attachments are strongly preferred over inlined patches. Attachments can be made by going to the Edit form of the ticket immediately after its creation, and then using the left-most button in the secondary navigation bar.

KEYWORDS

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

CATEGORY

Parsing and Grammars

COPYRIGHT

Copyright © 2009 Andreas Kupries

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