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EuroTcl/OpenACS 11 - 12 JULY 2024, VIENNA


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NAME

pt::peg::from::peg - PEG Conversion. Read PEG format

Table Of Contents

SYNOPSIS

package require Tcl 8.5 9
package require pt::peg::from::peg ?1.0.4?

pt::peg::from::peg convert text

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 PEG markup to parsing expression grammars.

It resides in the Import 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 import manager provided by pt::peg::import. The latter is intented for use in untrusted environments and done through the corresponding import plugin pt::peg::import::peg sitting between converter and import manager.

API

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

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, 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