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pt::peg::to::peg(3tcl) Parser Tools pt::peg::to::peg(3tcl) 





NAME


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






SYNOPSIS


package require Tcl 8.5 9


package require pt::peg::to::peg ?1.0.3?


package require pt::peg


package require pt::pe


package require text::write


pt::peg::to::peg reset


pt::peg::to::peg configure


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


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


pt::peg::to::peg 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 PEG 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::peg 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.



  

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

  

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

  

  
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.

  

  
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.

  

  
This command takes the canonical serialization of a parsing expression
      grammar, as specified in section PEG serialization format, and
      contained in serial, and generates PEG 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 PEG language recognizes the following options
    and changes its behaviour as they specify.



  

  
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.

  

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

  

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

  

  
The value of this option is a string into which to put the generated text
      and the values of the other options. 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 PEG.

  
@file@

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

  
@name@

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

  
@code@

  
To be replaced with the generated text.










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.



  

  







  
[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







  

  
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







  

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

  

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







  

  
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.

  

  
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.

  

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











  

  
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.







  

  
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.



  

  







  

  







  
[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.







  

  







  
[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.









  

  
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
    [http://core.tcl.tk/tcllib/reportlist]. 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 (c) 2009 Andreas Kupries <andreas_kupries@users.sourceforge.net>








  

    1.0.3
    tcllib