pt_peg_export_peg(3)



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

______________________________________________________________________________

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

SYNOPSIS
       package require Tcl  8.5

       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.

       IMAGE: arch_core_eplugins

       While the direct use of this package with a regular interpreter is pos-
       sible, this is strongly disrecommended and requires a number of contor-
       tions 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  pro-
              vides.

       [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.

       export serial configuration
              This command takes the canonical serialization of a parsing  ex-
              pression grammar, as specified in section PEG serialization for-
              mat, and contained in serial, the configuration,  a  dictionary,
              and  generates  PEG  markup  encoding  the grammar.  The created
              string is then returned as the result of the command.

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

       string template
              If this configuration variable is set it is assumed to contain a
              string into which to put the generated text and other configura-
              tion  data.  The  various locations are expected to be specified
              with the following placeholders:

              @user@ To be replaced with the value of the configuration  vari-
                     able user.

              @format@
                     To be replaced with the the constant PEG.

              @file@ To  be replaced with the value of the configuration vari-
                     able file.

              @name@ To be replaced with the value of the configuration  vari-
                     able name.

              @code@ To be replaced with the generated text.

       If  this  configuration  variable is not set, or empty, then the plugin
       falls back to a standard template, which is defined as "@code@".

       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 de-
       fined 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   <- "\\" [0-2][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 <- '(' Expression ')' / (Factor (MulOp Factor)*);
                  MulOp      <- [*/];
                  Factor     <- Term (AddOp Term)*;
                  AddOp      <- [-+];
                  Term       <- Number;
              END;

PEG SERIALIZATION FORMAT
       Here we specify the format used by the Parser Tools to serialize  Pars-
       ing  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  expres-
                     sion. 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 dictio-
                                   nary 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  serial-
                                          ization format.

                                   mode   The value can be one of three values
                                          specifying how a parser should  han-
                                          dle  the  semantic value produced by
                                          the symbol.

                                          value  The  semantic  value  of  the
                                                 nonterminal  symbol is an ab-
                                                 stract 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  chil-
                                                 dren.

                                          leaf   The  semantic  value  of  the
                                                 nonterminal symbol is an  ab-
                                                 stract 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 seman-
                                                 tic value. Any ASTs generated
                                                 by the  symbol's  right  hand
                                                 side are discarded (as well).

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

              [4]    The terminal symbols of the grammar are specified implic-
                     itly 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 spec-
              ified 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 con-
                     tain 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  Pars-
       ing 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 expres-
                            sion. 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  charac-
                            ter.  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  expres-
                            sion.  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 charac-
                            ter. 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, includ-
                            ing 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 charac-
                            ter. This is a custom extension of  PEs  based  on
                            Tcl's builtin command string is.

                     [14]   The  string  wordchar is an atomic parsing expres-
                            sion. It matches any Unicode word character.  This
                            is any alphanumeric character (see alnum), and any
                            connector  punctuation  characters  (e.g.   under-
                            score). 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 ex-
                            pression. It matches the terminal string x.

                     [18]   The expression [list n A] is an atomic parsing ex-
                            pression. 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 repeti-
                            tions.

                     [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 for-
              mat 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 con-
                     tain 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 out-
       put 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 gram-
       mar, plugin, 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>

tcllib                                 1            pt::peg::export::peg(3tcl)

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