Provided by: tcllib_1.21+dfsg-1_all bug

NAME

       pt::peg - Parsing Expression Grammar Serialization

SYNOPSIS

       package require Tcl  8.5

       package require pt::peg  ?1?

       package require pt::pe

       ::pt::peg verify serial ?canonvar?

       ::pt::peg verify-as-canonical serial

       ::pt::peg canonicalize serial

       ::pt::peg print serial

       ::pt::peg merge seriala serialb

       ::pt::peg equal seriala serialb

_________________________________________________________________________________________________

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  provides  commands  to  work  with the serializations of parsing expression
       grammars as managed by the Parser  Tools,  and  specified  in  section  PEG  serialization
       format.

       This is a supporting package in the Core Layer of Parser Tools.

       IMAGE: arch_core_support

API

       ::pt::peg verify serial ?canonvar?
              This  command  verifies  that  the  content of serial is a valid serialization of a
              parsing expression and will throw an error if that is not the case. The  result  of
              the command is the empty string.

              If  the  argument canonvar is specified it is interpreted as the name of a variable
              in the calling context. This variable will be written to if and only if serial is a
              valid regular serialization. Its value will be a boolean, with True indicating that
              the serialization is not only valid, but also canonical. False will be written  for
              a valid, but non-canonical serialization.

              For the specification of serializations see the section PE serialization format.

       ::pt::peg verify-as-canonical serial
              This command verifies that the content of serial is a valid canonical serialization
              of a PEG and will throw an error if that is not the case. The result of the command
              is the empty string.

              For the specification of canonical serializations see the section PEG serialization
              format.

       ::pt::peg canonicalize serial
              This command assumes that the content of serial is a valid regular serialization of
              a PEG and will throw an error if that is not the case.

              It  will  then  convert the input into the canonical serialization of the contained
              PEG and return it as its result. If the input  is  already  canonical  it  will  be
              returned unchanged.

              For  the  specification of regular and canonical serializations see the section PEG
              serialization format.

       ::pt::peg print serial
              This command assumes that the argument serial contains a valid serialization  of  a
              parsing  expression  and  returns  a  string containing that PE in a human readable
              form.

              The exact format of this form is not specified and cannot be relied on for  parsing
              or other machine-based activities.

              For the specification of serializations see the section PEG serialization format.

       ::pt::peg merge seriala serialb
              This  command  accepts  the regular serializations of two grammars and uses them to
              create their union.  The result of the command is the  canonical  serialization  of
              this unified grammar.

              A  merge  errors  occurs  if  for  any  nonterminal symbol S occuring in both input
              grammars the two input grammars specify different semantic modes.

              The semantic mode of each nonterminal symbol S is the semantic mode of S in any  of
              its  input  grammars. The previous rule made sure that for symbols occuring in both
              grammars these values are identical.

              The right-hand side of each nonterminal symbol S occuring in both input grammars is
              the  choice  between  the  right-hand  sides  of  S in the input grammars, with the
              parsing expression of S in seriala coming first, except  if  both  expressions  are
              identical. In that case the first expression is taken.

              The  right-hand side of each nonterminal symbol S occuring in only one of the input
              grammars is the right-hand side of S in its input grammar.

              The start expression of the  unified  grammar  is  the  choice  between  the  start
              expressions  of  the  input  grammars,  with the start expression of seriala coming
              first, except if both expressions are identical.  In that case the first expression
              is taken

       ::pt::peg equal seriala serialb
              This  command  tests  the two grammars seriala and serialb for structural equality.
              The result of the command is a boolean value.  It  will  be  set  to  true  if  the
              expressions are identical, and false otherwise.

              String  equality is usable only if we can assume that the two grammars are pure Tcl
              lists and dictionaries.

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
       [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, expression, grammar, matching, parser,
       parsing expression, parsing expression grammar, push down  automaton,  recursive  descent,
       state, top-down parsing languages, transducer

CATEGORY

       Parsing and Grammars

COPYRIGHT

       Copyright (c) 2009 Andreas Kupries <andreas_kupries@users.sourceforge.net>