pt_json_language(3)



pt::json_language(3tcl)          Parser Tools          pt::json_language(3tcl)

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NAME
       pt::json_language - The JSON Grammar Exchange Format

SYNOPSIS
       package require Tcl  8.5

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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.

       The json format for parsing expression grammars was written as  a  data
       exchange  format not bound to Tcl. It was defined to allow the exchange
       of grammars with PackRat/PEG based parser  generators  for  other  lan-
       guages.

       It is formally specified by the rules below:

       [1]    The JSON of any PEG is a JSON object.

       [2]    This object 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 JSON object holding the set of
              nonterminal  symbols  and  the starting expression. The relevant
              keys and their values are

              rules  The value is a JSON object 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 sym-
                            bol.

                     [3]    The value for each symbol is a JSON object itself.
                            The relevant keys and their values in this dictio-
                            nary are

                            is     The value is a JSON string holding the  Tcl
                                   serialization of the parsing expression de-
                                   scribing the symbols sentennial  structure,
                                   as  specified  in the section PE serializa-
                                   tion format.

                            mode   The value is a JSON holding holding one  of
                                   three values specifying how a parser should
                                   handle the semantic value produced  by  the
                                   symbol.

                                   value  The  semantic value of the nontermi-
                                          nal 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  nontermi-
                                          nal  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  dis-
                                          carded (as well).

              start  The  value is a JSON string holding the Tcl serialization
                     of the start parsing expression of the grammar, as speci-
                     fied 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.

       As an aside to the advanced reader, this is pretty much the same as the
       Tcl serialization of PE grammars, as specified in section  PEG  serial-
       ization format, except that the Tcl dictionaries and lists of that for-
       mat are mapped to JSON objects and arrays. Only the parsing expressions
       themselves  are  not  translated further, but kept as JSON strings con-
       taining a nested Tcl list, and there is no concept  of  canonicity  for
       the JSON either.

   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;

       a JSON serialization for it 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 {t (} {n Expression} {t )}} {x {n Factor} {* {x {n MulOp} {n Factor}}}}",
                              "mode" : "value"
                          },
                          "Factor"    : {
                              "is"   : "x {n Term} {* {x {n AddOp} {n Term}}}",
                              "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"   : "n Number",
                              "mode" : "value"
                          }
                      },
                      "start" : "n Expression"
                  }
              }

       and a Tcl serialization of the same 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}
              }

       The similarity of the latter to the JSON should be quite obvious.

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, 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>

tcllib                                 1               pt::json_language(3tcl)

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