pt::peg::import::json(n)         Parser Tools         pt::peg::import::json(n)



______________________________________________________________________________

NAME
       pt::peg::import::json - PEG Import Plugin. Read JSON format

SYNOPSIS
       package require Tcl  8.5

       package require pt::peg::import::json  ?1?

       package require pt::peg::to::json

       import text

_________________________________________________________________

DESCRIPTION
       Are you lost ?  Do you have trouble understanding this document ?  In
       that case please read the overview provided by the Introduction to
       Parser Tools. This document is the entrypoint to the whole system the
       current package is a part of.

       This package implements the parsing expression grammar import plugin
       processing JSON markup.

       It resides in the Import section of the Core Layer of Parser Tools and
       is intended to be used by pt::peg::import, the import manager, sitting
       between it and the corresponding core conversion functionality provided
       by pt::peg::from::json.

       IMAGE: arch_core_iplugins

       While the direct use of this package with a regular interpreter is
       possible, this is strongly disrecommended and requires a number of
       contortions 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::import and the import manager objects it
              provides.

       [2]    In a trusted environment however simply use the package
              pt::peg::from::json 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 Import API specification.

       import text
              This command takes the JSON markup encoding a parsing expression
              grammar and contained in text, and generates the canonical
              serialization of said grammar, as specified in section PEG
              serialization format.  The created value is then returned as the
              result of the command.

JSON GRAMMAR EXCHANGE FORMAT
       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
       languages.

       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
                            symbol.

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

                            is     The value is a JSON string holding the Tcl
                                   serialization of the parsing expression
                                   describing the symbols sentennial
                                   structure, as specified in the section PE
                                   serialization 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
                                          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 a JSON string holding the Tcl 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.

       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
       serialization format, except that the Tcl dictionaries and lists of
       that format are mapped to JSON objects and arrays. Only the parsing
       expressions themselves are not translated further, but kept as JSON
       strings containing 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 <- '(' Expression ')' / (Factor (MulOp Factor)*);
                  MulOp      <- '*' / '/';
                  Factor     <- Term (AddOp Term)*;
                  AddOp      <- '+'/'-';
                  Term       <- 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 {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}
              }


       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
       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 <- '(' Expression ')' / (Factor (MulOp Factor)*);
                  MulOp      <- '*' / '/';
                  Factor     <- Term (AddOp Term)*;
                  AddOp      <- '+'/'-';
                  Term       <- 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 {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}
              }


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 <- '(' Expression ')'
                              / Factor (MulOp Factor)*


       then its canonical serialization (except for whitespace) is

                  {/ {x {t (} {n Expression} {t )}} {x {n Factor} {* {x {n MulOp} {n Factor}}}}}


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 SF Trackers [http://sourceforge.net/tracker/?group_id=12883].
       Please also report any ideas for enhancements you may have for either
       package and/or documentation.

KEYWORDS
       EBNF, JSON, LL(k), PEG, TDPL, context-free languages, expression,
       grammar, import, matching, parser, parsing expression, parsing
       expression grammar, 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>




pt                                     1              pt::peg::import::json(n)
