pt::json_language(n)						   Parser Tools 					      pt::json_language(n)

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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 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  sen-
				   tennial 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  dis-
					  carded.

				   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 seri-
       alization format, except that the Tcl dictionaries and lists of that format are mapped to JSON objects and arrays. Only the parsing expres-
       sions 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 rel-
		     evant 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 serial-
			    ization 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 con-
	      straints 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 super-
		     fluous 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  exten-
			    sion 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  exten-
			    sion 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 cus-
			    tom 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 cus-
			    tom 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 exten-
			    sion 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 clo-
			    sure, 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 predi-
			    cate.

		     [6]    For a parsing expression e the result of [list ! e] is a parsing expression as well.  This is the not lookahead predi-
			    cate.

		     [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  satis-
	      fies 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 superflu-
		     ous 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, 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>

pt									 1						      pt::json_language(n)