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EuroTcl/OpenACS 11 - 12 JULY 2024, VIENNA


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NAME

pt::json_language - The JSON Grammar Exchange Format

Table Of Contents

SYNOPSIS

package require Tcl 8.5 9

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 sentennial
          structure, as specified in the section [PE serialization
          format](#section3)\.
        
        * __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 <- 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 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.

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.

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. 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 © 2009 Andreas Kupries