pt::peg::export::json(n) Parser Tools pt::peg::export::json(n)
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NAME
pt::peg::export::json - PEG Export Plugin. Write JSON format
SYNOPSIS
package require Tcl 8.5
package require pt::peg::export::json ?1?
package require pt::peg::to::json
export serial configuration
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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.
This package implements the parsing expression grammar export plugin for the generation of JSON markup.
It resides in the Export section of the Core Layer of Parser Tools and is intended to be used by pt::peg::export, the export manager, sit-
ting between it and the corresponding core conversion functionality provided by pt::peg::to::json.
IMAGE: arch_core_eplugins
While the direct use of this package with a regular interpreter is possible, this is strongly disrecommended and requires a number of con-
tortions 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::export and the export manager objects it provides.
[2] In a trusted environment however simply use the package pt::peg::to::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 Export API specification.
export serial configuration
This command takes the canonical serialization of a parsing expression grammar, as specified in section PEG serialization format,
and contained in serial, the configuration, a dictionary, and generates JSON markup encoding the grammar. The created string is
then returned as the result of the command.
CONFIGURATION
The JSON export plugin recognizes the following configuration variables and changes its behaviour as they specify.
boolean indented
If this flag is set the plugin will break the generated JSON code across lines and indent it according to its inner structure, with
each key of a dictionary on a separate line.
If this flag is not set (the default), the whole JSON object will be written on a single line, with minimum spacing between all ele-
ments.
boolean aligned
If this flag is set the generator ensures that the values for the keys in a dictionary are vertically aligned with each other, for a
nice table effect. To make this work this also implies that indented is set.
If this flag is not set (the default), the output is formatted as per the value of indented, without trying to align the values for
dictionary keys.
Note that this plugin ignores the standard configuration variables user, format, file, and name, and their values.
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 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, JSON, LL(k), PEG, TDPL, context-free languages, export, expression, grammar, 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::export::json(n)