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)