'\"
'\" Generated from file 'plugin\&.inc' by tcllib/doctools with format 'nroff'
'\" Copyright (c) 2009 Andreas Kupries <andreas_kupries@users\&.sourceforge\&.net>
'\"
.TH "pt::peg::export::json" n 1 tcllib "Parser Tools"
.\" The -*- nroff -*- definitions below are for supplemental macros used
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.el \{\
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.\}
.el \}\
\h'-1.5n'\L'|\\n(^yu-1v'\h'\\n(^lu+3n'\L'\\n(^tu+1v-\\n(^yu'\l'|0u-1.5n\(ul'
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See the \\*(So manual entry for details on the standard options.
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.BS
.SH NAME
pt::peg::export::json \- PEG Export Plugin\&. Write JSON format
.SH SYNOPSIS
package require \fBTcl 8\&.5 9\fR
.sp
package require \fBpt::peg::export::json ?1?\fR
.sp
package require \fBpt::peg::to::json\fR
.sp
\fBexport\fR \fIserial\fR \fIconfiguration\fR
.sp
.BE
.SH DESCRIPTION
.PP
Are you lost ?
Do you have trouble understanding this document ?
In that case please read the overview provided by the
\fIIntroduction to Parser Tools\fR\&. This document is the
entrypoint to the whole system the current package is a part of\&.
.PP
This package implements the parsing expression grammar export plugin
for the generation of JSON markup\&.
.PP
It resides in the Export section of the Core Layer of Parser Tools and
is intended to be used by \fBpt::peg::export\fR, the export
manager, sitting between it and the corresponding core conversion
functionality provided by \fBpt::peg::to::json\fR\&.
.PP
IMAGE: arch_core_eplugins
.PP
.PP
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:
.IP [1]
In an untrusted environment the proper access is through the package
\fBpt::peg::export\fR and the export manager objects it
provides\&.
.IP [2]
In a trusted environment however simply use the package
\fBpt::peg::to::json\fR and access the core
conversion functionality directly\&.
.PP
.SH API
The API provided by this package satisfies the specification of the
Plugin API found in the \fIParser Tools Export API\fR
specification\&.
.TP
\fBexport\fR \fIserial\fR \fIconfiguration\fR
This command takes the canonical serialization of a parsing expression
grammar, as specified in section \fBPEG serialization format\fR,
and contained in \fIserial\fR, the \fIconfiguration\fR, a dictionary,
and generates JSON markup encoding the grammar\&.
The created string is then returned as the result of the command\&.
.PP
.SH CONFIGURATION
The JSON export plugin recognizes the following configuration
variables and changes its behaviour as they specify\&.
.TP
boolean \fIindented\fR
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\&.
.sp
If this flag is not set (the default), the whole JSON object will be
written on a single line, with minimum spacing between all elements\&.
.TP
boolean \fIaligned\fR
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 \fBindented\fR
is set\&.
.sp
If this flag is not set (the default), the output is formatted as per
the value of \fBindented\fR, without trying to align the values for
dictionary keys\&.
.PP
\fINote\fR that this plugin ignores the standard configuration
variables \fBuser\fR, \fBformat\fR, \fBfile\fR, and \fBname\fR, and
their values\&.
.SH "JSON GRAMMAR EXCHANGE FORMAT"
The \fBjson\fR 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\&.
.PP
It is formally specified by the rules below:
.IP [1]
The JSON of any PEG is a JSON object\&.
.IP [2]
This object holds a single key, \fBpt::grammar::peg\fR, and its
value\&. This value holds the contents of the grammar\&.
.IP [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
.RS
.TP
\fBrules\fR
The value is a JSON object whose keys are the names of the nonterminal
symbols known to the grammar\&.
.RS
.IP [1]
Each nonterminal symbol may occur only once\&.
.IP [2]
The empty string is not a legal nonterminal symbol\&.
.IP [3]
The value for each symbol is a JSON object itself\&. The relevant
keys and their values in this dictionary are
.RS
.TP
\fBis\fR
The value is a JSON string holding the Tcl serialization of the
parsing expression describing the symbols sentennial structure, as
specified in the section \fBPE serialization format\fR\&.
.TP
\fBmode\fR
The value is a JSON holding holding one of three values specifying how
a parser should handle the semantic value produced by the symbol\&.
.RS
.TP
\fBvalue\fR
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\&.
.TP
\fBleaf\fR
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\&.
.TP
\fBvoid\fR
The nonterminal has no semantic value\&. Any ASTs generated by the
symbol's right hand side are discarded (as well)\&.
.RE
.RE
.RE
.TP
\fBstart\fR
The value is a JSON string holding the Tcl serialization of the start
parsing expression of the grammar, as specified in the section
\fBPE serialization format\fR\&.
.RE
.IP [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\&.
.PP
.PP
As an aside to the advanced reader, this is pretty much the same as
the Tcl serialization of PE grammars, as specified in section
\fBPEG serialization format\fR, 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\&.
.SS EXAMPLE
Assuming the following PEG for simple mathematical expressions
.PP
.CS


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;

.CE
.PP
a JSON serialization for it is
.PP
.CS


{
    "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"
    }
}

.CE
.PP
and a Tcl serialization of the same is
.PP
.CS


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}
}

.CE
.PP
The similarity of the latter to the JSON should be quite obvious\&.
.SH "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\&.
.PP
We distinguish between \fIregular\fR and \fIcanonical\fR
serializations\&.
While a PEG may have more than one regular serialization only exactly
one of them will be \fIcanonical\fR\&.
.TP
regular serialization
.RS
.IP [1]
The serialization of any PEG is a nested Tcl dictionary\&.
.IP [2]
This dictionary holds a single key, \fBpt::grammar::peg\fR, and its
value\&. This value holds the contents of the grammar\&.
.IP [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
.RS
.TP
\fBrules\fR
The value is a Tcl dictionary whose keys are the names of the
nonterminal symbols known to the grammar\&.
.RS
.IP [1]
Each nonterminal symbol may occur only once\&.
.IP [2]
The empty string is not a legal nonterminal symbol\&.
.IP [3]
The value for each symbol is a Tcl dictionary itself\&. The relevant
keys and their values in this dictionary are
.RS
.TP
\fBis\fR
The value is the serialization of the parsing expression describing
the symbols sentennial structure, as specified in the section
\fBPE serialization format\fR\&.
.TP
\fBmode\fR
The value can be one of three values specifying how a parser should
handle the semantic value produced by the symbol\&.
.RS
.TP
\fBvalue\fR
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\&.
.TP
\fBleaf\fR
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\&.
.TP
\fBvoid\fR
The nonterminal has no semantic value\&. Any ASTs generated by the
symbol's right hand side are discarded (as well)\&.
.RE
.RE
.RE
.TP
\fBstart\fR
The value is the serialization of the start parsing expression of the
grammar, as specified in the section \fBPE serialization format\fR\&.
.RE
.IP [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\&.
.RE
.TP
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\&.
.RS
.IP [1]
The keys found in all the nested Tcl dictionaries are sorted in
ascending dictionary order, as generated by Tcl's builtin command
\fBlsort -increasing -dict\fR\&.
.IP [2]
The string representation of the value is the canonical representation
of a Tcl dictionary\&. I\&.e\&. it does not contain superfluous whitespace\&.
.RE
.PP
.SS EXAMPLE
Assuming the following PEG for simple mathematical expressions
.PP
.CS


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;

.CE
.PP
then its canonical serialization (except for whitespace) is
.PP
.CS


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}
}

.CE
.PP
.SH "PE SERIALIZATION FORMAT"
Here we specify the format used by the Parser Tools to serialize
Parsing Expressions as immutable values for transport, comparison,
etc\&.
.PP
We distinguish between \fIregular\fR and \fIcanonical\fR
serializations\&.
While a parsing expression may have more than one regular
serialization only exactly one of them will be \fIcanonical\fR\&.
.TP
Regular serialization
.RS
.TP
\fBAtomic Parsing Expressions\fR
.RS
.IP [1]
The string \fBepsilon\fR is an atomic parsing expression\&. It matches
the empty string\&.
.IP [2]
The string \fBdot\fR is an atomic parsing expression\&. It matches
any character\&.
.IP [3]
The string \fBalnum\fR 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 \fBstring is\fR\&.
.IP [4]
The string \fBalpha\fR is an atomic parsing expression\&. It matches
any Unicode alphabet character\&. This is a custom extension of PEs
based on Tcl's builtin command \fBstring is\fR\&.
.IP [5]
The string \fBascii\fR 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 \fBstring is\fR\&.
.IP [6]
The string \fBcontrol\fR is an atomic parsing expression\&. It matches
any Unicode control character\&. This is a custom extension of PEs based
on Tcl's builtin command \fBstring is\fR\&.
.IP [7]
The string \fBdigit\fR 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 \fBstring is\fR\&.
.IP [8]
The string \fBgraph\fR 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 \fBstring is\fR\&.
.IP [9]
The string \fBlower\fR 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 \fBstring is\fR\&.
.IP [10]
The string \fBprint\fR 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 \fBstring is\fR\&.
.IP [11]
The string \fBpunct\fR is an atomic parsing expression\&. It matches
any Unicode punctuation character\&. This is a custom extension of PEs
based on Tcl's builtin command \fBstring is\fR\&.
.IP [12]
The string \fBspace\fR is an atomic parsing expression\&. It matches
any Unicode space character\&. This is a custom extension of PEs based
on Tcl's builtin command \fBstring is\fR\&.
.IP [13]
The string \fBupper\fR 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 \fBstring is\fR\&.
.IP [14]
The string \fBwordchar\fR 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 \fBstring is\fR\&.
.IP [15]
The string \fBxdigit\fR is an atomic parsing expression\&. It matches
any hexadecimal digit character\&. This is a custom extension of PEs
based on Tcl's builtin command \fBstring is\fR\&.
.IP [16]
The string \fBddigit\fR is an atomic parsing expression\&. It matches
any decimal digit character\&. This is a custom extension of PEs based
on Tcl's builtin command \fBregexp\fR\&.
.IP [17]
The expression
[list t \fBx\fR]
is an atomic parsing expression\&. It matches the terminal string \fBx\fR\&.
.IP [18]
The expression
[list n \fBA\fR]
is an atomic parsing expression\&. It matches the nonterminal \fBA\fR\&.
.RE
.TP
\fBCombined Parsing Expressions\fR
.RS
.IP [1]
For parsing expressions \fBe1\fR, \fBe2\fR, \&.\&.\&. the result of
[list / \fBe1\fR \fBe2\fR \&.\&.\&. ]
is a parsing expression as well\&.
This is the \fIordered choice\fR, aka \fIprioritized choice\fR\&.
.IP [2]
For parsing expressions \fBe1\fR, \fBe2\fR, \&.\&.\&. the result of
[list x \fBe1\fR \fBe2\fR \&.\&.\&. ]
is a parsing expression as well\&.
This is the \fIsequence\fR\&.
.IP [3]
For a parsing expression \fBe\fR the result of
[list * \fBe\fR]
is a parsing expression as well\&.
This is the \fIkleene closure\fR, describing zero or more
repetitions\&.
.IP [4]
For a parsing expression \fBe\fR the result of
[list + \fBe\fR]
is a parsing expression as well\&.
This is the \fIpositive kleene closure\fR, describing one or more
repetitions\&.
.IP [5]
For a parsing expression \fBe\fR the result of
[list & \fBe\fR]
is a parsing expression as well\&.
This is the \fIand lookahead predicate\fR\&.
.IP [6]
For a parsing expression \fBe\fR the result of
[list ! \fBe\fR]
is a parsing expression as well\&.
This is the \fInot lookahead predicate\fR\&.
.IP [7]
For a parsing expression \fBe\fR the result of
[list ? \fBe\fR]
is a parsing expression as well\&.
This is the \fIoptional input\fR\&.
.RE
.RE
.TP
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\&.
.RS
.IP [1]
The string representation of the value is the canonical representation
of a pure Tcl list\&. I\&.e\&. it does not contain superfluous whitespace\&.
.IP [2]
Terminals are \fInot\fR encoded as ranges (where start and end of the
range are identical)\&.
.RE
.PP
.PP
.SS EXAMPLE
Assuming the parsing expression shown on the right-hand side of the
rule
.PP
.CS


    Expression <- Term (AddOp Term)*

.CE
.PP
then its canonical serialization (except for whitespace) is
.PP
.CS


    {x {n Term} {* {x {n AddOp} {n Term}}}}

.CE
.PP
.SH "BUGS, IDEAS, FEEDBACK"
This document, and the package it describes, will undoubtedly contain
bugs and other problems\&.
Please report such in the category \fIpt\fR of the
\fITcllib Trackers\fR [http://core\&.tcl\&.tk/tcllib/reportlist]\&.
Please also report any ideas for enhancements you may have for either
package and/or documentation\&.
.PP
When proposing code changes, please provide \fIunified diffs\fR,
i\&.e the output of \fBdiff -u\fR\&.
.PP
Note further that \fIattachments\fR are strongly preferred over
inlined patches\&. Attachments can be made by going to the \fBEdit\fR
form of the ticket immediately after its creation, and then using the
left-most button in the secondary navigation bar\&.
.SH 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
.SH CATEGORY
Parsing and Grammars
.SH COPYRIGHT
.nf
Copyright (c) 2009 Andreas Kupries <andreas_kupries@users\&.sourceforge\&.net>

.fi
