Provided by: tcllib_1.21+dfsg-1_all 
NAME
pt::peg::export::container - PEG Export Plugin. Write CONTAINER format
SYNOPSIS
package require Tcl 8.5
package require pt::peg::export::container ?1?
package require pt::peg::to::container
export serial configuration
________________________________________________________________________________________________________________
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 CONTAINER
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, sitting between it and the corresponding core conversion
functionality provided by pt::peg::to::container.
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 contortions 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::container 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 CONTAINER markup encoding the grammar. The created string is then returned as the
result of the command.
CONFIGURATION
The CONTAINER export plugin recognizes the following configuration variables and changes its behaviour as
they specify.
enum mode
The value of this configuration variable controls which methods of pt::peg instances the plugin
will use to specify the grammar. There are two legal values
bulk In this mode the methods start, add, modes, and rules are used to specify the grammar in a
bulk manner, i.e. as a set of nonterminal symbols, and two dictionaries mapping from the
symbols to their semantic modes and parsing expressions.
This mode is the default.
incremental
In this mode the methods start, add, mode, and rule are used to specify the grammar
piecemal, with each nonterminal having its own block of defining commands.
string template
If this configuration variable is set it is assumed to contain a string into which to put the
generated code and other configuration data. The various locations are expected to be specified
with the following placeholders:
@user@ To be replaced with the value of the configuration variable user.
@format@
To be replaced with the the constant CONTAINER.
@file@ To be replaced with the value of the configuration variable file.
@name@ To be replaced with the value of the configuration variable name.
@mode@ To be replaced with the value of the configuration variable mode.
@code@ To be replaced with the generated code.
If this configuration variable is not set, or empty, then the plugin falls back to a standard template,
which is defined as "@code@".
Note that this plugin may ignore the standard configuration variables user, format, file, and their
values, depending on the chosen template.
The content of the standard configuration variable name, if set, is used as name of the grammar in the
output. Otherwise the plugin falls back to the default name a_pe_grammar.
GRAMMAR CONTAINER
The container format is another form of describing parsing expression grammars. While data in this format
is executable it does not constitute a parser for the grammar. It always has to be used in conjunction
with the package pt::peg::interp, a grammar interpreter.
The format represents grammars by a snit::type, i.e. class, whose instances are API-compatible to the
instances of the pt::peg::container package, and which are preloaded with the grammar in question.
It has no direct formal specification beyond what was said above.
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;
one possible CONTAINER serialization for it is
snit::type a_pe_grammar {
constructor {} {
install myg using pt::peg::container ${selfns}::G
$myg start {n Expression}
$myg add AddOp Digit Expression Factor MulOp Number Sign Term
$myg modes {
AddOp value
Digit value
Expression value
Factor value
MulOp value
Number value
Sign value
Term value
}
$myg rules {
AddOp {/ {t -} {t +}}
Digit {/ {t 0} {t 1} {t 2} {t 3} {t 4} {t 5} {t 6} {t 7} {t 8} {t 9}}
Expression {/ {x {t \50} {n Expression} {t \51}} {x {n Factor} {* {x {n MulOp} {n Factor}}}}}
Factor {x {n Term} {* {x {n AddOp} {n Term}}}}
MulOp {/ {t *} {t /}}
Number {x {? {n Sign}} {+ {n Digit}}}
Sign {/ {t -} {t +}}
Term {n Number}
}
return
}
component myg
delegate method * to myg
}
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 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 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 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.
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.
[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 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 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 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 expression. 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 character. 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, 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 custom 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 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 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 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 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.
[1] The string representation of the value is the canonical representation of a pure Tcl list.
I.e. it does not contain 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 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
CONTAINER, EBNF, 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>
tcllib 1 pt::peg::export::container(3tcl)