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
}
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}
}
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.
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>