NAME¶
grammar::aycock - Aycock-Horspool-Earley parser generator for Tcl
SYNOPSIS¶
package require
Tcl 8.5
package require
grammar::aycock ?1.0?
::aycock::parser grammar ?
-verbose?
parserName parse symList valList ?
clientData?
parserName destroy
parserName terminals
parserName nonterminals
parserName save
DESCRIPTION¶
The
grammar::aycock package implements a parser generator for the class
of parsers described in John Aycock and R. Nigel Horspool. Practical Earley
Parsing.
The Computer Journal, 45(6):620-630, 2002.
http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.12.4254
PROCEDURES¶
The
grammar::aycock package exports the single procedure:
- ::aycock::parser grammar ?-verbose?
- Generates a parser for the given grammar, and returns its name. If
the optional -verbose flag is given, dumps verbose information
relating to the generated parser to the standard output. The returned
parser is an object that accepts commands as shown in OBJECT
COMMAND below.
OBJECT COMMAND¶
- parserName parse symList valList
?clientData?
- Invokes a parser returned from ::aycock::parser. symList is
a list of grammar symbols representing the terminals in an input string,
and valList is a list of their semantic values. The result is the
semantic value of the entire string when parsed.
- parserName destroy
- Destroys a parser constructed by ::aycock::parser.
- parserName terminals
- Returns a list of terminal symbols that may be presented in the
symList argument to the parse object command.
- parserName nonterminals
- Returns a list of nonterminal symbols that were defined in the parser's
grammar.
- parserName save
- Returns a Tcl script that will reconstruct the parser without needing all
the mechanism of the parser generator at run time. The reconstructed
parser depends on a set of commands in the package
grammar::aycock::runtime, which is also automatically loaded when
the grammar::aycock package is loaded.
DESCRIPTION¶
The
grammar::aycock::parser command accepts a grammar expressed as a Tcl
list. The list must be structured as the concatenation of a set of
rules. Each
rule comprises a variable number of elements in the
list:
- •
- The name of the nonterminal symbol that the rule reduces.
- •
- The literal string, ::=
- •
- Zero or more names of terminal or nonterminal symbols that comprise the
right-hand-side of the rule.
- •
- Finally, a Tcl script to execute when the rule is reduced. Within the
given script, a variable called _ contains a list of the semantic
values of the symbols on the right-hand side. The value returned by the
script is expected to be the semantic value of the left-hand side. If the
clientData parameter was passed to the parse method, it is
available in a variable called clientData. It is permissible for
the script to be the empty string. In this case, the semantic value of the
rule will be the same as the semantic value of the first symbol on the
right-hand side. If the right-hand side is also empty, the semantic value
will be the empty string.
Parsing is done with an Earley parser, which is not terribly efficient in speed
or memory consumption, but which deals effectively with ambiguous grammars.
For this reason, the
grammar::aycock package is perhaps best adapted to
natural-language processing or the parsing of extraordinarily complex
languages in which ambiguity can be tolerated.
EXAMPLE¶
The following code demonstrates a trivial desk calculator, admitting only
+,
* and parentheses as its operators. It also shows the format
in which the lexical analyzer is expected to present terminal symbols to the
parser.
set p [aycock::parser {
start ::= E {}
E ::= E + T {expr {[lindex $_ 0] + [lindex $_ 2]}}
E ::= T {}
T ::= T * F {expr {[lindex $_ 0] * [lindex $_ 2]}}
T ::= F {}
F ::= NUMBER {}
F ::= ( E ) {lindex $_ 1}
}]
puts [$p parse {( NUMBER + NUMBER ) * ( NUMBER + NUMBER ) } {{} 2 {} 3 {} {} {} 7 {} 1 {}}]
$p destroy
The example, when run, prints
40.
KEYWORDS¶
Aycock, Earley, Horspool, parser, compiler
KEYWORDS¶
ambiguous, aycock, earley, grammar, horspool, parser, parsing, transducer
CATEGORY¶
Grammars and finite automata
COPYRIGHT¶
Copyright (c) 2006 by Kevin B. Kenny <kennykb@acm.org>
Redistribution permitted under the terms of the Open Publication License <http://www.opencontent.org/openpub/>