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grammar::aycock(n) 1.0 tcllib "Aycock-Horspool-Earley parser generator for Tcl"


grammar::aycock - Aycock-Horspool-Earley parser generator for Tcl

Table Of Contents



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=


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.


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.


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:

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.


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.


Aycock, Earley, Horspool, parser, compiler


ambiguous, aycock, earley, grammar, horspool, parser, parsing, transducer


Grammars and finite automata