A.9 Defining Definite Clause Grammars

The defgrammar macro can be used to define a definite clause grammar (DCG), which is a relation that determines whether the start of a list of tokens (a sentence ) matches a particular grammar. The remaining tokens in the list become the sentence tail .

The relation has the form

where the <extra argument> items are terms defined below.

The syntax of the defgrammar macro is

(defgrammar <grammar name>

  <rule>*)

<lhs> ::= <grammar name>

        | (<grammar name> <term>*)

<rhs> ::= <atom>

        | <var>

        | (<other grammar name> <term>*)

        | <lisp clause>

        | (call <term>)

        | (cut)

<grammar name> is the same symbol as the one naming the defgrammar

<other grammar name> is a symbol naming another defgrammar

<atom> is an atom, which forms the words of the sentence to be matched

<var> is a variable reference

<term> is any Common Prolog logic expression, including a variable

<lisp function name> is a symbol naming a Lisp function

<lisp arg> is any Lisp form, which is evaluated and passed to the function

Within the <lhs>, extra arguments can be added by specifying <term>s. Every <rule> must specify the same <grammar name> as the defgrammar form and have the same number of extra arguments.

The meaning of the various <rhs> items is as follows:

• <atom> matches that atom in the sentence
• <var> is unified with the next item in the sentence
• (<other grammar name> <term>*) calls the grammar relation <other grammar name> on the rest of the sentence. The optional <term> arguments are passed to the relation as its extra arguments.
• <lisp clause> evaluates the <non-atomic lisp form> as a Lisp form and unifies the values that it returns with the <term>s that follow it.
• (call <term>) calls <term> as a normal Prolog relation.
• (cut) calls the normal Prolog cut relation.

A.9.1 Examples

KnowledgeWorks and Prolog User Guide (Macintosh version) - 24 Mar 2017