next up previous contents
Next: To Use the Up: Parsing in Prolog Previous: A Second Approach

Prolog Grammar Rules

Prolog, as a convenience, will do most of the tedious work for you. What follows, is the way you can take advantage of Prolog.

This is how we can define the simple grammar which is accepted `as is' by Prolog.

sentence   		  --> 		  noun_phrase, verb_phrase.

noun_phrase --> determiner, noun.

verb_phrase --> verb, noun_phrase.

determiner --> [a].

determiner --> [the].

noun --> [man].

noun --> [cake].

verb --> [ate].

It is very easy to extend if we want to include adjectives.
noun_phrase 		 --> 		  determiner, adjectives, noun.

adjectives --> adjective.

adjectives --> adjective, adjectives.

adjective --> [young].

This formulation is sometimes known as a Definite Clause Grammar (DCG).

We might later think about the ordering of these rules and whether they really capture the way we use adjectives in general conversation but not now.

Essentially, the Prolog Grammar Rule formulation is syntactic sugaring. This means that Prolog enables you to write in:

sentence 		    --> 		  noun_phrase, verb_phrase.

and Prolog turns this into:


verb_phrase(S1,S0). [-5pt]

adjective 		   --> 		  [young].


'C'(A,young,A0). [-5pt]

where 'C'/3 is a built in Prolog Predicate which is defined as if:

Paul Brna
Mon May 24 20:14:48 BST 1999