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A Second Approach

We now try an approach which is less non-deterministic. We will start by looking at:

 
sentence(In,Out)
 [-5pt]

The idea is that sentence/2 takes in a list of words as input, finds a legal sentence and returns a result consisting of the input list minus all the words that formed the legal sentence.

We can define it:

 
sentence(S,S0):-

		       noun_phrase(S,S1),

		       verb_phrase(S1,S0).

Here is a rough semantics for sentence/2.

A sentence can be found at the front of a list of words if there is a noun phrase at the front of the list and a verb phrase immediately following.

This declarative reading should help to bridge the gap between what we want to be a sentence and the procedure for finding a sentence.

Here is the rest of the parser:

 
noun_phrase(NP,NP0):-

		       determiner(NP,NP1),

		       noun(NP1,NP0).

verb_phrase(VP,VP0):-

		       verb(VP,VP1),

		       noun_phrase(VP1,VP0).

determiner([aRest],Rest).

determiner([theRest],Rest).

noun([manRest],Rest).

noun([cakeRest],Rest).

verb([ateRest],Rest).

As you can see, there is a remarkable sameness about each rule which, once you see what is going on, is fairly tedious to type in every time. So we turn to a facility that is built in to Prolog.

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