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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