Search mechanisms in Prolog
We can utilize this simple Prolog program to describe how Prolog searches:
president(X) :- first_name(X, georgedubya), second_name(X, bush).
prime_minister(X) :- first_name(X, maggie), second_name(X, thatcher).
prime_minister(X) :- first_name(X, tony), second_name(X, blair).
first_name(tony,tonyblair).
first_name(georgedubya,georgebush).
second_name(tonyblair, blair).
second_name(georgebush, bush).
If we loaded this into a Prolog implementation likeSicstus, and queried the database:
?- prime_minister(P).
then in the following mannerSicstus would search: it would run through it's database until it came across a Horn clause (or fact) for which the head was prime_minister and the arity of the predicate was 1. It would first search at the president clause and reject this because the name of the head does not match with the head in the query. Next it would find that the clause, through:
prime_minister(X) :- first_name(X, maggie), second_name(X, thatcher).
fits the bill. It would then lookin the body of the clause at the predicates and see if it could satisfy them. In this instance, it would try to find a match for first_nameX, maggie). However, it would fail, because no this type of information may be found in the database. That means that the total clause fails, and Sicstus would backtrack, for example, it would go back to looking for a clause with the same head as the query. Of course, it would next find this clause:
prime_minister(X) :- first_name(X, tony), second_name(X, blair).
Then it would look at the body again, and try to discover a match for first_name(X, tony). It would look through the database and discover X=tonyblaira good assignment, because the factfirst_name(tonyblair, tony) is got towards the end of the database. Similarly, having assigned X=tonyblair, then it would search for a match to: second_name(tonyblair, blair), and would succeed. So, the answer tonyblair would make the query succeed, and this would be reported back to us.
The essential thing to remember is that Prolog implementations search from the top to the bottom of the database, and in the body try each term of a clause in the order in which they appear. We say that Sicstus has verified the query prime_minister(P) by finding something which satisfied the declaration of what a prime minister is: Tony Blair. It is also good to remembering that Sicstus assumes negation as failure. This means that if it cannot verify a predicate, then the predicate is false. So the query is:
?- \+ president(tonyblair).
Returns an answer of 'true', because Sicstus cannot prove that Tony Blair is a president.