Search mechanisms in Prolog:
Here we can needs this simple Prolog program to describe how Prolog searches as: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(tonyblair, tony).
first_name(georgebush, georgedubya).
second_name(tonyblair, blair).
second_name(georgebush, bush).
If there we loaded this in a Prolog implementation into as Sicstus, or queried the database as:
?- prime_minister(P).
In fact then Sicstus would search in the following manner as: it would run through it is database when pending for it came across a Horn clause or fact for that the head was prime_minister and the arity of the predicate was 1. Than there It would first look at the president clause, so after that reject this to see the name of the head doesn't match with the head in the query. moreover, next it would find the clause as:
prime_minister(X) :- first_name(X, maggie), second_name(X, thatcher).
There fits the bill. After than it would look at the predicates in the body of the clause or see if it could satisfy them. In fact in this case, we see that it would try to find a match for first_name(X, maggie). But, it would fail, it means that there no such information can be found in the database. Because it means that the entire clause fails Sicstus would backtrack, thai is., so it would go back just to looking for a clause with the same head as the query. But it would next find this clause such as:
prime_minister(X) :- first_name(X, tony), second_name(X, blair).
So there then it would look at the body again, than try to find a match for first_name(X, tony). According to that it would look through the datatabase and find X=tonyblair a good assignment hence it means the fact first_name(tonyblair, tony) is found towards the end of the database. as well having assigned X=tonyblair, so it would then look for a match to: second_name(tonyblair, blair), and would succeed. Likewise, the answer tonyblair would make the query succeed, for this would be reported back to us.