Science Computer 255: Artificial Intelligence
Introduction to Prolog
Basic Prolog Syntax Programs are made of two kinds of sentences, including facts, simple predicates that are true about the world and the objects in it: male(fred). male(bob). male(frank). female(sue). female(mary). female(diana). parent(sue,fred). parent(sue,bob). Note: Predicates and named objects use lower case. Variables are upper case. The other kind of sentence is Prolog is a Horn sentence, a disjunction containing at most one positive literal: A B C D written in another order, this would be: B C D A which is equivalent to: (B C D) A which is equivalent to: B C D A In Prolog, you write this like A :- B, C, D. Note:
Not all logical sentences can be translated as Horn clauses
However, having this form lends itself very nicely to resolution... which is what PROLOG uses to prove things
Prolog Handout Page 1 of 3
Connective only if and or not is equal to is not equal to
First-Order Logic ~ = <>
Prolog :, ; not == \==
Examples: mother(X,Y) :- parent(X,Y), female(Y). sister(X,Y) :- parent(X,Z),parent(Y,Z),female(Y), X /== Y. sibling(X,Y) :- sister(X,Y);brother(X,Y). If you want a variable that you will not use elsewhere in the expression, an use underline. ismother(X) :- parent(X,_), female(X). Queries in Prolog The basic interaction with Prolog is to load in a file containing facts and Horn sentences and then to "query" the environment to obtain information:
Yes/no questions (ground queries) contain only ground terms, or named objects in the world,
and basically are questions asking: Is this sentence true of these objects? ?- sister(bob,sue)
Other questions (non-ground queries) contain variables and are asking: What bindings to
these variables will make these sentences true? ?- sister(diana,X) ?- sister(diana,X);brother(diana,X) ?- not(mother(diana,X)) Lists You can do much more than that with Prolog. In fact, Prolog's major power is based on...