Rules
Deriving new knowledge with :- (if), commas (and) and semicolons (or).
Facts are things we state. Rules let Prolog derive new facts. A rule has a head and a body, separated by :- which you read as "if":
head :- condition1, condition2, ... .The comma means and. Here is a family database, and a rule defining grandparent/2:
parent(tom, bob).
parent(tom, liz).
parent(bob, ann).
parent(bob, pat).
parent(pat, jim).
male(tom). male(bob). male(jim).
female(liz). female(ann). female(pat).
% G is a grandparent of C if G is a parent of some P, and P is a parent of C.
grandparent(G, C) :- parent(G, P), parent(P, C).?- grandparent(tom, Who)?- grandparent(Who, jim)Variables in a rule are local to that rule. P above links the two conditions: whatever person makes parent(G, P) true must also be the one in parent(P, C). Prolog searches for a value of P that satisfies both.
More rules, and a new operator
A \= B means "A and B cannot be made equal", handy to say "two different people".
sibling(A, B) :- parent(P, A), parent(P, B), A \= B.
father(F, C) :- parent(F, C), male(F).
mother(M, C) :- parent(M, C), female(M).?- sibling(ann, Who)?- father(Who, ann)?- mother(Who, Kid)That last query asked for every mother–child pair. liz has no children in this data, so only pat (mother of jim) qualifies.
"Or": several clauses, or a semicolon
Writing several rules with the same head gives you or: Prolog tries each in turn.
person(X) :- male(X).
person(X) :- female(X).
% a semicolon inside one body also means "or"
related(X, Y) :- parent(X, Y) ; parent(Y, X).?- person(X)?- related(bob, Who)Exercise: Define an aunt
The family facts and sibling/2 are already loaded for you. Write a rule aunt(A, N): A is an aunt of N if A is female and a sibling of one of N's parents.