Prolog

Prolog is a general purpose logic programming language associated with artificial intelligence and computational linguistics. Prolog has its roots in first-order logic, a formal logic, and unlike many other programming languages, Prolog is declarative: the program logic is expressed in terms of relations, represented as facts and rules. A computation is initiated by running a query over these relations. The language was first conceived by a group around Alain Colmerauer in Marseille, France, in the early 1970s and the first Prolog system was developed in 1972 by Colmerauer with Philippe Roussel. Prolog was one of the first logic programming languages, and remains among the most popular such languages today, with many free and commercial implementations available. While initially aimed at natural language processing, the language has since then stretched far into other areas like theorem proving, expert systems, games, automated answering systems, ontologies and sophisticated control systems. Modern Prolog environments support creating graphical user interfaces, as well as administrative and networked applications.

mother(trude, sally).

father(tom, sally).
father(tom, erica).
father(mike, tom).

sibling(X, Y) :- parent(Z, X), parent(Z, Y).

parent(X, Y) :- father(X, Y).
parent(X, Y) :- mother(X, Y).