Prolog

Prolog is a high-level programming language associated with logic programming and artificial intelligence. It is based on formal logic, particularly first-order predicate logic and Horn clauses. A Prolog program consists of a database of facts and rules that define relations, and a query asks whether a given goal can be inferred from those clauses. The language relies on unification, a pattern-matching process that binds variables, and on backtracking to explore alternative proofs, providing a declarative approach where the programmer specifies what is true rather than how to compute it.

Execution proceeds in a goal-driven manner: given a query, the system attempts to prove it by selecting a clause whose head matches the goal and recursively proving the body. This search is implemented by SLD resolution. Prolog supports recursion and lists, and provides control constructs such as the cut operator, negation as failure, and the use of conjunctions and disjunctions within rule bodies. Programs are typically concise and readable, and often rely on pure definite clauses though impure features and meta-programming extend the language’s expressiveness.

History and standards: Prolog was developed in the early 1970s by Alain Colmerauer and colleagues at the University of Marseille, with significant contributions from Robert Kowalski. It gained prominence through various implementations, including the Edinburgh tradition and Warren Abstract Machine (WAM) based systems. ISO Prolog standards (spanning several decades) sought portability across implementations and environments.

Impact and use: Prolog remains influential in symbolic artificial intelligence, natural language processing, and knowledge representation, as well as rapid prototyping of rule-based systems. Modern Prolog systems include SWI-Prolog, SICStus, Yap, and GNU Prolog, providing efficient backtracking engines, modules, definite clause grammars, and advanced language features.