Prolog is a language based upon programming in logic. It is possible because Alan Robinson's resolution theorem prover can be implemented efficiently. Prolog does not have a notion of a program as such. There is a database of facts and rules. Rules are Horn clauses with a head consisting of a single functor and a body which can be a disjunction of terms. A fact is a rule whose body is empty (or more properly the single term true.
A goal is a conjunction a1,a2, ..., an of terms. To satisfy a goal the prolog inference engine chooses the leftmost term in the goal (a1 in this case) and tries to match it with a clause in the database. If such a clause (eg).
is found then a1 in the goal is replaced by the body of the clause so that the goal becomes.
Note that if the clause is a fact then the matched head is discarded.
The goal is satisfiable if this process continues until the goal is empty. If a particular choice of matching clause fails the system backtracks and uses the next clause in the database that matches. Various techniques of prolog programming are described in this collection. Amongst these are
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