Concurrent logic programming

Concurrent logic programming is a variant of logic programming in which programs are sets of guarded Horn clauses of the form:

H :- G₁, …, G_n | B₁, …, B_n.

The conjunction G₁, … , G_n is called the guard of the clause, and | is the commitment operator.

Declaratively, guarded Horn clauses are read as ordinary logical implications:

H if G₁ and … and G_n and B₁ and … and B_n.

However, procedurally, when there are several clauses whose heads H match a given goal, then all of the clauses are executed in parallel, checking whether their guards G₁, … , G_n hold. If the guards of more than one clause hold, then a committed choice is made to one of the clauses, and execution proceeds with the subgoals B₁, …, B_n of the chosen clause. These subgoals can also be executed in parallel. Thus concurrent logic programming implements a form of "don't care nondeterminism", rather than "don't know nondeterminism".