- H :- G1, …, Gn | B1, …, Bn.
The conjunction G1, … , Gn 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 Gn and B1 and … and Bn.
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, … , Gn 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, …, Bn 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".
The first concurrent logic programming language was the Relational Language of Keith L. Clark and Steve Gregory, which was an offshoot of IC-Prolog. Later versions of concurrent logic programming include Ehud Shapiro's Concurrent Prolog and Ueda's Guarded Horn Clause language.
The development of concurrent logic programming was given an impetus when Guarded Horn Clause was used to implement KL1, the systems programming language of the Japanese Fifth Generation Project (FGCS). The FGCS Project was a $400M initiative by Japan's Ministry of International Trade and Industry, begun in 1982, to use massively parallel computing/processing for artificial intelligence applications. The choice of concurrent logic programming as the “missing link” between the hardware and the applications was influenced by a visit to the FGCS Project in 1982 by Ehud Shapiro, who invented Concurrent Prolog.
- Clark, K. L. and Gregory, S. (1981). A relational language for parallel programming. In Proceedings of the 1981 conference on Functional programming languages and computer architecture (pp. 171-178). ACM.