BProlog

B-Prolog is a high-performance implementation of the standard Prolog language with several extended features including matching clauses, action rules for event handling, finite-domain constraint solving, arrays and hash tables, declarative loops, and tabling. First released in 1994, B-Prolog is now a widely used CLP system. The constraint solver of B-Prolog was ranked top in two categories in the Second International Solvers Competition, and it also took the second place in P class in the second ASP solver competition and the second place overall in the third ASP solver competition. B-Prolog underpins the PRISM system, a logic-based probabilistic reasoning and learning system. B-Prolog is a commercial product, but it can be used for learning and non-profit research purposes free of charge (since version 7.8 for individual users, including commercial individual users, B-Prolog is free of charge ).

A matching clause is a form of a clause where the determinacy and input/output unifications are denoted explicitly. The compiler translates matching clauses into matching trees and generates indexes for all input arguments. The compilation of matching clauses is much simpler than that of normal Prolog clauses because no complex program analysis or specialization is necessary; and the generated code tends to be more compact and faster. The B-Prolog compiler and most of the library predicates are written in matching clauses.

A matching clause takes the following form:

where H is an atomic formula, G and B are two sequences of atomic formulas. H is called the head, G the guard, and B the body of the clause. No call in G can bind variables in H and all calls in G must be in-line tests. In other words, the guard must be flat. The following gives an example predicate in matching clauses that merges two sorted lists:

The cons [Y|Ys] occurs in both the head and the body of the third clause. To avoid reconstructing the term, we can rewrite the clause into the following:

The call Ys=[Y|_] in the guard matches Ys against the pattern [Y|_].

The lack of a facility for programming "active" sub-goals that can be reactive to the environment has been considered one of the weaknesses of logic programming. To overcome this, B-Prolog provides a simple and yet powerful language, called Action Rules (AR), for programming agents. An agent is a subgoal that can be delayed and can later be activated by events. Each time an agent is activated, some action may be executed. Agents are a more general notion than delay constructs in early Prolog systems and processes in concurrent logic programming languages in the sense that agents can be responsive to various kinds of events including instantiation, domain, time, and user-defined events.

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