Herbrand Constraint Solving in HAL

HAL is a new constraint logic programming language specifically designed to support construction of and experimentation with constraint solvers. One of the most important constraint solvers in any logic programming language is the Herbrand (or term) constraint solver. HAL programs are compiled to Mercury but, while Mercury provides only assignment, tests for equality and construction and deconstruction of ground terms, HAL supports full unification (without the occurs check). In this paper we describe the HAL Herbrand constraint solver and show how by using PARMA bindings, rather than the standard term representation used in the WAM, we can implement a solver which is compatible with Mercury's term representation. Like Mercury, HAL supports type, mode and determinism declarations. HAL uses information from these declarations to reduce the overhead of Herbrand constraint solving wherever possible. In this paper we also systematically evaluate the effect of each kind of declaration on the efficiency of HAL programs