Early Projection in CLP(R)

During the evaluation of a constraint logic program, many local variables become inaccessible, or dead. In Prolog and other programming languages, the data bound to local variables can be removed automatically by garbage collection. The case of CLP is more complex, as the variables may be involved in several constraints. We can consider dead variables to be existentially quantiied. Removing an existential variable from a set of constraints is then a problem of quantiier elimination , or projection. Eliminating variables not only allows recovery of space but also can decrease the cost of further consistency tests. Surprisingly , the existing systems do not exploit these advantages. Instead, the primary use of projection is as a mechanism for obtaining answer constraints. In this paper, we will give a general system architecture for automatic early projection and specify the heuristics for CLP(R) together with an in-situ removal method. We then show the eeectiveness of early projection by applying it to some practical planning problems.