A Declarative Alternative to "Assert" in Logic Programming

The problem with the standard means by which Prolog programs are extended — assert — is that the construct is not semantically well-behaved. A more elegant alternative (adopted, for example, in λProlog) is implication with its intuitionistic meaning, but the assumptions so added to a logic program are of limited applicability. We propose a new construct rule, which combines the declarative semantics of implication with some of the power of assert. Operationally, rule provides for the extension of the logic program with results that deductively follow from that program. rule, used in conjunction with higher-order programming techniques such as continuationpassing style, allows the natural and declarative formulation of a whole class of logic programs which previously required assert. Example applications include memoization, partial evaluation combined with reflection, resolution, ML type inference, and explanation-based learning.