LOGIC PROGRAMMING IN A FRAGMENT OF INTUITIONISTIC LINEAR LOGIC

When logic programming is based on the proof theory of intuitionistic logic, it is natural to allow implications in goals and in the bodies of clauses. Attempting to prove a goal of the form D superset-of G from the context (set of formulas) GAMMA leads to an attempt to prove the goal G in the extended context GAMMA or {D}. Thus contexts, represented as the left-hand side of intuitionistic sequents, grow as stacks during the bottom-up search for a cut-free proof. While such an intuitionistic notion of context provides for elegant specifications of many computations, contexts can be made more expressive and flexible if they are based on linear logic. After presenting two equivalent formulations of a fragment of linear logic, we show that the fragment has a goal-directed interpretation, thereby partially justifying calling it a logic program-ming language. Logic programs based on the intuitionistic theory of hereditary Harrop formulas can be modularly embedded into this linear logic setting. Programming examples taken from theorem proving, natural language parsing, and data base programming are presented: each example requires a linear, rather than intuitionistic, notion of context to be modeled adequately. An interpreter for this logic programming language must address the problem of splitting contexts; that is, in the attempt to prove a multiplicative conjunction (tensor), say G1 X G2, from the context DELTA, the latter must be split into disjoint contexts DELTA1 and DELTA2 for which G1 follows from DELTA1 and G2 follows from DELTA2. Since there is an exponential number of such splits, it is important to delay the choice of a split as much as possible. A mechanism for the lazy splitting of contexts is presented based on viewing proof search as a process that takes a context, consumes part of it, and returns the rest (to be consumed elsewhere). In addition, we use collections of Kripke interpretations indexed by a commutative monoid to provide models for this logic programming language and show that logic programs admit canonical models. (C) 1994 Academic Press, Inc.