Constructive negation under the well-founded semantics

Abstract

Constructive negation derives constraint answers for non-ground negative literals. Its incorporation into query evaluation under the well-founded semantics introduces two problems. One is the detection of repeated subgoals and the elimination of redundant answers, which is required in order to guarantee termination. The other is the interaction between constraint answers of non-ground negative literals and recursion through negation. This paper presents SLG_CN for effective query evaluation with constructive negation under the well-founded semantics. It has two unique features. First, it supports reduction of constraint answers and redundant answer elimination and provides the first termination result for goal-oriented query evaluation with constructive negation for function-free programs. Second, it avoids repeated computation in a subgoal. Even if a non-ground negative literal depends upon some ground negative literals whose truth values are not completely determined when they are selected, the constraints and bindings for variables in the non-ground negative literal can still be propagated once and for all.