Default Rules: An Extension of Constructive Negation for Narrowing-based Languages

In this paper an extension of narrowing-based functional logic languages is proposed: Every partial deenition of a function can be completed with a default rule. In a concrete function call, the default rule is applicable when the normal ones determine that they cannot compute the value of the call. The use of the default rule, in the presence of a goal with variables, is constructive. The operational semantics provides constraints to the variables to make the default rule applicable. Narrowing semantics are modiied extending the technique of constructive negation 3, 4, 17].