Dahl V. "Two solutions for the negation problem." Proc. Logic Programming Workshop, Hungary, 1980.  Home/Search   Document Not in Database   Summary   Related Articles   Check

....to terminate immediately. We should note at this point that there are other approaches to the problem of handling negation. Loveland and Reed, for example [LR91] define a resolution method by which queries against programs with negation can be evaluated in a sound and complete manner. Dahl [Dah80] defines an approach which delays the evaluation of a negated goal until it becomes ground, and an approach which, within a negated goal s computation, blocks only the unification of variables which are free 17 outside the scope of the negation. The latter methods have been implemented in a ....

....solution behaviour which if has inherited from cut. We could evidently get closer to the liberal general semantics by allowing B to be computed with free variables, as long as those variables do not get bound in the course of the computation, as suggested by one of Dahl s negation strategies [Dah80] Since this would complicate the operational semantics and our analysis, we have decided to stick with the conservative semantics as given. 7 The Witness Properties In this section, we prove the witness properties of the conservative semantics. These properties are similar in intent to one ....

Ver'onica Dahl. Two solutions for the negation problem. In Workshop on Logic Programming, Debrecen, Hungary, July 1980.

....is preferable to the unsound negation of most Prologs for some applications, but Naish points out [Nai86] that even the slightly more liberal form of NAF used in IC Prolog gives the error indication frequently. More useful is the form of NAF in which the call to :G is delayed until G is ground [Dah80, Nai84]. However, it is theoretically interesting that this form of negation can be given a logical semantics based on a four valued logic. The four truth values are T (true) F (false) U (undefined) and N ( floundering on negation , needed to help characterize the other truth values) They are ....

....can be computed without floundering in the operational semantics described here, since negated atoms in clauses often correspond to tests done on ground terms. However, it would still be preferable to be able to characterize other, more complete negation strategies, such as delaying negation [Dah80, Nai84], and it may appear that we could do so with a simple modification of the semantics given here. Unfortunately, this appears not to be the case; only a simple variant, in which floundering closures are discarded and new closures allowed to succeed or fail, seems easily characterizable. For ....

Ver'onica Dahl. Two solutions for the negation problem. In Workshop on Logic Programming, Debrecen, Hungary, July 1980.

....in which negation is defined by proof failure (i.e. the failure to prove a given fact is taken as proof of its negative counterpart) This is the case for most practical data bases. As a discussion of this problem falls outside the scope of this paper, the interested reader is referred to [ 12]. 2.1.1 Contextual Typing The use of variable typing to constrain the parse and aid in disambiguation is not new. Many language processors such as LUNAR [34] CO OP [17] LADDER [16] PHLIQA1 [1] resort to some kind of typing to provide these capabilities. In our particular approach, the ....

Dahl V. "Two solutions for the negation problem." Proc. Logic Programming Workshop, Hungary, 1980.

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