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10.26.7 DB-Spec—Details

A db-spec has the form of a SpecList:


= [spec1, …, specM]


= functor(argspec1, …, argspecN)


= + | -

where functor is a Prolog atom. The case N = 0 is allowed.

A spec F(argspec1, …, argspecN) is applicable to any nonvar term with principal functor F/N.

When storing a term T, we generate a hash code for every applicable spec in the db-spec, and a reference to T is stored with each of them. (More precisely, with each element of the set of generated hash codes). If T contains nonvar elements on each + position in the spec, then the hash code depends on each of these elements. If T does contain some variables on + position, then the hash code depends only on the functor of T.

When fetching a term Q, we look for an applicable spec for which there are no variables in Q on positions marked +. If no applicable spec can be found, a domain error is raised. If no spec can be found where on each + position a nonvar term occurs in Q, an instantiation error is raised. Otherwise, we choose the spec with the most + positions, breaking ties by choosing the leftmost one.

The terms that contain nonvar terms on every + position will be looked up using indexing based on the principal functor of the term and the principal functor of terms on + positions. The other (more general) terms will be looked up using an indexing based on the principal functor of the term only.

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