31.7 The DB-Spec

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 maked +. 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 the spec with the most + postitions in it breaking ties by choosing the leftmost one.

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

As can be seen, storing and fetching terms with variables on + positions are not vigorously supported operations.