Node:The DB-Spec, Previous:An Example Session, Up:BDB
A db-spec has the form of a speclist:
[spec1, ..., specM]
(argspec1, ..., argspecN)
+ | -
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.