The predicates described in this section store arbitrary terms in a per-module repository known as the “blackboard”. The main purpose of the blackboard was initially to provide a means for communication between branches executing in parallel, but the blackboard works equally well during sequential execution. The blackboard implements a mapping from keys to values. Keys are restricted to being atoms or small integers, whereas values are arbitrary terms. In contrast to the predicates described in the previous sections, a given key can map to at most a single term.
Each Prolog module maintains its own blackboard, so as to avoid name clashes if different modules happen to use the same keys. The “key” arguments of these predicates are subject to module name expansion, so the module name does not have to be explicitly given unless multiple Prolog modules are supposed to share a single blackboard.
The predicates below implement atomic blackboard actions.
A copy of Term is stored under Key. See mpg-ref-bb_put.
If a term is currently stored under Key, then a copy of it is
unified with Term. Otherwise,
If a term is currently stored under Key, then the term is
deleted, and a copy of it is unified with Term. Otherwise,
bb_delete/2 silently fails.
bb_update(:Key, ?OldTerm, ?NewTerm)
If a term is currently stored under Key and unifies
with OldTerm, then the term is replaced by a copy of
bb_update/3 silently fails. This
predicate provides an atomic swap operation.
If the term being stored contains attributed variables
(see lib-atts) or suspended goals (see ref-sem-sec),
then those attributes are not stored. To retain the
attributes, you can use
copy_term/3 (see ref-lte-cpt).
The following example illustrates how these primitives may be used to
implement a “maxof” predicate that finds the maximum value
computed by some nondeterminate goal. We use a single key
We assume that Goal does not produce any “false” solutions that
would be eliminated by cuts in a sequential execution. Thus,
Goal may need to include redundant checks to ensure that its
solutions are valid, as discussed above.
maxof(Value, Goal, _) :- bb_put(max, -1), % initialize max-so-far call(Goal), update_max(Value), fail. maxof(_, _, Max) :- bb_delete(max, Max), Max > 1. update_max(New):- bb_get(max, Old), compare(C, Old, New), update_max(C, Old, New). update_max(<, Old, New) :- bb_update(max, Old, New). update_max(=, _, _). update_max(>, _, _).
This is not necessarily a good example of using
the blackboard. For instance, the implementation is not reentrant,
e.g. it will not work if the Goal itself uses
reentrant implementation would need to ensure that multiple nested
maxof/3 do not interfer with each other.