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|Tabling for impure programs|
Tabling guarantees logically correct results and termination provided the computation only involves terms of bounded size on pure Prolog programs, i.e., Prolog programs without side effects or pruning of choice points (cut, ->/2, etc.). Notably pruning choice points of an incomplete tabled goal may cause an incomplete table and thus cause subsequent queries for the same goal to return an incomplete set of answers. The current SWI-Prolog implementation provides several mechanisms to improve on this situation.
Minimal SCCs imply that goals are completed as early as possible.
This implies that tabled goals may be embedded in e.g., findall/3
or be used as a condition as long as there is no dependency (loop)
with goals outside the findall/3
or condition. For example, the code below misbehaves when called as
because the argument of findall/3
calls a variant of the goal that initiated the findall goal. A
p(1) however is ok as
p(1) is not a
p(X) :- findall(Y, p(Y), Ys), ...