delayed execution - Prolog systems with occurs check aware freeze

Question

David Tonhofer posed an interesting problem about occurs check. An occurs check aware coroutine can be realized as follows with a predicate with_occurs_check/2 which can be easily implemented as a refinement of with_occurs_check/1:

occurs_check_aware_freeze(X, G) :-
    current_prolog_flag(F), 
    freeze(X, with_occurs_check(F, G)). 

Namely by using an with_occurs_check/2 with a further parameter. But the main problem of a realiable with_occurs_check/[1,2] that also works with backtracking remains. Since woken up goals are allowed to leave choice points and we don't want leaking.

Would Prolog engines in some way solve the problem as David Tonhofer speculates? Are there any Prolog systems around that already implement an occurs check aware freeze?

question from:https://stackoverflow.com/questions/66050655/prolog-systems-with-occurs-check-aware-freeze

Answer 1

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