CODEWITHSUNDEEP
prolog
Viko Riféo
Viko Riféo

Reputation: 119

How do I make a rule fail without proving that something is false?

I am trying to make an invert/2. I want it to ignore the order of its arguments, so that you don't need to worry about which half of the inversion is the one you know, and so that you don't have to repeat yourself when stating inversions as facts. I currently have

invert(Left, Right) :-
    call_with_depth_limit(invert(Right, Left), 1, Result),
    Result \= depth_limit_exceeded.

as the first invert rule. This mostly works, but if invert(a, b)., then ?- invert(b, X). gives both X = a and false, which is not what I want at all.

Upvotes: 0

Views: 99

Answers (1)

lurker
lurker

Reputation: 58244

There are two things I'd consider. First, make sure that the predicate you create for establishing the symmetry of the relationship in your facts has a different name than your facts. Having the name be the same usually leads to issues such as infinite recursion. Secondly, if the facts are under your control to declare, be consistent about whether the facts explicitly declare the symmetrical case or not. If they do, then you don't need the extra predicate. If they don't, then you need an extra predicate.

For example, with symmetries explicitly declared:

opposite(a, b).
opposite(b, a).
opposite(c, d).
opposite(d, c).

You don't need an extra predicate to get the symmetrical solutions for opposite.

An example where the symmetries are not explicitly declared:

opposite(a, b).
opposite(c, d).

inverted(X, Y) :- opposite(X, Y).
inverted(X, Y) :- opposite(Y, X).

In this case, querying inverted/2 instead of opposite/2 will handle the symmetry in the relationships.

Upvotes: 1

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