Prolog implying a negative predicate

Question

How can I write the following rule in PROLOG: if P then not Q

I understand that you can easily write if P then Q the predicates like q(X) :- p(X)

Answer 1

"...if P then not Q" can be represented via the -> if-then control-flow predicate (e.g., GNU) , along with the \+ negation (or 'not-provable') operator (e.g., GNU), as follows:

(P -> \+ Q),

Note that, typically, \+ will implement what is known as negation-as-failure; i.e., the subgoal/expression \+ Q will succeed iff Q cannot. Note that the evaluation of Q under \+ will not affect the bindings of any variables present in the expression Q at execution.

For example, consider:

foo(a).
bar(b).

Given these facts, the following hold:

foo(a) -> \+ bar(a). % succeeds, as bar(a) is not provable.
foo(a) -> \+ bar(b). % fails, as bar(b) is provable.
foo(a) -> \+ bar(X). % fails, as bar(X) is provable; note that X remains unbound.
foo(X) -> \+ bar(X). % succeeds, as bar(X) where X unified to 'a' isn't provable.

Implementing something akin to \+ q(X) :- p(X) as you might want (in terms of a 'rule') isn't straightforward, as you describe, however a potential hack is:

q(X) :- p(X), !, fail.

This definition will only reflect the intention that q(X) is to fail for all X where p(X) succeeds iff it is asserted before any other clauses of q(X), but may not be ideal.