There is a lot of hype around the dif/2 constraint, especially as a rescue for some non-declarativity of (\=)/2 and (\==)/2. This non-declarativity is often characterized as non-monotonicity and examples of non-communtativity are given.
But what would be the means to test whether test cases involving dif/2 are commutative. Here is a meta explanation what I want to do:
I make a commutativity test, and I want to probe that both variants give the same result:
?- A, B. -- versus -- ?- B, A.
So usually you can check monotonicity, if it boils down to checking commutativity, with the (==)/2 built-in predicate. Since this predicate follows instantiated variables.
But if you are testing cases that produce constraints, call_with_residue/2 is not enough, you need also to have equality of constraints. Which can be tricky, as the following example shows:
Welcome to SWI-Prolog (Multi-threaded, 64 bits, Version 7.3.23)
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?- dif(f(X,X),f(a(U,g(T)),a(g(Z),U))), X=a(g(Z),U).
X = a(g(Z), U),
dif(f(a(g(Z), U), U, Z, U, T), f(a(U, g(T)), g(Z), T, g(Z), Z)).
?- X=a(g(Z),U), dif(f(X,X),f(a(U,g(T)),a(g(Z),U))).
X = a(g(Z), U),
dif(f(U, T), f(g(Z), Z)).
Any ideas how to proceed?
Disclaimer, its a trap:
I don't endorse commutativity testing as a good testing method, where you can separate good and bad predicates versus a specification. Since usually both the good and bad predicates might have no problems with commutativity.
I am using commutativity testing as a vehicle to find out about methods for equality of dif/2 constraints. This equality can then be used in more traditional test cases as a validation point.