It looks like since this commit difference calculation is not working as expected.

Example:

S.difference (S.fromFoldable [1,2,3,4]) (S.fromFoldable [2,3]) == S.empty

Whereas the above expression should equal to S.fromFoldable [1,4]
In fact, I could not find any other inputs that would not yield the empty set for this operation.

Previous versions of that library give expected results.