Proposal: Add a method disjoint :: IntSet → IntSet → Bool, to Data.IntSet, such that prop> disjoint a b ≡ null (intersection a b) Alternatively or additionally, add a method overlaps :: IntSet -> IntSet -> Bool such that prop> a `overlaps` b ≡ not.null$ intersection a b Rationale: - I have found it useful to have such a method when keeping track of the set of free-variables in a term. I believe that there can be more use cases. - There are already similar specialized implementations in the library. For example, isSubsetOf :: IntSet -> IntSet -> Bool is such that prop> a `isSubsetOf` b ≡ (a == (a `intersection` b)). - Having `disjoint`, the user can also check whether two sets overlap with `(not.).disjoint`. This is shorter and more self-explaining than (not.null$ intersection). - A specialized implementation of `disjoint` (vs. (null.).intersection) can shortcircuit as soon as the sets overlap on one element. This leads to significant speed-ups; for example, 23× when checking that the sets {1,2,3…,2¹²} and {2,4,6,…,2¹²} are not disjoint [1]. Discussion: I would like to get some comments on this proposal. In particular: - Should any of these methods be added? - Should both `overlaps` and `disjoint` be added? - If only one of them is added, which one should it be? I hope a decision about the proposal can be reached before 2018-01-09. See also: - Proposed patch [2] - Previous discussion [3] [1] https://github.com/haskell/containers/pull/377#issuecomment-352585171 [2] https://github.com/haskell/containers/pull/377/files [3] https://github.com/haskell/containers/pull/377