Actually, I currently get a much worse error message: Prelude> fmap (+1) (1,2,3) <interactive>:2:1: Non type-variable argument in the constraint: Functor ((,,) t t) (Use FlexibleContexts to permit this) When checking that ‘it’ has the inferred type it :: forall b t t1. (Functor ((,,) t t1), Num b, Num t, Num t1) => (t, t1, b) That there is a *lousy* error message. I understand that there are newcomers who struggle to understand the difference between tuples and lists; I don't think that's a good enough reason to omit a good and valid instance. On Mon, Jan 18, 2016 at 3:32 PM, Henning Thielemann <lemming@henning-thielemann.de> wrote:
On Mon, 18 Jan 2016, David Feuer wrote:
I'd be strongly +1 on that too! Traversable is strong magic.
I am opposed to all these Functor, Foldable, Traversable stuff on tuples (including pairs). Why should the last element of a tuple get a special treatment? I suspect that if you use such instances you are making something dirty. I am afraid that those instance may hide type errors or make type errors incomprehensible. E.g. if you get a stack of fmap's wrong, you do not get a "no instance for Functor ((,) a)" but instead the type mismatch occurs at a different level.
I would never use such an instance. Can I be warned if I accidentally use it anyway?