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
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?