I've addressed this here: http://bitemyapp.com/posts/2015-10-19-either-is-not-arbitrary.html The thousand-papercuts opposition to typeclass instances on the premise that a Functor for (a, b, c) maps over the final type not making sense is a rejection of how higher kinded types and typeclasses work together. This is natural and predictable if one bothers to explain it. On Mon, Jan 18, 2016 at 2:42 PM, Henning Thielemann < lemming@henning-thielemann.de> wrote:
On Mon, 18 Jan 2016, David Feuer wrote:
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.
indeed
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.
I would not qualify myself as a newcomer. For me these instances are a problem, not a help. _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
-- Chris Allen Currently working on http://haskellbook.com