24 Feb
2016
24 Feb
'16
11:22 a.m.
Edward Kmett
You can wish for such an instance all you want, but it can't exist.
The definition of Functor is forced on you by the a combination of the laws, and the way the type system works. You can't have this "other" instance even if you want it, even if the instance we have today wasn't there.
(,) e :: * -> *
That is a legal choice of 'f' for
class Functor f where fmap :: (a -> b) -> f a -> f b
I see where I made a mistake, yes. Are you also saying that this cannot be resolved by some kind of a type families-based type-level 'flip'? -- с уважениeм / respectfully, Косырев Сергей