Haskell Foldable Wats

[Manuel Gómez]

On Thu, Feb 18, 2016 at 9:58 AM, Andreas Abel <andreas.abel at ifi.lmu.de> wrote:
> I also do not see why (a,a) could not be a container of exactly two
> elements, as opposed to current GHC which insists it is a container of
> exactly one element.  (Case for the mental institution, I'd say.)

Because, given

    data T a = T a a
    data T' a b = T a b

these two types have essential structural differences at the type
level, despite being isomorphic:

    forall a. T a
    forall a. T' a a

Type classes exist at the type level.  These things are decidedly
different at the type level.  It’s to be expected that their behavior
under Functor differs.