Haskell Foldable Wats

Marcin Mrotek marcin.jan.mrotek at gmail.com
Thu Feb 18 14:37:32 UTC 2016

> 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 this isn't how types in Haskell works. `Foo a a` always parsed to
`(Foo a) a` and no instance of any class can implement it differently than
`(Foo b) a`, unless you want to throw parametricity out of the window. This
has been long before `Foldable`.

Best regards,
Marcin Mrotek
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