Am Mittwoch, 1. August 2007 14:41 schrieb apfelmus:
> […]
> The problem with the Functor/Cofunctor instances is that they are
> ambiguous as soon as a type constructor X is made an instance of both
> Functor and Cofunctor . Of course, such an X cannot exist in a
> mathematically useful way (really ?)
I think, it can:
newtype Const a b = MkConst a
instance Functor (Const a) where
fmap fun (MkConst a) = MkConst a
instance Cofunctor (Const a) where
cofmap fun (MkConst a) = MkConst a
> […]
Best wishes,
Wolfgang
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