Dan Doel wrote:
If you do want to generalize (.), you have to decide whether you
want to generalize it as composition of arrows, or as functor application.
The former isn't a special case of the latter (with the current Functor, at
least).
By annotating functors with the category they operate on, you can
reconcile both seemingly different generalizations
class Category (~>) => Functor (~>) f where
(.) :: (a ~> b) -> (f a -> f b)
-- functor application
instance Functor (->) [] where
(.) = map
-- arrow composition
instance Category (~>) => Functor (~>) (d ~>) where
(.) = (<<<)
Regards,
apfelmus
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