Re: Suggestion regarding (.) and map

25 Apr 2008


      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