CODEWITHSUNDEEP
haskellfunctorcategory-theory
nicolas
nicolas

Reputation: 9825

removing ad-hoc polymorphism

What is the best way to remove ad-hoc polymorshism in haskell ?

80% of the time, I don't need fmap to be polymorphic in Functor f, I actually know which instance I apply it to. Replacing it with a specific instance gives me :

What is the best way to apply, like in category theory, a functor F to a morphism in haskell using its name ?

-- F is a functor : it maps objects of * to objects of *
data F r = Z | Suc r

-- F is a functor : it maps arrows of *  to arrows of *
-- generic fmap will be found for this type, I inherit much code for free, great
instance Functor F where
  fmap f Z       = Z
  fmap f (Suc n) = Suc (f n)

-- But I care writing code specific for this functor only
-- Applies F for arrows of *
fmapF = fmap :: (a -> b) -> (F a -> F b)

-- an arrow in *, aka a function -- (and also a value as * is CCC)
f = id :: a -> a

-- works for values F a, not any functor f
r = fmapF f Z :: F a
r' = fmap f "hi" -- as opposed to

Upvotes: 1

Views: 109

Answers (1)

chi
chi

Reputation: 116174

I think you actually want

{-# LANGUAGE TypeApplications #-}
r = fmap @ F f Z

The @ F part specifies that we want the fmap of functor F.

Upvotes: 7

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