CODEWITHSUNDEEP
haskellfunctortype-constructor
Chris
Chris

Reputation: 710

How to make a Type-constructor part of the Functor typeclass if it has more than one Type-parameter?

I am trying to understand the way the Functor-Typeclass works in Haskell. If you have a function f :: a -> b -> c and you want to partially apply it to the argB to get a function that takes one argument, you can simply do: 

f' :: a -> c
f' x = f x argB

and use that. Is it possible to get such behavior when making something part of the Functor-Typeclass with something like this:

instance Functor (MyTypeconstructor _ argB) where
   fmap <implementation of fmap>

I know you can partially apply a Type-constructor to its first type-parameter (standart currying):

instance Functor (MyTypeconstructor argA) where
   fmap <implementation of fmap>

but how can you partially apply it to its second / third / all except one type-parameter, if this is possible?

thank you.

Upvotes: 3

Views: 289

Answers (2)

Alexey Romanov
Alexey Romanov

Reputation: 170805

More generally, you can write 

newtype FlippedF a b = FlippedF { unFlip :: F b a }

and then 

instance Functor (FlippedF argB)

Or even more generally,

newtype Flip f a b = Flip { unFlip :: f b a }

instance Functor (Flip F argB)

Flip and many more type-level combinators are defined in e.g. http://hackage.haskell.org/package/TypeCompose-0.9.12/docs/Control-Compose.html#g:9 (and also http://hackage.haskell.org/package/bifunctors-5.4.1/docs/Data-Bifunctor-Flip.html#t:Flip)

Upvotes: 2

jakubdaniel
jakubdaniel

Reputation: 2223

Suppose you have data F a b = ..., define

newtype Fx a = Fx { unFx :: F a X }

to partially apply F to X in the second argument. Now you can use

instance Functor Fx where
    fmap f fa = ...

to define your instance for F _ X.

Upvotes: 5

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