haskellcategory-abstractions

Generalising ($) like Control.Category generalises (.)


I had a thought to generalise ($) like Control.Category generalises (.), and I've done so with the code at the end of this post (also ideone).

In this code I've created a class called FunctionObject. This class has a function ($) with the following signature:

($) :: f a b -> a -> b

Naturally I make (->) an instance of this class so $ continues to work with ordinary functions.

But this allows you to make special functions that, for example, know their own inverse, as the example below shows.

I've concluded there's one of three possibilities:

  1. I'm the first to think of it.
  2. Someone else has already done it and I'm reinventing the wheel.
  3. It's a bad idea.

Option 1 seems unlikely, and my searches on hayoo didn't reveal option 2, so I suspect option 3 is most likely, but if someone could explain why that is it would be good.

import Prelude hiding ((.), ($))
import Control.Category ((.), Category)

class FunctionObject f where
  ($) :: f a b -> a -> b

infixr 0 $

instance FunctionObject (->) where
  f $ x = f x

data InvertibleFunction a b = 
   InvertibleFunction (a -> b) (b -> a)

instance Category InvertibleFunction where
  (InvertibleFunction f f') . (InvertibleFunction g g') =
    InvertibleFunction (f . g) (g' . f')

instance FunctionObject InvertibleFunction where
  (InvertibleFunction f _) $ x = f $ x

inverse (InvertibleFunction f f') = InvertibleFunction f' f

add :: (Num n) => n -> InvertibleFunction n n
add n = InvertibleFunction (+n) (subtract n)

main = do
  print $ add 2 $ 5 -- 7
  print $ inverse (add 2) $ 5 -- 3

Solution

  • There are two abstractions used for things like this in Haskell, one usings Arrows and the other Applicatives. Both can be broken down into smaller parts than those used in base.


    If you go in the Arrow direction and break down the capabilities of Arrows into component pieces, you'd have a separate class for those arrows that are able to lift arbitrary functions into the arrow.

    class ArrowArr a where
        arr :: (b -> c) -> a b c
    

    This would be the opposite of ArrowArr, arrows where any arbitrary arrow can be dropped to a function.

    class ArrowFun a where
        ($) :: a b c -> (b -> c)
    

    If you just split arr off of Arrow you are left with arrow like categories that can construct and deconstruct tuples.

    class Category a => ArrowLike a where
        fst   :: a (b, d) b
        snd   :: a (d, b) b
        (&&&) :: a b c -> a b c' -> a b (c,c')
    

    If you go in the Applicative direction this is a Copointed "Applicative without pure" (which goes by the name Apply).

    class Copointed p where Source
        copoint :: p a -> a
    
    class Functor f => Apply f where
      (<.>) :: f (a -> b) -> f a -> f b
    

    When you go this way you typically drop the Category for functions and instead have a type constructor C a representing values (including function values) constructed according to a certain set of rules.