[Haskell-cafe] Closed classes

[David Feuer]

Yes, closed type families *can* be used to achieve the same result even
now, but it tends to be fairly verbose. There are a number of minor
variations in how it can be done, but here's one example:

{-# LANGUAGE TypeFamilies, DataKinds, MultiParamTypeClasses,
      FlexibleInstances, FlexibleInstances, ScopedTypeVariables,
      UndecidableInstances #-}
import Data.Proxy

class Closed a b where
  fun :: a -> b -> Int

-- Names for the instances
data InstanceChoice = ChooseFirst | ChooseSecond | IgnoreBoth

-- Determine which instance should be used
type family Choose a b where
  Choose Int y = 'ChooseFirst
  Choose x Int = 'ChooseSecond
  Choose x y = 'IgnoreBoth

-- Auxiliary class with instance-choice parameter
class Closed' (choice :: InstanceChoice) a b where
  fun' :: proxy choice -> a -> b -> Int

-- The actual instances
instance Closed' 'ChooseFirst Int y where fun' _ x _ = x
instance Closed' 'ChooseSecond x Int where fun' _ _ y = y
instance Closed' 'IgnoreBoth x y where fun' _ _ _ = 0

-- Note that ScopedTypeVariables is necessary to make this typecheck.
instance (choice ~ Choose a b, Closed' choice a b) => Closed a b where
  fun = fun' (Proxy :: Proxy choice)

Then

> fun (3 :: Int) 'a'
3
> fun 'a' (4 :: Int)
4
> fun 'a' 'b'
0



On Wed, Feb 17, 2016 at 7:21 PM, Imants Cekusins <imantc at gmail.com> wrote:

> This would be very useful indeed.
>
> Can closed type families not be used to achieve the same result even now,
> already?
>
> I tried to use type families recently to explicitly pick an (otherwise
> overlapping) instance in specified order but could not figure out, how.
>
> If someone could give a complete simple example of primary intended use of
> closed type families with class instances,  this would help a lot.
>
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