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@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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