Convertible.hs

{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE UndecidableInstances #-}
module Convertible where

import Control.Applicative
import Data.Proxy

class Convertible a b where
    convert :: a -> b

instance Convertible a b => Convertible (Const a x) (Const b y) where
    convert = Const . convert . getConst

instance {-# OVERLAPPABLE #-} (a ~ b) => Convertible a b where
    convert = id

apply :: forall a b proxy. (Convertible a b, Show b) => proxy b -> a -> IO ()
apply _ = print . (convert :: a -> b)

t :: Const String Int -> IO ()
t = apply (Proxy @(Const String Int))