qwe.hs

{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeApplications #-}

import Unsafe.Coerce

class ToString a where
  toString :: a -> String
  tag :: String

instance ToString Int where
  toString x = "int: " <> show x
  tag = "default"

data ToString' a = ToString'
  { toString' :: a -> String,
    tag' :: String
  }

toStringInt' :: ToString' Int
toStringInt' = ToString' {toString' = \x -> "int: " <> show (x * 10), tag' = "custom"}

newtype Wrapper a b = Wrapper (ToString a => b)

f :: forall a. ToString a => a -> (String, String)
f x = (tag @a, toString x)

f' :: forall a. ToString' a -> a -> (String, String)
f' = unsafeCoerce (Wrapper f :: Wrapper a (a -> (String, String)))

main = do
  print $ f (3 :: Int)               -- ("default","int: 3")
  print $ f' toStringInt' (3 :: Int) -- ("custom","int: 30")