Main.hs

{-# LANGUAGE DerivingVia #-}
{-# LANGUAGE DerivingStrategies #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE UndecidableInstances #-}

module Main where


class Foo a b
instance Foo Int ()

newtype BarInt = Bar Int

-- This doesnt work because BarInt is _not_ the last type class parameter.
-- deriving newtype instance Foo BarInt ()

class Foo a b => FlippedFoo b a

instance {-# OVERLAPPING #-} FlippedFoo b BarInt => Foo BarInt b

deriving newtype instance FlippedFoo () BarInt

bleh :: Foo BarInt () => ()
bleh = ()

main :: IO ()
main = print bleh