Undecidable.hs

{-# LANGUAGE 
    MultiParamTypeClasses,
    FunctionalDependencies,
    FlexibleInstances,
    TypeFamilies #-}

class Mul' a b where
  type MulT a b
  mul' :: a -> b -> MulT a b

instance Mul' a b => Mul' a [b] where
  type MulT a [b] = [MulT a b]
  mul' a = map (mul' a)

g b x y = if b then mul' x [y] else y

main :: IO ()
main = return ()