exists.hs

{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE KindSignatures #-}

import GHC.Exts (Constraint)

data Exists (t :: * -> Constraint) = forall a. t a => Exists a

instance Show (Exists Show) where
    show (Exists x) = show x

showables :: [Exists Show]
showables = [Exists 1, Exists "2", Exists '3']

main = mapM_ print showables