List indexed by constraint

ConstraintList

{-# LANGUAGE GADTs #-}
{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE FlexibleInstances #-}

data ConstraintList c where
    Nil :: ConstraintList c
    Cons :: (c a) => a -> ConstraintList c -> ConstraintList c

instance Show (ConstraintList Show) where
    show Nil         = "[]"
    show (Cons x xs) = show x ++ " : " ++ show xs

conls :: ConstraintList Show
conls = Cons 1 $ Cons "Hello, World!" $ Cons True Nil

main = print conls

{-# LANGUAGE UnicodeSyntax, ExistentialQuantification,
             ConstraintKinds, FlexibleInstances #-}

module Exists where

data Exists c = ∀ a. c a ⇒ Exists a

stuff ∷ [Exists Show]
stuff = [Exists 3, Exists 3.0, Exists "three"]

instance Show (Exists Show) where
    showsPrec n (Exists a) = showsPrec n a

main = mapM_ print stuff