existential.purs

import Data.Exists

data Showable_ a = Showable_ a
data Showable = Showable (Exists Showable_)
  
instance showable_ :: (Show a) => Show (Showable_ a) where
    show (Showable_ x) = show x
    
instance showable :: (Show (Showable_ a)) => Show Showable where
    show (Showable v) = runExists show' v
      where
      show' :: forall a. (Show a) => Showable_ a -> String
      show' x = show x
    
showable :: forall a. (Show a) => a -> Showable
showable = Showable <<< mkExists <<< Showable_ 
  
[showable 1, showable "2"]

-- The `Show Showable` will result in the following error:

-- No type class instance was found for

--     Prelude.Show a0


-- in value declaration showable

-- where a0 is a rigid type variable