Example of ExistentialQuantification usage inside of a data declaration.

Kris.hs

{-# LANGUAGE ExistentialQuantification #-}
module Kris where

data A = A
data B = B
data C = C

instance Show A where
  show _ = "This is A"

instance Show B where
  show _ = "B"

instance Show C where
  show _ = "and C"

data Showable = forall s. Show s => Showable s

instance Show Showable where
   show (Showable s) = show s

main :: IO ()
main = print [Showable A, Showable B, Showable C]

Output is of course:

[This is A,B,and C]