http://blog.begriffs.com/2013/08/tricking-haskell-into-being-dynamic.html

DynamicEquals2.hs

{-# LANGUAGE ExistentialQuantification #-}

import Data.Typeable (Typeable, TypeRep, typeOf)
import Unsafe.Coerce (unsafeCoerce)

data Magic = forall a. Magic
  (a -> a -> Bool)
  TypeRep
  a

instance Eq Magic where
  Magic eq typeRepX x == Magic _ typeRepY y
    = typeRepX == typeRepY && x `eq` unsafeCoerce y

magic :: (Eq a, Typeable a) => a -> Magic
magic x = Magic (==) (typeOf x) x

-- > magic (10::Int) == magic ("hello"::String)
-- False
-- > magic (10::Int) == magic (10::Int)
-- True
-- > magic (10::Int) == magic (10::Double)
-- False