{-# LANGUAGE ExistentialQuantification #-}

data Value = A Int

data ForallFunc = forall a. Eq a => Forall (Value -> a) 

unpackA (A int) = int

equalityTest :: Value -> Value -> ForallFunc -> Bool            
equalityTest arg1 arg2 (Forall unpacker) =
  let a1 = unpacker arg1
      a2 = unpacker arg2 in
    a1 == a2

main = putStrLn $ show $ equalityTest (A 1) (A 1) (Forall unpackA)