Haskell forall 
    Why the following function CAN NOT be compiled

    liftTup :: (x -> f x) -> (a, b) -> (f a, f b)
    liftTup f (t, v) = (f t, f v)
Compilation Error  The default function signature is the following 
    liftTup :: forall x (f :: * -> *) a b. (x -> f x) -> (a, b) -> (f a, f b)
    liftTup f (t, v) = (f t, f v)     ↑            ↑
                                      |            |
                                      a ←   +    → x
                                            ↑
                                            + -> GHC treats a and x are different type

    e.g   f::Int -> Int
          x :: Int

          let a = "abc"
          f a  ⇒  ERROR
ERROR: Couldn't match expected type ‘x’ with actual type ‘a’ GHC treats 'x' and 'a' are different type ERROR: Couldn't match expected type ‘x’ with actual type ‘b’ GHC treats 'x' and 'b' are different type Rewrite the following, it should be OK. 
    {-# LANGUAGE RankNTypes #-}
    liftTup :: (forall x. x -> f x) -> (a, b) -> (f a, f b)
    liftTup f (t, v) = (f t, f v)

    -- x is 'any' type
    -- (forall x. x -> f x)
    -- f is polymorphic function