Can some explain me the error and a possibly  a fix : 

data Tree a = NULL | Node (Tree a) a (Tree a) deriving Show 

instance Functor Tree where

............fmap f NULL = NULL

............fmap f (Node left x right) | (left  == NULL) && (right == NULL) = Node left (f x) right

................................................... | (left  /= NULL) = Node (fmap f left) x right

................................................... | (right /= NULL) = Node left x (fmap f right)

ghci gives:

 

Prelude> :load programs\tree.hs

[1 of 1] Compiling Main             ( programs\tree.hs, interpreted )

programs\tree.hs:32:78:

Couldn't match type `a' with `b'

      `a' is a rigid type variable bound by

          the type signature for fmap :: (a -> b) -> Tree a -> Tree b

          at programs\tree.hs:31:7

      `b' is a rigid type variable bound by

          the type signature for fmap :: (a -> b) -> Tree a -> Tree b

          at programs\tree.hs:31:7

    Expected type: Tree b

      Actual type: Tree a

    In the first argument of `Node', namely `left'

    In the expression: Node left (f x) right

    In an equation for `fmap':

        fmap f (Node left x right)

          | (left == NULL) && (right == NULL) = Node left (f x) right

          | (left /= NULL) = Node (fmap f left) x right

          | (right /= NULL) = Node left x (fmap f right)

Failed, modules loaded: none.