[Haskell-beginners] Functor on Tree data constructor

[Tony Morris]

On 03/04/14 00:37, Gesh wrote:
>> On 2014-04-02 12:53, Nishant wrote:
>>> Can some explain me the error and a possibly  a fix : 
>>>
>>> data Tree a = NULL | Node (Tree a ) a (Tree a)
>>>
>>> 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)
> Note that you can simplify your instance declaration to:
>> instance Functor Tree where
>>     fmap f NULL = NULL
>>     fmap f (Node left x right) = Node (f' left) (f x) (f' right)
>>       where f' x = if x == NULL then NULL else fmap f x
> Also note that the NULL in the then clause differs from x. Let f :: a -> b, then x :: Tree a and the NULL in that clause :: Tree b. These values are as distinct as 2 :: Int and 2.0 :: Double.
> HTH,
> Gesh
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instance Functor Tree where
    fmap f NULL = NULL
    fmap f (Node left x right) = Node (f' left) (f x) (f' right)
      where f' = fmap f



-- 
Tony Morris
http://tmorris.net/