Re: [Haskell-beginners] Functor on Tree data constructor

Thanks Vlatko ... That analogy helped ..... I created my own functor typeclass where i defined fmap :: MyFucntor f => (a->a) -> f a ->f a and that worked ... On Wed, Apr 2, 2014 at 7:24 PM, Vlatko Basic wrote:

Try to visualize with concrete types

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

Tree Int = Node (Tree Int) Int (Node Int) -- OK Tree String = Node (Tree String) String (Node String) -- OK

f :: Int -> String f i = show i

and see what you get:

instance Functor Tree where fmap f NULL = NULL fmap f (Node left x right) -- Node (Tree Int) Int (Node Int) | (left == NULL) && (right == NULL) = Node left (f x) right -- Node (Tree Int) String (Node Int) === Not OK | (left /= NULL) = Node (fmap f left) x right -- Node (Tree String) Int (Node Int) === Not OK | (right /= NULL) = Node left x (fmap f right) -- Node (Tree Int) Int (Node String) === Not OK

-------- Original Message -------- Subject: [Haskell-beginners] Functor on Tree data constructor From: Nishant To: beginners@haskell.org Date: 02.04.2014 12:53

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)

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-- Nishant