2011/7/23 Александр
I built binary tree with:
data Tree a = Empty | Node a (Tree a) (Tree a) deriving (Eq, Ord, Read, Show) How can i make Monad type class instance for this tree? And can i make it on not?
If you had a monad for Tree a, then you'd have join :: Tree (Tree a) -> Tree a join x = x >>= id It is non-obvious for me how such function would behave. (I'm not saying it's impossible.) However, you may try with a different binary tree, having data only on the leafs: data Tree a = Leaf a | Node (Tree a) (Tree a) It's pretty easy to define it as a monad: instance Monad Tree where return = Leaf Leaf a >>= f = f a Node l r >>= f = Node (l >>= f) (r >>>= r) Then you get basically: join :: Tree (Tree a) -> Tree a join (Left t) = t join (Node l r) = Node (join l) (join r) In other words, join just glues the trees on the leafs to the tree itself. It's kinda straightforward. Cheers, -- Felipe.