On Tue, May 6, 2008 at 8:20 AM, patrik osgnach <patrik.osgnach@gmail.com> wrote:

Hi. I'm learning haskell but i'm stuck on a generic tree folding exercise. i must write a function of this type
treefoldr::(Eq a,Show a)=>(a->b->c)->c->(c->b->b)->b->Tree a->c
Tree has type
data (Eq a,Show a)=>Tree a=Void | Node a [Tree a] deriving (Eq,Show)
as an example treefoldr (:) [] (++) [] (Node '+' [Node '*' [Node 'x' [], Node 'y' []], Node 'z' []])
must return "+∗xyz"
any help?
(sorry for my bad english)

Having a (Tree a) parameter, where Tree is defined as an algebraic data type, also immediately suggests that you should do some pattern-matching to break treefoldr down into cases:

treefoldr f y g z Void = ?
treefoldr f y g z (Node x t) = ?

-Brent