Re: [Haskell-cafe] help in tree folding

On Tue, May 6, 2008 at 6:20 AM, patrik osgnach wrote:

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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)

Functions like this are very abstract, but are also quite nice in that there's basically only one way to write them given the types.

What have you tried so far? This function needs to be recursive, so what arguments should it give to its recursive calls, and where should it plug the results?

It also helps, as you're writing, to keep meticulous track of the the types of everything you have and the type you need, and that will tell you what you need to write next.

Luke so far i have tried this

Luke Palmer ha scritto: treefoldr f x g y Void = x treefoldr f x g y (Node a []) = f a y treefoldr f x g y (Node a (t:ts)) = treefoldr f x g (g (treefoldr f x g y t) y) (Node a ts) but it is clearly incorrect. this functions takes as arguments two functions and two zeros (one for the empty tree and one for the empty tree list). thanks for the answer Patrik