Heinrich,
A canonical example is the following solution to the problem of labeling all the leaves in a tree with the total leaf count:
data Tree a = Branch (Tree a) (Tree a) | Leaf a
labelLeaves :: Tree a -> Tree Int labelLeaves tree = tree' where (n, tree') = label n tree -- n is both result and argument!
label n (Branch a b) = (na+nb, Branch a' b') where (na,a') = label n a (nb,b') = label n b label n (Leaf _) = (1, Leaf n)
This looks completely freaky to me... how does it work? Is it the laziness that allows the sum to be calculated first while preserving the structure (as thunks?), and then once the value of n is known it is propagated back down the tree and the actual tree values constructed? Anyways this is really amazing to my newbie eyes... Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada