On 2006-06-19 at 15:24-0000 "C Rodrigues" wrote:
Here's a puzzle I haven't been able to solve. Is it possible to write the initlast function?
There are functions "init" and "last" that take constant stack space and traverse the list at most once. You can think of traversing the list as deconstructing all the (:) [] constructors in list.
init (x:xs) = init' x xs where init' x (y:ys) = x:init' y ys init' _ [] = []
last (x:xs) = last' x xs where last' _ (y:ys) = last' y ys last' x [] = x
Now, is there a way to write initlast :: [a] -> ([a], a) that returns the result of init and the result of last, takes constant stack space, and traverses the list only once? Calling reverse traverses the list again. I couldn't think of a way to do it, but I couldn't figure out why it would be impossible.
il [] = error "foo" il [x] = ([], x) il (x:xs) = cof x (il xs) where cof x ~(a,b) = (x:a, b) -- ! Should do it, I think. -- Jón Fairbairn Jon.Fairbairn at cl.cam.ac.uk