On 8/16/07, ok
We're going to have to keep track of whether we have a last element or not. The obvious candidate for this is Maybe x. Initially there is no element, Nothing. f x Empty = Just x f x (Just y) = Just y This picks up a new value (x) when there wasn't one (Nothing) and keeps the old last element (Just y) when there was one (Just y). But this gives us a Maybe x, when we want an x, so we'll have to finish off with a fromJust.
last = fromJust . foldr f Nothing where f _ r@(Just _) = r f x Nothing = Just x
I had this idea as well, but the questioner said the chapter with the exercise preceded any use of Maybe, although I admit my suggestion of using foldr to compose a processing function is more complicated for a beginner. Here's a way to use the above idea without Maybe: myLast = head . foldr f [] where f x [] = [x] f _ [x] = [x]