16 Aug
2007
16 Aug
'07
9:03 p.m.
Let's start by reminding ourselves what foldr does. foldr f z [x1,x2,...,xn] = f x1 (f x2 ... (f xn z) ...) Now let's ask about last: last [] = error ... last [x1,...,xn] = xn 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