As a project mostly motivated by personal curiosity, I've been implementing a Sequence class: class Sequence s where front :: s a -> (a, s a) back :: s a -> (s a, a) null :: s a -> Bool empty :: s a prepend :: a -> s a -> s a append :: s a -> a -> s a concat :: s a -> s a -> s a The intent is to make it almost as easy to use, via pattern guards, as pattern matching on lists: map :: (a -> b) -> [a] -> [b] map _ [] = [] map f (x:xs) = f x:map f xs smap :: Sequence s => (a -> b) -> s a -> s b smap f s | null s = empty | (a, s') <- front s = prepend (f a) $ smap f s' It is, of course, trivial to implement this for lists. I've run into a snag, however, when trying to implement this for Arrays (as in Data.Array) - I can't seem to find a way to represent an empty array, which makes implementing 'empty' and 'null' impossible. Suggestions? Abe