Maybe we should replace data Stack a = Empty | Node { focus :: !a -- focused thing in this set , left :: [a] -- clowns to the left , right :: [a] } -- jokers to the right deriving (Show, Read, Eq) with something like: type Stack a = Maybe (NonEmptyStack a) data NonEmptyStack a = Node { focus :: !a -- focused thing in this set , left :: [a] -- clowns to the left , right :: [a] } -- jokers to the right Then (modify Empty f) could just be (fmap f), and (modify d f) becomes (Just . maybe d . fmap f), which seems cleaner to me. This also allows nice use of pattern matching in the Maybe monad. I like Maybe. -- David Roundy Department of Physics Oregon State University