[Haskell-cafe] Short-circuiting a fold

Here's a bit of Thursday afternoon fun. Mission: Define "ssfold," a short-circuiting fold. It evaluates to the "folded value" that first satisfies the given predicate.

ssfold :: ( a -> Bool ) -> ( a -> b -> a ) -> a -> [b] -> a

Here are two of mine. Straightforward:

ssfold p f z = head . dropWhile ( not . p ) . scanl f z

Monadic:

data Done a b = Done { undone :: a } | NotDone b instance Monad ( Done a ) where ( NotDone i ) >>= f = f i ( Done r ) >>= _ = Done r return = NotDone

ssfold p f z = undone . foldM (\ v e -> if p v then Done v else NotDone ( f v e ) ) z

I want to see some real creativity here, so pull out all the stops. Kurt