It looks quite neat to use the Maybe monoid here:
import Data.Monoid searchList p = foldr (\x -> if p x then mappend (Just [x]) else id) Nothing
but it seems that the Maybe Monoid instance keeps this strict. I fiddled with this a bit, and came up with the following:
instance (Monoid m) => Monoid (Maybe m) where mempty = Nothing -- as usual mappend (Just x) y = Just $ mappend x (fromMaybe mempty y) mappend Nothing y = y
which results in the expected behaviour (it's unsatisfyingly asymmetric, since it should (but can't) produce a Just if the second argument is Just without pattern-matching on the first, but there is only so much one can do without involving scary things like unamb). I'd be interested in what people thought of the relative merits of this alternative Monoid instance, but perhaps that would be a subject for a different thread.