mapMaybe package:pipes

mapMaybe :: Functor m => (a -> Maybe b) -> Pipe a b m r
pipes Pipes.Prelude
(mapMaybe f) yields Just results of f. Basic laws: 
mapMaybe (f >=> g) = mapMaybe f >-> mapMaybe g

mapMaybe (pure @Maybe . f) = mapMaybe (Just . f) = map f

mapMaybe (const Nothing) = drain
As a result of the second law, 
mapMaybe return = mapMaybe Just = cat