map -package:containers -package:blaze-html -package:bytestring -package:conduit -package:imports -is:exact -package:basement -package:filepath package:pipes

map :: Functor m => (a -> b) -> Pipe a b m r

Apply a function to all values flowing downstream

map id = cat

map (g . f) = map f >-> map g

Apply a function to all values flowing downstream, and forward each element of the result.

Apply a monadic function to all values flowing downstream

mapM return = cat

mapM (f >=> g) = mapM f >-> mapM g

Consume all values using a monadic function

(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