Maps a side effect over an
InputStream.
mapM_ f s produces a new input stream that passes all output
from
s through the side-effecting IO action
f.
Example:
ghci> Streams.fromList [1,2,3] >>=
Streams.mapM_ (putStrLn . show . (*2)) >>=
Streams.toList
2
4
6
[1,2,3]