Re: mapM_ for bytestring

On Sun, 1 Sep 2013, Artyom Kazak wrote:

On Sun, 01 Sep 2013 15:18:32 +0400, Erik de Castro Lopo wrote:

...

Err, mapM is defined as:

mapM :: Monad m => (a -> m b) -> [a] -> m [b]

Yeah, the signatures for mapM_ and mapM would respectively be

mapM_ :: Monad m => (Word8 -> m b) -> ByteString -> m ()

This one should not be a big problem, but could be done by calling mapM_ on the result of ByteString.unpack.

mapM :: Monad m => (Word8 -> m Word8) -> ByteString -> m ByteString

This one can become very inefficient. Think of m = [], then many ByteStrings have to be constructed by repreated ByteString.cons which requires a lot of copying. For lists "cons" aka (:) is efficient because the tails are shared.