mapM_ for bytestring
yom at artyom.me
Sun Sep 1 13:31:00 CEST 2013
On Sun, 01 Sep 2013 15:18:32 +0400, Erik de Castro Lopo
<mle+hs at mega-nerd.com> 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 ()
mapM :: Monad m => (Word8 -> m Word8) -> ByteString -> m ByteString
After all, the signature for ByteString-y `map` too requires a function
of type `Word8 -> Word8`, so it seems perfectly reasonable for mapM to
> in that the second parameter is a list. ByteStrings are not lists,
> they're chunks of bytes. Besides, you can just do:
> let bs = "This is a bytestring"
> mapM someFUnctions $ BS.unpack bs
> or define a function:
> mapBSM :: Monad m => (Char -> m a) -> ByteString -> m [a]
> mapBSM f bs = mapM f $ BS.unpack bs
> and similar for mapBSM_.
There’s even a RULE which would optimise this. However, it still would be
*four times slower* than necessary if the monad in question is IO or ST.
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