Proposal [Trac #1212]: add IPv6 support to network library

[Bryan O'Sullivan]

Sven Panne wrote:

>    * You use an internal class HostAddr and make two type synonyms an instance 
> of it.

You must have downloaded the first version of the patch that I posted a 
few weeks ago.  The updated version doesn't have this rather unpleasant 
wart.  Unfortunately, Trac won't let me drop old versions of a patch.

>    * The types and values of AddrInfoFlags/NameInfoFlags are not very 
> Haskell-like. Using
> 
>         data AddrInfoFlag = Passive | CanonName | NumericHost | ...
> 
>      and [AddrInfoFlag] instead of AddrInfoFlags is much nicer.

OK, I'll update the interface to look like this.  It would be nice if 
there were helper functions in Foreign.Marshal.Utils or Data.Bits to 
make this a more mindless operation:

import Data.Bits
import Data.List (foldl')

packBits :: (Eq a, Bits b) => [(a, b)] -> [a] -> b

packBits mapping xs = foldl' pack 0 mapping
     where pack acc (k, v) | k `elem` xs = acc .|. v
                           | otherwise   = acc

unpackBits :: Bits b => [(a, b)] -> b -> [a]

unpackBits mapping bits = foldl' unpack [] mapping
     where unpack acc (k, v) | bits .&. v == 0 = acc
                             | otherwise       = k:acc

It's simple boilerplate, but nice not to need to write.  This would make 
the conversion process very tidy:

aiMapping = [(AI_PASSIVE, #const AI_PASSIVE),
              (AI_CANONNAME, #const AI_CANONNAME),
              ...and so on...]

packAIFlags   = packBits aiMapping
unpackAIFlags = unpackBits aiMapping

Assuming people like the interface, should I submit this as a separate 
proposal, or just fold it into Network.Socket?

> Alas, the X11 package is not nice regarding the last item, either... :-(

True.  But there's an SoC proposal to add XCB bindings, which could 
presumably use this bit-swizzling code :-)

	<b