missing multicast support

[zoranbosnjak]

Please consider adding UDP multicast support to the network library.
There is a separate library network-multicast here:
https://hackage.haskell.org/package/network-multicast
but the multicast is so basic function that it could as well be integral part of the network.

[kazu-yamamoto]

We are trying to make network truely cross-platform and keep it as small as possible. So, we should answer the following questions for this kind of issues:

• Is the code truely cross-platoform? If no, we cannot merge the code into network.
• Is it impossible to implement the code without modifications of network? If the answer is possible, such network-xxx packags are our intention.

[eborden]

@kazu-yamamoto This might point to a deficiency in documentation. Just as we point to high level libraries in network's description, we may want to point to the existence of a network-xxx ecosystem.

[zoranbosnjak]

I agree on the arguments. However, there is still room for improvement. The network-multicast has 2 parts:

• High level functions multicastSender and multicastReceiver.
• Low level socket manipulation functions (setting socket options with some multicast specific constants).

Only the low level part is a candidate for the network base library. I am almost certain that it can remain cross-platform. On the other hand, a network-multicast can still provide added value, but without digging low level.

network-multicast source code is very short. Please have a look at the low level part and see how/if it could be translated to the base library.

[kazu-yamamoto]

We should read https://tools.ietf.org/html/rfc3678.
Unfortunately, the support is not straightforward because setSocketOption allows an Int value only.
We need a more generic API.

[kazu-yamamoto]

When #445 is merged, we have the generic API.

[kazu-yamamoto]

Not correct. master already has getSockOpt and setSockOpt!