`socketPort` does not handle Unix sockets

[eborden]

socketPort is a partial function. It only supports AF_INET and AF_INET6. Its type signature is not really indicative of this. IO informs that this function could throw from an exception, but partiality should be communicated with Maybe.

Originally reported by @RyanGlScott here #318 (comment)

[kazu-yamamoto]

I'm not sure which is better:

1. Keep this code and update the documentation
2. Change the signature to Socket -> IO (Maybe PortNumber)

[eborden]

@kazu-yamamoto It is a tough call. If we don't want to change the type signature then we must update the documentation.

Option 1

Pro

• This does not cause API breakage.
• It doesn't require pointer indirection introduced by Maybe

Con

• We continue to have a partial function.
• Documentation can easily get out of sync with reality.

Option 2

Pro

• It communicates partiality within the type.
• It leaves how to handle that partiality up to the consumer.
• It doesn't mix true exceptions with non exceptional failure.

Con

• It causes API breakage.
• It requires pointer indirection, so will be less efficient.

[kazu-yamamoto]

I would suggest to create a new API, for instance, socketPortSafe :: Socket -> IO (Maybe PortNumber) and update the doc of socketPort.

[kazu-yamamoto]

socketPortSafe is implemented.