Refactoring ProtocolNumber as newtype

[archaephyrryx]

In the current implementation of network, the ProtocolNumber type, defined in Network.Socket.Types, is given as

type ProtocolNumber = CInt

which means that it can only ever be displayed using the show method for Foreign.C.Types.CInt.

I initially was considering working on redefining it along the lines of

newtype ProtocolNumber = ProtocolNumber CInt
    deriving (Eq, Ord, Enum, Real, Num, Integral)

so that the implicit constraint Num a => a on numeric literals would be maintained, and so that it would be possible to define custom Read/Show instances that used pattern synonyms for commonly used protocols (e.g. TCP and UDP) for human-readability. (Along the lines of the pattern-synonym read/show work I implemented in #465 and #466)

Values that correspond to obsolete or extremely uncommon protocols could just be represented numerically, without any constructor syntax, as they are currently represented (and parsed) by default.

However, @vdukhovni pointed out that this would break the API for anyone using either variables or literals with explicit :: CInt signatures where ProtocolNumber values are expected, which could be either a trivial issue or a major problem depending on how common such usage is in the countless projects that have a dependency on network.

In any case, it seemed like enough of an obstacle that I wanted to consult with the project maintainers before I embark on a doomed mission that would break an indeterminate fraction of network-dependent code.

[kazu-yamamoto]

I encourage you to continue this work. Your work would be merged into the next major release (probably soon) because of compatibility issue.