Unavailable system functions in `unix`

[Herbert Valerio Riedel]

On 2013-12-28 at 01:11:40 +0100, Ben Millwood wrote:
> On Sun, Dec 22, 2013 at 12:02:16PM -0800, John Lato wrote:
>>On Dec 22, 2013 7:28 AM, "Herbert Valerio Riedel" <hvr at gnu.org> wrote:
>>>
>>> On 2013-12-22 at 11:32:20 +0100, Thomas Schilling wrote:
>>> > Either I should be able to check at compile time, or I should get a
>>> > defined exception call which then must be handled by the library.  In
>>> > either case it should be documented with the function.
>>>
>>> It might be worth adding more information to the Haddock-comments
>>> mentioning the respective `HAVE_*` CPP symbol and whether a
>>> fallback-implementation is used.
>>>
>>> Moreover, the use of `error` to signal non-existing implementations
>>> could be reconsidered (e.g. maybe switch to a properly thrown
>>> "call-not-implemented" exception)
>>
>>This would definitely be better than calling error. Personally I would
>>prefer conditional exports, as it would turn up errors sooner.
>>
>>John L.
>
> What does client code look like, if this is your implementation
> strategy? Note that the network package recently changed to *stop*
> conditionally exporting things, because it led to bad error messages
> (yes, your error is caught at compile time, but it's not so clear what
> it *is*) and ugly client code. See
> e.g. https://github.com/haskell/network/issues/40

btw, as a compromise, one could export the symbol always (even if its
body is exception-throwing stub) and at the same time enable a '{-#
WARNING #-}' for that symbol for the not-implemented; that way one gets
both, a stable API as well as compile-time warnings...