Unavailable system functions in `unix`

[John Lato]

On Sat, Dec 28, 2013 at 2:04 AM, Herbert Valerio Riedel <hvr at gnu.org> wrote:

> 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...
>
> I didn't see this before I replied to Ben, but I think this is a fine
solution.
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