Unavailable system functions in `unix` (was: Pull request for inclusion in `unix' module of fsync(2), fdatasync(2), posix_fadvise(2) and posix_fallocate(2))

[Ricardo Catalinas Jiménez]

On Sun, Dec 22, 2013 at 8:02 PM, John Lato <jwlato at gmail.com> 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:
> > > Looks like GHC HQ maintains this package.  I don't think it'll make it
> > > into the version released with 7.8, though.
> >
> > fwiw, I've been wanting to review that submission but didn't get to it
> > yet;
> >
> > > I do have another question, though.  How should Haskell code using the
> > > `unix` library check for whether `fsync` are implemented or not?
> >
> > there's a HsUnixConfig.h include file generated (and installed) you can
> > include from your Haskell code, and use CPP
> >
> > > If I call `fsync` I wouldn't want to get a runtime `error` call if the
> > > platform doesn't support it.
> >
> > To be fair, `unix` does not seem consistent with respect to handling
> > potentially missing lib/syscalls; I've seen the following three cases:
> >
> >  a) fallback to implement semantics via different calls
> >     (e.g. `System.Posix.Env.unsetEnv`), or
> >
> >  b) the implementation returns bottom via an `error`-value
> >     (e.g. `System.Posix.Temp.mkstemps`), or
> >
> >  c) the symbol is conditionally exported
> >     (e.g. `System.Posix.Directory.tellDirStream`)
> >
> > > 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.
>

OK, sounds good. Now everything is conditionally exported:

https://github.com/jimenezrick/unix/compare/master...file-utils.patch

Yeah, the unix library is messy as it is having to much preprocessor
infection and different ways of handling this situations.
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