[Haskell-cafe] ifdef based on which OS you're on

[Dan Cross]

On Fri, Feb 15, 2013 at 9:05 AM, Andrew Cowie <
andrew at operationaldynamics.com> wrote:

> I've got a piece of code that looks like this:
>
>         baselineContextSSL :: IO SSLContext
>         baselineContextSSL = do
>             ctx <- SSL.context
>             SSL.contextSetDefaultCiphers ctx
>         #if defined __MACOSX__
>             SSL.contextSetVerificationMode ctx SSL.VerifyNone
>         #elif defined __WIN32__
>             SSL.contextSetVerificationMode ctx SSL.VerifyNone
>         #else
>             SSL.contextSetCADirectory ctx "/etc/ssl/certs"
>             SSL.contextSetVerificationMode ctx $
>                 SSL.VerifyPeer True True Nothing
>         #endif
>             return ctx
>
> all very nice (this being necessary because apparently the non-free
> operating systems don't store their certs in a reliably discoverable
> place; bummer).
>
> That, however, is not the problem. After all, this sort of thing is what
> #ifdefs are for. The problem is needing to get an appropriate symbol
> based on what OS you're using defined.
>
> I naively assumed there would be __LINUX__ and __MACOSX__ and __WIN32__
> defined by GHC because, well, that's just the sort of wishful thinking
> that powers the universe.
>
> So my question is: what's an appropriate Haskell mechanism for building
> code that is OS / arch  / distro specific? It's not like I have autoconf
> running generating me a config.h I could #include, right?
>
> This feels simple and an appropriate use of CPP; even the symbol names
> look just about like what I would have expected; stackoverflow said so,
> must be true. Just need to get the right symbol defined at build time.
>
> Any suggestions?


Things like this have been the bane of Unix programmers for decades; the C
pre-processor has always been kind of a hack, mostly because of things like
#ifdef and the huge messes they create.  For more on why this is so bad
from the Unix side of the house, see:
http://static.usenix.org/publications/library/proceedings/sa92/spencer.pdf

A better solution is to define a standard interface called by your code and
a per-system module that implements that interface and does the things you
need, then simply include one for the appropriate system at compile time.
 E.g., a Darwin and Win32 modules that provide a function that calls
'SSL.contextSetVerificationMode ctx SSL.VerifyNone', and some kind of
Generic module that provides a function that does the rest.

        - Dan C.
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