On Fri, Feb 15, 2013 at 9:05 AM, Andrew Cowie < and...@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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