Where can I learn about #ifdef?

Question

I see this used often to make modules compatible with GHC and Hugs, but google is not helping me learn more about it.

What can I put inside the conditional? Can I m

Answer 1

In addition to the very useful flags defined by GHC (OS, architecture, etc), when using cabal other flags and macros are defined.

Check Package Versions

Here's a use from crypto-api that checks the version of the tagged package being used:

#if MIN_VERSION_tagged(0,2,0)
import Data.Proxy
#endif

Custom CPP Defines Based on Cabal Flags

You can define CPP symbols dependent on cabal flags. Here's an (unnecessarily complex) example from pureMD5 (from the .cabal file):

 if arch(i386) || arch(x86_64)
   cpp-options: -DFastWordExtract

Inside the .hs module you can then use #ifdef, for example:

#ifdef FastWordExtract
getNthWord n b = inlinePerformIO (unsafeUseAsCString b (flip peekElemOff n . castPtr))
#else
... other code ...
#endif

For more information you can see the Cabal users guide. This page has the "conditional compilation" information you're probably looking for.