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

If you run your Haskell compiler with the -cpp option, it will first preprocess the source files with the CPP (C Pre Processor).

Take a look at the section 4.11.3. Options affecting the C pre-processor here.