Where can I learn about #ifdef?

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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 make parts of a module conditional on what version of 'base' is in use?

EDIT 3/2017: This is a great resource: https://guide.aelve.com/haskell/cpp-vww0qd72

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