Where can I learn about #ifdef?

I see this used often to make modules compatible with GHC and Hugs, but google is not helping me le开发者_开发问答arn 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

The GHC documentation has a section relating to the C pre-processor that documents some of the predefined pre-processor macros.

The Cabal documentation has a section relating to conditional compilation that gives an example relating to base. If you are writing a portable package, you should be using Cabal, anyway.

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.

#ifdef and friends are used by the C preprocessor (CPP). They provide a way to compile code conditionally. You can enable the use of the CPP by adding the pragma {-# LANGUAGE CPP #-} on top of a file.

Many programs that deal with Haskell code set some macros for the preprocessor (eg. GHC sets __GLASGOW_HASKELL__ to the version of GHC), so one can conditionally compile code, for instance to use different properitary libraries for Hugs and GHC.

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.