Where can I find out about #ifdef?

Question

I see that this is often used to make modules compatible with GHC and Hugs, but Google doesn't help me know more about this.

What can I set inside the conditional? Can I make parts of the module conditional, which version of the "base" is used?

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

Answer 1

The GHC documentation has a section related to the C preprocessor that documents some of the predefined macros of the preprocessor.

The Cabal documentation has a section related to conditional compilation , which gives an example related to base . If you are writing a portable package, you should still use Cabal.

Answer 2

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

Check package version

This uses crypto-api , which 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 characters depending on cabal flags. Here's a (optionally complicated) example from pureMD5 (from a .cabal file):

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

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

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

For more information, you can see the Cabal user guide. This page contains conditional compilation information that you are probably looking for.

Answer 3

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

Many Haskell-related programs install some macros for the preprocessor (for example, GHC sets __GLASGOW_HASKELL__ to the GHC version), so you can conditionally compile code, for example, to use various native libraries for Hugs and GHC.

Answer 4

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

Take a look at section 4.11.3. Options affecting the C preprocessor are here .