How do I use cabal's MIN_VERSION_ and other macros with ghci?

Question

When I use Cabal's various MIN_VERSION_ macros in a Haskell project, how can I ensure they are all correctly defined when I am not using cabal, e.g. when testing in GHCi?

Answer 1

Nowadays, cabal supports a cabal repl subcommand, which does all the setup for you, so at least for ghci the following is unnecessary. Nevertheless:

The cabal build command generates the file dist/build/autogen/cabal_macros.h, which contains all the definitions you need. In order to include that file in a ghc invocation, you'll need the flags -optP-include -optPdist/build/autogen/cabal_macros.h.

For convenience, you can place the following in a .ghci file in the project directory:

:set -optP-include -optPdist/build/autogen/cabal_macros.h

so that you don't have to type out the options every time you want to use ghci.

Beware, though: the macros will be defined according to the configuration when you last ran cabal build, and will not be updated when you install new packages or use a different GHC version: for that you'd need to re-configure and rebuild the package.

(Thanks to Simon Hengel on the libraries list for this wisdom: http://www.haskell.org/pipermail/libraries/2012-September/018491.html).