I wanted the --version flag in an application to return the version from the cabal file. Unable to find solution for this on the net, I ventured into the darcs source code to for a solution. It's actually pretty easy:

Step 1

Change the Build-Type field in the cabal file to be "Custom". This means cabal will look for a Setup.hs file to control the build.

Step 2

Create a Setup.hs that autogenerates a haskell module containing the version number. Here's mine:

import Distribution.Simple(defaultMainWithHooks, UserHooks(..), simpleUserHooks )
import Distribution.Simple.Utils(rewriteFile)
import Distribution.Package(packageVersion)
import Distribution.Simple.BuildPaths(autogenModulesDir)
import System.FilePath((</>))
import Data.Version(showVersion)
generateVersionModule pkg lbi = do
let dir = autogenModulesDir lbi
let version = packageVersion pkg

rewriteFile (dir </> "Version.hs") $ unlines
["module Version where"
,"version :: String"
,"version = \"" ++ showVersion version ++ "\""
]

myBuildHook pkg lbi hooks flags = do
generateVersionModule pkg lbi
buildHook simpleUserHooks pkg lbi hooks flags

main = defaultMainWithHooks simpleUserHooks {
buildHook=myBuildHook
}

Step 3

Change your program to access the created Version module. It's actually generated in the ./dist/build./autogen directory, but this seems to be correctly on the source path by default.