How can my Haskell program or library find its version number?

I would like my cabalised program to have a --version switch.

I would like it to report the same version as is present in the .cabal file.

If I have to update the version number separately in my Haskell source code as well as in the .cabal file, I will eventually get them out of sync.

So, how can my program, while being compiled under cabal, get its version number from the .cabal file?

Solution 1:

This is well supported with Cabal. As follows (from xmonad):

Import Paths_$myprogram - a file Cabal creates with lots of metadata from the .cabal file, along with a the module for handling version numbers:

import Paths_xmonad (version)
import Data.Version (showVersion)

Add a print statement to print the 'version' field provided by Paths_$myprogram:

case args of
     ["--version"] -> putStrLn ("xmonad " ++ showVersion version)

In general, Cabal's generated Paths file contains the following values, in dist/build/autogen/

version,
getBinDir, getLibDir, getDataDir, getLibexecDir,
getDataFileName