Haskell: Correct practice to specify version in source?

Question

What is the best/correct practice to specify version within your source code tree? What I want is, for instance, to put VERSION file in the top level of the source tree and

Answer 1

Cabal automatically generates a module for each package named Paths_packagename. Just import this package and look at the version value it exports.

Edit: For example:

module Data.Blah where

import Paths_t

func :: IO ()
func = print version

And an example run:

> func
Version {versionBranch = [0,1], versionTags = []}

Be sure to put Paths_packagename in your Other-Modules section of the cabal file.

Answer 2

Best practice is to put the version number in your cabal file, as you have already noted.

I am not aware of any good practices by which you can maintain a single point of truth about the version, yet make the number available both to cabal and to your application.

I would recommend a single file Version.hs in the sources with these contents:

 module Version 
 where
 version :: String
 version = "3.14159"

You could then, if you wished, use some kind of script to update the cabal file with this number, but I don't know how to do this from within cabal itself.

This strategy will work only for application packages; if you are building a library, you'll need to give some thought to where in the namespace of hierarchical modules your Version.hs will go.

On the whole, I suspect the game is not worth the candle.

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P.S. The version number should be immutable, so you want a value, not a function.