Getting your Version

The Paths_foo has your version, but beware

Posted on 2020-04-07

This just a brief note to self, and y’all, on how an app can discover its own version.

Often it is useful to be able to expose your version, possibly in response to --version request from the command line. Clearly it is desirable that this should not rely on manual intervention, leaving two options:

• the Haskell build system makes it available somehow or

• the versions get burned into the source as part of the project’s release mechanism.

For smaller projects the first variant is particularly welcome as they are unlikely to have the kind of release mechanism that can ensure the version is reliable made available to the running program.

Yay, the Toolchain has your back, mostly

If your app is called foo then it will be found in the Paths_foo module automagically maintained by the build system. (The Paths_foo module is documented in the Cabal section of past GHC doc sets.)

If you need to use the version in an executable then it will have to list foo in its Cabal dependencies.

The recipe I use

1. Add Paths_foo to the library’s exposed modules

library
  exposed-modules:
      Paths_foo

2. Import Paths_foo and Data.Version

Import version from Paths_foo and Version from Data.Version into any module in the library needing access to the version. Your package’s version will be of type Version.

import Data.Version(Version)
import Paths_foo(version)

vrn :: Version
vrn = version

Why?

You can just list Paths_foo in the other-modules: of your executable but then ghci will not be able to follow along.

Using hpack?

If you don’t need ghci to see the Paths_foo then your generated cabal file should always have Paths_foo listed in other-modules and you just can import it. If you need ghci to see it will have to be listed in the library’s exposed-modules.

Got an issue with any of this? Please share or drop me a line (see below).