Transition to NixOS

Posted on by nek0 in english, administration, programming

Just recently I made a transition, I was thinking of for quite some time. What I’m speaking of is my transition to NixOS.

Two days ago I just installed NixOS on my laptop out of the blue. Now I’m trying to get my programming projects to build and work again. On may way to accomplish this, I learned a lot about the system and its configuration.

Overall I must say, that I am quite pleased with the outcome. It is a fairly usable system, which holds some cool advantages for me as a Haskell developer over my old Debian system setup. Te biggest one is, That I can scrap all cabal sandboxes from all my projects and hand the management of the right dependency versions over to Nix.

What I want to share is a small snippet of a nix expression, that lets you include local dependencies to your Haskell project’s nix shell (like you would do with cabal sandbox add-source.

First you need to create a nix shell configuration. There is a handy tool called cabal2nix which you can install simply by invoking nix-env -iA nixos.cabal2nix. This is the main work horse for converting any Haskell package to nix and it does its job very well.

To build your Nix shell configuration you call cabal2nix --shell . > shell.nix from your project directory. This generates the file shell.nix for you. Check it for any errors. If you want to use a local package, you can add its definition to the shell.nix file. To get the definition of that package, you can again use cabal2nix. Simply invoke cabal2nix file://<path to Package> and copy the resulting block into the let block of your existing shell.nix like this:

<packageName> = with haskellPackages; callPackage (
  ) {};

Where <packageName> is the variable name for the package inside this Nix expression. <packagDefinition> is the package definition of the local package we just obtained from cabal2nix. Then delete the original package name form the arguments of the f function in the let block of the Nix expression. Finally align everything to look nicely, and save the file. To see an example of that, you can look at the Nix expression of my latest game project.

To test your setup, just invoke nix-shell shell.nix and watch the system build a temporary environment for your project and give you a shell to it, if everything went well. Now all dependencies of your project, including your local ones are installed in this environment, so you only need to invoke cabal build.

I hope You find this snippet of my Nix experience helpful, it sure took me some time to find out.

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