Using the Nix Package Manager for Haskell Development from Arch Linux

I recently installed and configured NixOS on a laptop and had to learn how to develop Haskell on it. The Nix community uses something called cabal2nix (version 2.0 and up!) and nix-shell to get the job done. While things work quite smoothly right now in NixOS, I was wondering if I could do the same on my desktop Arch Linux box.

The answer is yes — you can easily use Nix to create a ‘system sandbox’ of sorts (the Nix store) that is completely isolated from Arch’s own Haskell packages/GHC. To be clear, what we are trying to do is install the Nix package manager (which is composed of many satellite programs like nix-env, nix-shell, etc.) so that we can develop Haskell programs with all the advantages that come with it.

For myself, I have several different Haskell projects, and I wanted to avoid redownloading and recompiling the same packages for each project’s Cabal sandbox environment. Using Nix, I still have the same Cabal sandboxes (one for each project root), but Nix allows all the different sandboxes to share the same packages if the versions and dependencies are the same. And plus, because the Nix store (where Nix stores everything — /nix/store) is independent of Arch’s pacman tool, there is no fear of conflict or things breaking whenever you upgrade Arch Linux’s own Haskell packages.¹

Use the Nix Manual to install Nix

The Nix manual has up-to-date documentation on how to get Nix. When we say Nix, we are talking about the collection of console programs (with a nix- prefix in their names) that make up to form the Nix package management system — much like how Git is made up of smaller programs that work as a team. There is a nix package on the AUR, but I suggest simply following this guide.

The first step is to run the install script from the NixOS site (which hosts Nix and other related programs) as a normal user:

$ bash <(curl https://nixos.org/nix/install)

. You will now have a directory called /nix in your system. This is where everything related to Nix will be stored. In addition, the script will create some hidden files under your user’s home directory with the .nix- prefix. The most important file for now is ~/.nix-profile, because it links to a shell script that initializes the Nix environment (to bring in nix-* utilities into the current shell’s scope). You will get a message from the shell script to source this file, like this:

$ . /home/l/.nix-profile/etc/profile.d/nix.sh

. For me, I put the whole thing into an alias for my shell called nix, like this:

# somewhere in my ~/.zshrc
alias nix='. /home/l/.nix-profile/etc/profile.d/nix.sh'

.² So, whenever I want access to Nix utilities, I just type in nix and go on my merry way.

Install cabal2nix and cabal

Now, use your alias to enable Nix.

$ nix

You now have access to all the nix-* utilities that make up to provide the Nix package management system. You can list all Nix-packaged packages with nix-env -qaP. For us, we’re interested in the cabal2nix package. As of the time of this writing, it is called nixpkgs.haskellPackages.cabal2nix. However, the haskellPackages prefix refers to the old system that has been more or less deprecated as of January 2015. We need to use the haskellngPackages (note the ng) prefix instead. I know that nixpkgs.haskellngPackages.cabal2nix isn’t listed with the nix-env -qaP command, but I believe that’s for legacy reasons. You can still install it! Let’s do that now:

$ nix-env -iA nixpkgs.haskellngPackages.cabal2nix

. This will give you the very useful cabal2nix binary which you can use to convert any .cabal file into something that Nix can understand! Let’s also install cabal for Nix:

$ nix-env -iA nixpkgs.haskellngPackages.cabal-install

. This will install cabal to ~/.nix-profile/bin/cabal. This step is not really necessary if you have cabal-install already installed on the Arch Linux side with pacman. However, I still recommend it because

1. if you’re using Nix for Haskell development, there is no longer a need to use cabal outside of the Haskell/Nix development process;
2. it just makes sense to use the cabal package that comes from the same source tree as cabal2nix (i.e., from the same haskellngPackages set³); and
3. as of the time of this writing the cabal-install version from Nix packages set is newer than the Arch version.

At the end of the day, your cabal binary should be writing to ~/.cabal so take care to use one version and stick with it.

Nixify your project

Create a .cabal file

If you haven’t done so already, create a Cabal file your_project.cabal in your project’s root folder to describe the dependencies in the traditional Haskell way. This step is mandatory!

Create a shell.nix file

Go to your project’s root folder that contains your_project.cabal, and do

$ cabal2nix --shell . > shell.nix

. The actual syntax is cabal2nix --shell path/to/cabal/file, which prints out the contents of the .nix file to STDOUT. In the case above, we redirect it to a file named shell.nix. The name of this file is important because it is what nix-shell expects.

Now just invoke

$ nix-shell

and you’re set. You will be dropped into a bash instance that has knowledge of the Nix store. The first time you run nix-shell, Nix will identify any missing dependencies and install them for you. Because your project’s shell.nix file describes a Haskell project, nix-shell will install GHC along the way. So when it’s ready, you can start ghci. Because you installed cabal2nix earlier, you have access to cabal (i.e., cabal is a dependency of cabal2nix).

To build your binary just do cabal build! Personally I like to instantiate a Cabal sandbox with cabal sandbox init first, and then do cabal configure, cabal repl, cabal build, etc.

Local dependencies

If you’re like me, you might have a Haskell library you wrote for yourself (let’s call it “Private Project X” (PPX)) which is not on Hackage. If you just want to build PPX on its own, you can use the same steps outlined above. But what if your other project depends on PPX?

The trick is to use cabal2nix, and to set up your ~/.nixpkgs folder. You should already have ~/.nixpkgs created by now as a result of installing Nix. Make a folder called ~/.nixpkgs/my-local-hs. Now do

$ cabal2nix path/to/ppx > ~/.nixpkgs/my-local-hs/ppx.nix

. This will create a Nix expression that can be used to build PPX with Nix. It’s like creating a PKGBUILD file. The next step is to create a ~/.nixpkgs/config.nix file, as follows:

 ■

config.nix

# Taken from
# http://lists.science.uu.nl/pipermail/nix-dev/2015-January/015601.html.
{
  packageOverrides = super: let self = super.pkgs; in
  {
    haskellngPackages = super.haskellngPackages.override {
      overrides = self: super: {

        # Enable profiling. Taken from
        # http://lists.science.uu.nl/pipermail/nix-dev/2015-January/015620.html.
        # Comment out this line if you do not want to enable profiling!
        mkDerivation = expr: super.mkDerivation (expr // {
          enableLibraryProfiling = true; });

        # Private package
        ztile = self.callPackage ./my-local-hs/ppx.nix {};
      };
    };
  };
}

. Now, invoke cabal2nix --shell for your other project that depends on PPX. When you invoke nix-shell for this other project, Nix should be able to resolve the dependency, based on the information you gave it in ~/.nixpkgs/config.nix. That’s it!

Conclusion

I recommend trying Nix out for Haskell development, or just as a secondary package manager in general. Right now, everything “Just Works” and it’s a pleasure to see different Haskell projects re-use the same packages, even when they are Cabal-sandboxed, as long as you are doing everything within nix-shell.

Even though the title of this post suggests that this is an Arch Linux guide to Nix, there is nothing Arch-specific about it. You should be able to use the steps in this post for any Linux distribution.

Happy hacking!

Copyright (C) 2013-2018 Linus Arver. All rights reserved.