Haskell with Stack and Nix

Haskell is one of those languages that fascinates me, I had the first contact with it in 2011, and since then, from time to time, I end up revisiting just to play around. I nothing close to an expert on it, since its mental modal is wholly different from other languages, but I know one thing or two about it.

The most complex thing that I did on it was a 2D game, to help a friend complete a college appointment. It was hard, but fun at the same time. I probably would take less time today since I learn some concepts from lambda calculus and category theory; two heavily mathematical topics presents in each line that you write in Haskell.

One of the things that stressed me out a lot with Haskell is the lake of modern tooling that makes it easy to maintain the software that we write; this is one of the main reasons why a love Rust, Cargo is a complete tool set for all your needs while creating a Rust project. It can be your:

Dependency manager;
Testing tool;
Documentation;
Build system;
and, a lot more.

Fortunately, something have improved last years, and we have now tools like Stack that are wonderful to manage your Haskell project; however, I don’t want to install another tool directly on my machine, but fortunately, I already used a solution for that for quite some time - Nix.

Solution

Nix is a package manager that provides – among other benefits – the nix-shell, a sort of virtual environment for everything.

Nix in a way can solve the same challenges as Docker, in particular, as a solution to the dreaded “it works on my machine” class of problems often encountered by teams working on a project. However, I have other reasons to use it daily, as being a replacement for my OS package manager, and a way to isolate installed tools.

To create a Nix Shell with the needed Haskell tools is pretty simple, we just create a shell.nix file that will contain the shell definition.

let
  pkgs = import <nixpkgs> { };
  stack-wrapped = pkgs.symlinkJoin {
    name = "stack";
    paths = [ pkgs.stack ];
    buildInputs = [ pkgs.makeWrapper ];
    postBuild = ''
      wrapProgram $out/bin/stack \
        --add-flags "\
            --nix \
            --no-nix-pure \
            "
    '';
  };
in
pkgs.mkShell {
  # Do NOT use `stack`, otherwise system dependencies like `zlib` are missing at compilation
  buildInputs = [ stack-wrapped ];
  NIX_PATH = "nixpkgs=" + pkgs.path;
}

The contents of the above file will create a new wrapper around Stack that will pass two new flags --nix and --no-nix-pure, that will tell Stack to use Nix to fetch any missing library/executable and allow running Stack in a non-pure way, correspondingly. The latest, mining two things:

environment variables will be forwarded from the shell into nix session;
the build will use host libraries to build the artifacts.

Now, you can simply navigate to the directory where the shell.nix lives and run nix-shell; Nix will automatically set up a Haskell environment for you, without changing your host system. All the tools only will exist inside this shell. 🎉

Haskell with Stack and Nix

Solution

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