+++
date = '2025-04-06T14:49:03+02:00'
draft = false
title = 'Why I do not use flake-utils'
tags = ['nix']
+++

I have been using [Nix] for a while now. Around a year ago, I switched everything from the [servicepoint] library to my [machine configuration] over to flakes.

For me the biggest advantage flakes bring is not additional functionality. Instead, they are an easier and semi-standardized way to do what you could before.

When learning flakes, you often see [flake-utils] being used. With it, you can shorten your flakes by not having to specify everything per system.

### Without anything

```nix
{
  description = "Flake utils demo - without flakes";

  outputs = { self, nixpkgs, flake-utils }:
  {
    packages."x86_64-linux" = rec {
          hello = pkgs.hello;
          default = hello;
    }
    packages."aarch64-linux" = rec {
          hello = pkgs.hello;
          default = hello;
    }

    # more systems ...
  }
}
```

### With flake-utils

```nix
{
  description = "Flake utils demo";

  inputs.flake-utils.url = "github:numtide/flake-utils";

  outputs = { self, nixpkgs, flake-utils }:
    flake-utils.lib.eachDefaultSystem (system:
      let pkgs = nixpkgs.legacyPackages.${system}; in
      {
        packages = rec {
          hello = pkgs.hello;
          default = hello;
        };
      }
    );
}
```

### With function in flake

To make a long story short, here is what I usually do instead:

```nix
{
  description = "forAllSystems demo";

  outputs = { self, nixpkgs }:
    let
      supported-systems = [ # [1]
        "x86_64-linux"
        "aarch64-linux"
      ];
      forAllSystems =
        f:
        nixpkgs.lib.genAttrs supported-systems /* [2] */ (
          system:
          f rec {
            pkgs = nixpkgs.legacyPackages.${system}; # [3]
            inherit system;
          }
        );
    in
    {
      packages = forAllSystems ({ pkgs, ... }: rec {
        hello = pkgs.hello;
        default = default;
      });
    }
}
```

That’s definitely more code! - Yes, but it also includes more information in the flake, while getting rid of an external dependency.
While more code can be intimidating for beginners, it actually helps remove a barrier to understanding how the flake works in this case.
For me, it wasn’t a problem to ignore boilerplate like this at first, slowly learning more language features until I finally understood everything.

At **[1]**, the supported systems are specified. I personally use `x86_64-linux` and `aarch64-linux`, but I also usually support `x86_64-darwin` and `aarch64-darwin` in public projects.
If you want to support any system, you can use [`nixpkgs.lib.system.flake-exposed`] at **[2]** instead of defining your own list.

Because the definition is right inside the flake, you can tweak what gets passed to the function. For example, the [flake for RedoxOS-development] I contributed[^1] this to passes the custom rust-toolchain.
An example for how to do it is already right there: at **[3]**, `pkgs` is provided.

Another possible tweak: You may want to define separate supported systems for each output.
This is useful, for example, if the target environment you're developing for cannot support a development shell.

For me, the trade-offs are worth it, as they provide greater transparency and control over the flake configuration.

That being said, I fully acknowledge that `flake-utils` can still be a great choice for many people. It simplifies things and reduces the need to write boilerplate code, which can be a big plus depending on your needs and workflow. Ultimately, it's a matter of personal preference and the specific requirements of your project.

[^1]: If you check the history, you will see I am not mentioned. I am still a bit salty about that, as it was my first contribution to a bigger OSS project.

[Nix]: https://nixos.org/
[servicepoint]: https://git.berlin.ccc.de/servicepoint/servicepoint
[machine configuration]: https://git.berlin.ccc.de/vinzenz/nixos-configuration
[flake-utils]: https://github.com/numtide/flake-utils
[`nixpkgs.lib.system.flake-exposed`]: https://github.com/NixOS/nixpkgs/blob/374e6bcc403e02a35e07b650463c01a52b13a7c8/lib/systems/default.nix#L58
[flake for RedoxOS-development]: https://gitlab.redox-os.org/redox-os/redox/-/blob/cb34b9bd862f46729c0082c37a41782a3b1319c3/flake.nix#L38