Flake Parts

Core of a distributed framework for writing Nix Flakes.

flake-parts provides the options that represent standard flake attributes and establishes a way of working with system. Opinionated features are provided by an ecosystem of modules that you can import.

Why Modules?

Flakes are configuration. The module system lets you refactor configuration into modules that can be shared.

It reduces the proliferation of custom Nix glue code, similar to what the module system has done for NixOS configurations.

Unlike NixOS, but following Flakes' spirit, flake-parts is not a monorepo with the implied goal of absorbing all of open source, but rather a single module that other repositories can build upon, while ensuring a baseline level of compatibility: which core attribute make up a flake and how these are represented as module options.

Getting Started

If your project does not have a flake yet:

nix flake init -t github:hercules-ci/flake-parts

Otherwise, add the input,

    flake-parts.url = "github:hercules-ci/flake-parts";

then slide mkFlake between your outputs function head and body,

  outputs = { self, flake-parts, ... }:
    flake-parts.lib.mkFlake { inherit self; } {
      flake = {
        # Put your original flake attributes here.
      };
      systems = [
        # systems for which you want to build the `perSystem` attributes
        "x86_64-linux"
        # ...
      ];
    };

Now you can add the remaining module attributes like in the the template.

Example

See the template.

Options Reference

See flake.parts

Top-level module parameters

config, options, lib, ...: standard module system parameters.
getSystem: function from system string to the config of the appropriate perSystem.

moduleWithSystem: function that brings the perSystem module arguments. This allows a module to reference the defining flake without introducing global variables (which may conflict).

{ moduleWithSystem, ... }:
{
  nixosModules.default = moduleWithSystem (
    perSystem@{ config }:  # NOTE: only explicit params will be in perSystem
    nixos@{ ... }:
    {
      services.foo.package = perSystem.config.packages.foo;
      imports = [ ./nixos-foo.nix ];
    }
  );
}

withSystem: enter the scope of a system. Worked example:

{ withSystem, ... }:
{
  # perSystem = ...;

  nixosConfigurations.foo = withSystem "x86_64-linux" (ctx@{ pkgs, ... }:
    pkgs.nixos ({ config, lib, packages, pkgs, ... }: {
      _module.args.packages = ctx.config.packages;
      imports = [ ./nixos-configuration.nix ];
      services.nginx.enable = true;
      environment.systemPackages = [
        packages.hello
      ];
    }));
}

`perSystem` module parameters

pkgs: Defaults to inputs.nixpkgs.legacyPackages.${system}. Can be set via config._module.args.pkgs.
inputs': The flake inputs parameter, but with system pre-selected. Note the last character of the name, ', pronounced "prime".

system selection is handled by the extensible function perInput.
self': The flake self parameter, but with system pre-selected. This might trigger an infinite recursion (#22), so prefer config.
system: The system parameter, describing the architecture and platform of the host system (where the thing will run).

Equivalences

Getting the locally defined hello package on/for an x86_64-linux host:
- nix build #hello (assuming systems has x86_64-linux)
- config.packages.hello (if config is the perSystem module argument)
- allSystems."x86_64-linux".packages.hello (assuming systems has x86_64-linux)
- (getSystem "x86_64-linux").packages.hello)
- withSystem "x86_64-linux" ({ config, ... }: config.packages.hello)

Why so many ways?

Flakes counterintuitively handles system by enumerating all of them in attribute sets. flake-parts does not impose this restriction, but does need to support it.
flake-parts provides an extensible structure that is richer than the flakes interface alone.

How do I define my own flake output attribute?

Have a look at the source for some examples.

Whether directly or indirectly, you'll be defining an attribute inside the flake option.

If you want the attribute to be derived from perSystem you can start with packages.nix as an example, or formatter.nix if you need to do some filtering.

If you really don't care about your attribute, you may temporarily use transposition.<name>.adHoc = true to create and expose a perSystem option without merging support, type checking or documentation.

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