{
  # Makes outputs available on multiple platforms using a hand-rolled
  # `forAllSystems` helper (no extra dependencies).
  # Compare with the `flake-utils` approach described in notes/005-multi-system.md.
  description = "Multi-system outputs with forAllSystems";

  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
  };

  outputs = { self, nixpkgs, ... }:
    let
      # List every system you want to support.
      supportedSystems = [
        "x86_64-linux"
        "aarch64-linux"
        "aarch64-darwin"
        "x86_64-darwin"
      ];

      # Helper: apply a function to each system and collect results into an attrset.
      # `nixpkgs.lib.genAttrs` turns a list of keys + a function into { key = f key; … }.
      forAllSystems = f:
        nixpkgs.lib.genAttrs supportedSystems (system:
          f (import nixpkgs { inherit system; })
        );
    in {
      # Every output that varies per system goes through `forAllSystems`.
      packages = forAllSystems (pkgs: {
        default = pkgs.writeShellScriptBin "hello-multi" ''
          echo "hello from $(uname -s)/$(uname -m)"
        '';
      });

      devShells = forAllSystems (pkgs: {
        default = pkgs.mkShell {
          packages = with pkgs; [ jq ripgrep ];
          shellHook = ''
            echo "dev shell on $(uname -s)/$(uname -m)"
          '';
        };
      });

      # `nix flake check` evaluates everything under `checks`.
      # Wrapping a simple test here shows the pattern.
      checks = forAllSystems (pkgs: {
        runs = pkgs.runCommand "hello-multi-runs" {} ''
          ${self.packages.${pkgs.system}.default}/bin/hello-multi > $out
        '';
      });
    };
}