{
  description = "Calculator with exact arithmetic, unit conversion and a programmer mode, as a CLI and TUI";

  inputs.nixpkgs.url = "nixpkgs/master";

  outputs = { self, nixpkgs }:
    let
      supportedSystems = [ "x86_64-linux" "aarch64-linux" "x86_64-darwin" "aarch64-darwin" ];

      # Helper to generate an attrset '{ x86_64-linux = f "x86_64-linux"; ... }'.
      forAllSystems = nixpkgs.lib.genAttrs supportedSystems;
    in
    {
      packages = forAllSystems (system:
        let
          pkgs = nixpkgs.legacyPackages.${system};
          tally = pkgs.callPackage ./nix/package.nix {
            # Pinned deliberately rather than tracking `pkgs.zig`: build.zig.zon
            # declares 0.16.0 as the minimum, and 0.16 was a large standard
            # library refactor, so a newer default would not build unchanged.
            zig = pkgs.zig_0_16;
            # The git-tracked tree, which keeps .zig-cache, zig-out, zig-pkg and
            # the Android build outputs out of the derivation.
            src = self;
          };
        in
        {
          inherit tally;
          default = tally;
        });

      devShells = forAllSystems (system:
        let pkgs = nixpkgs.legacyPackages.${system};
        in
        {
          # `nix develop` for the CLI/TUI without mise. The versions here are the
          # same ones .mise.toml pins; the Android toolchain stays with mise.
          default = pkgs.mkShell {
            packages = [
              pkgs.zig_0_16
              pkgs.zls
            ];
          };
        });
    };
}