Pass system explicitly to allow pure evaluation

[akirak]

Hello,

Since adoption of niv for tracking sources in nix-emacs-ci (16c43b7), I cannot use the overlay in a Nix flake.

The error message is as follows:

error: attribute 'currentSystem' missing
at /nix/store/2iz88y66ln6yfhr4k2nlr50535g64fz2-source/nix/sources.nix:165:16:
164| , sources ? if isNull sourcesFile then {} else builtins.fromJSON (builtins.readFile sourcesFile)
165| , system ? builtins.currentSystem
| ^
166| , pkgs ? mkPkgs sources system
(use '--show-trace' to show detailed location information)

This is because builtins.currentSystem is not allowed in pure evaluation mode, which is the default in flakes.

To prevent this error, you can pass the system explicitly, as in this PR. This API seems to be safe to use (see nmattia/niv#134 (comment)), and even it it is not, you can easily rollback the change.

To reproduce the error, you can create the following flake.nix in a Git repository:

{
  description = "Minimal flake project";

  inputs.emacs-ci = {
    url = "github:purcell/nix-emacs-ci";
    flake = false;
  };

  outputs = {
    self,
    nixpkgs,
    flake-utils,
    emacs-ci,
  }:
    flake-utils.lib.eachDefaultSystem
    (system: let
      pkgs = import nixpkgs {
        inherit system;
        overlays = [
          (import (emacs-ci + "/overlay.nix"))
        ];
      };
    in {
      packages = flake-utils.lib.flattenTree {
        default = pkgs.emacs-snapshot;
      };
    });
}

When you run nix build, it throws the error.

When you run the following command, it successfully builds:

nix build --override-input emacs-ci github:akirak/nix-emacs-ci/pure-eval

[purcell]

Makes sense, thanks! Sorry it broke things for you. I'd like to find time to play with Flakes more soon (I have lots of colleagues at Tweag who can help me).

[akirak]

Thank you for merging this PR.

I have lots of colleagues at Tweag who can help me

That's fantastic. I read several blog posts from Tweag. I may possibly apply for one of those FP-embracing companies in the future, but not now.