Evaluation failure using flakes and using nixpkgs release-20.03

[jsoo1]

If I use the 20.03 release, there is an evaluation error when building this system:

{
  inputs = {
    nixpkgs.url = "git+https://github.com/nixos/nixpkgs?ref=release-20.03";
    darwin.url = "github:LnL7/nix-darwin";
  };
  outputs = { self, darwin, nixpkgs, ... }: {
    darwinConfigurations.my-hostname = let system = "x86_64-darwin"; in
      darwin.lib.darwinSystem {
        inherit system;
        modules = [{ nixpkgs = import nixpkgs { inherit system; }; }];
      };
  };
}

{
  "nodes": {
    "darwin": {
      "inputs": {
        "nixpkgs": "nixpkgs"
      },
      "locked": {
        "lastModified": 1634994402,
        "narHash": "sha256-xmlCVVOYGpZoxgOqsDOVF0B0ASrnbNGVAEzID9qh2xo=",
        "owner": "LnL7",
        "repo": "nix-darwin",
        "rev": "44da835ac40dab5fd231298b59d83487382d2fab",
        "type": "github"
      },
      "original": {
        "owner": "LnL7",
        "repo": "nix-darwin",
        "type": "github"
      }
    },
    "nixpkgs": {
      "locked": {
        "lastModified": 1602411953,
        "narHash": "sha256-gbupmxRpoQZqL5NBQCJN2GI5G7XDEHHHYKhVwEj5+Ps=",
        "owner": "LnL7",
        "repo": "nixpkgs",
        "rev": "f780534ea2d0c12e62607ff254b6b45f46653f7a",
        "type": "github"
      },
      "original": {
        "id": "nixpkgs",
        "type": "indirect"
      }
    },
    "nixpkgs_2": {
      "locked": {
        "lastModified": 1625876338,
        "narHash": "sha256-pys6cebTmrZ7FHT4JUAFl4wk2IBff9aDE3hD6BcPYho=",
        "ref": "release-20.03",
        "rev": "eb73405ecceb1dc505b7cbbd234f8f94165e2696",
        "revCount": 216226,
        "type": "git",
        "url": "https://github.com/nixos/nixpkgs"
      },
      "original": {
        "ref": "release-20.03",
        "type": "git",
        "url": "https://github.com/nixos/nixpkgs"
      }
    },
    "root": {
      "inputs": {
        "darwin": "darwin",
        "nixpkgs": "nixpkgs_2"
      }
    }
  },
  "root": "root",
  "version": 7
}

$ nix build -L --show-trace .#darwinConfigurations.my-hostname.system
error: The option `nixpkgs.AAAAAASomeThingsFailToEvaluate' does not exist. Definition values:
       - In `<unknown-file>'

       … while evaluating 'evalModules'

       at /nix/store/bq10m17mq4si9m2ai5h06b9zc27bbn3r-source/lib/modules.nix:21:17:

           20|      evalModules) and the less declarative the module set is. */
           21|   evalModules = { modules
             |                 ^
           22|                 , prefix ? []

       … from call site

       at /nix/store/g7kjd5mkr6g2yydcrm1l8iih13gpwq0d-source/eval-config.nix:35:10:

           34|
           35|   eval = libExtended.evalModules (builtins.removeAttrs args [ "inputs" "system" ] // {
             |          ^
           36|     modules = modules ++ [ inputsModule pkgsModule ] ++ baseModules;

I would love to use nix-darwin but we have an old revision of nixpkgs we unfortunately have to use. If nix-darwin does not support 20.0x (or some other revision), it would be nice to know, at least.

[jsoo1]

(sorry for my tone, I totally forgot how experimental flakes are) I guess I am going to learn about nixos module internals now! :)

[jsoo1]

Looks like the issue is the deprecated check attribute of evalModules in nixpkgs. I'm not sure if this is a documentation issue, or something else. (see https://github.com/NixOS/nixpkgs/blob/9e8961e5607402ed0ff98e283ef485ac8f22e226/lib/modules.nix#L88)

Using 20.0x, adding the attribute to the darwin configuration fixed my issue. i.e.:

{
  inputs = {
    nixpkgs.url = "git+https://github.com/nixos/nixpkgs?ref=release-20.03";
    darwin.url = "github:LnL7/nix-darwin";
  };
  outputs = { self, darwin, nixpkgs, ... }: {
    darwinConfigurations.my-hostname = let system = "x86_64-darwin"; in
      darwin.lib.darwinSystem {
        inherit system;
        modules = [{ nixpkgs = import nixpkgs { inherit system; }; }];
        check = false;
      };
  };
}

[roberth]

The current expressions seems to be kind of geared towards flakes, but also not really using flakes as intended. Initializing pkgs is currently a mess in the whole ecosystem. I don't think any single module can do a good job at it. I think we should cut it out, but the next best thing is to use lib.mkForce. You could try adding a module { config._module.args.pkgs = lib.mkForce (import nixpkgs { system = "x86_64-darwin"; }); } for example.

[lilyball]

If you want to override nixpkgs from a flake, the correct way to do this is to pass it in as inputs. This would look like

{
  inputs = {
    nixpkgs.url = "git+https://github.com/nixos/nixpkgs?ref=release-20.03";
    darwin.url = "github:LnL7/nix-darwin";
  };
  outputs = { self, darwin, nixpkgs, ... }: {
    darwinConfigurations.my-hostname = let system = "x86_64-darwin"; in
      darwin.lib.darwinSystem {
        inherit system;
        modules = [ /* where's your config module? */ ];
        inputs = { inherit nixpkgs; };
      };
  };
}

This way your config.nixpkgs will continue to work as well.

You can simplify this slightly by just passing all of your inputs too, like so:

{
  inputs = {
    nixpkgs.url = "git+https://github.com/nixos/nixpkgs?ref=release-20.03";
    darwin.url = "github:LnL7/nix-darwin";
  };
  outputs = { self, darwin, nixpkgs, ... }@inputs: {
    darwinConfigurations.my-hostname = let system = "x86_64-darwin"; in
      darwin.lib.darwinSystem {
        inherit system inputs;
        modules = [ /* where's your config module? */ ];
      };
  };
}

[emilazy]

Hey @jsoo1, I was looking into why we have this option in eval-config.nix. Do you know how https://github.com/LnL7/nix-darwin/pull/397/commits/cb1b24cff1e5fc682785952127aa530bf9322d29 fixed this issue? It's unclear to me how adding an unused parameter to our eval-config.nix would stop the nixpkgs.AAAAAASomeThingsFailToEvaluate error.

[jsoo1]

Hm. I'm not sure anymore! I won't be mad if it gets removed. We have definitely moved on from 20.x releases and I am not sure it's worth supporting them any longer.