cabal2nix prevents `nix flake show` (IFD)

[spikespaz]

The formatter is specified like this:

{
  inputs = {
    nixpkgs-lib.url = "github:nix-community/nixpkgs.lib";
    systems.url = "github:nix-systems/default";
    nixfmt.url = "github:serokell/nixfmt";
  };
  outputs = { self, nixpkgs-lib, systems, nixfmt }:
    let
      inherit (self.lib) lib;
      eachSystem = lib.genAttrs (import systems);
    in {
      # ...
      formatter = eachSystem (system: nixfmt.packages.${system}.default);
    };
}

When you run nix flake show:

warning: formatter.aarch64-darwin omitted (use '--all-systems' to show)
warning: formatter.aarch64-linux omitted (use '--all-systems' to show)
warning: formatter.x86_64-darwin omitted (use '--all-systems' to show)
error:
       … while evaluating the attribute 'packages.x86_64-linux.default'

         at /nix/store/wfjzvlbfj7ccczbphcbd16ncgxs2bj24-source/flake.nix:73:11:

           72|         packages = rec {
           73|           default = nixfmt;
             |           ^
           74|           nixfmt = pkgs.haskellPackages.nixfmt;

       … while evaluating the attribute 'haskellPackages.nixfmt'

         at /nix/store/wfjzvlbfj7ccczbphcbd16ncgxs2bj24-source/flake.nix:31:15:

           30|             packageOverrides = self: super: {
           31|               nixfmt = self.callCabal2nix "nixfmt" src { };
             |               ^
           32|             };

       (stack trace truncated; use '--show-trace' to show the full trace)

       error: cannot build '/nix/store/770ly489hm8i079paqjq2153c0izx7k6-cabal2nix-nixfmt.drv' during evaluation because the option 'allow-import-from-derivation' is disabled

[infinisil]

Unfortunately not really fixable nicely here, this is a Nix bug: https://github.com/NixOS/nix/issues/4265

[yorickvP]

We could commit the cabal2nix output..

[infinisil]

Yeah that would work, it's just not nice, since it requires manual updates (or setting up automated ones). I don't think that's worth it for a bug that should really be fixed in Nix.

[spikespaz]

For the sake of having this formatter be adopted by anyone, I wouldn't be waiting for Nix to solve this particular issue for us. It's been an issue for quite some time now, and I can't say "run nix flake show to see what this provides" and instead I have to say "use this complicated Nix eval piped to sed".

[spikespaz]

Also prevents nix flake check do that you have to come up with something other than checking the status code in an integration script.

[SuperSandro2000]

I would also like to see IFD not being fully required as it prevents usage in hydra.

[infinisil]

Fair enough, let's reconsider!

[yorickvP]

Note that this package is already in nixpkgs without IFD via hackage

[remi-gelinas]

This is actually blocking my ability to use nix fmt as well - specifying the nixfmt package here: https://github.com/remi-gelinas/rosetta/blob/trunk/flake.nix#L45

Results in nix fmt failing. Targeting the aarch64-darwin package on an M1 machine, it still attempts to evaluate the formatters for other systems, and encounters IFD as aarch64-darwin can't build the cabal2nix package for x86_64-linux.

Not sure if this specifically is a Nix issue or a nixfmt issue, or one that I can work around, but it's really blocking my attempt to adopt nixfmt :/

[infinisil]

@remi-gelinas Hmm I just tried it on an aarch64-darwin machine, but couldn't reproduce the problem. I also don't see why it would need to build anything for x86_64-linux. Can you share exactly how you're getting the problem along with the error message?

[remi-gelinas]

I just did an isolated reproduction attempt with a barebones flake like so:

{
  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs?rev=c4ce542c774aa505fd80d86d9d5de51349328e43";
    nixfmt.url = "github:NixOS/nixfmt?rev=3bcb63c13e7aaf0b8e14081cf0c14c44f62e840a";
  };
  outputs = {nixpkgs, nixfmt, ...}: let
        supportedSystems = [
        "aarch64-darwin"
        "x86_64-linux"
      ];

      forAllSystems = nixpkgs.lib.genAttrs supportedSystems;
  in {
    formatter = forAllSystems (system: nixfmt.packages.${system}.nixfmt);
  };
}

and it works correctly, totally as expected. I am assuming the problem lies somewhere in my usage of flake-parts to generate the flake options then. Seems to not be an actual issue! That being said, I do still have to explicitly allow IFD for flake operations to work as expected, so this issue is still a problem