Should overriding haskellPackages also override the cabal2nix version?

[wolverian]

I have a Haskell project that uses Cabal 3.8. Using the flake configuration from the README results in:

% nix build
trace: warning: @NixOS/haskell: Remove jailbreak for graphql
error: builder for '/nix/store/7hr7x6h9z814lslnvvy2piv543bzzf33-cabal2nix-mars.drv' failed with exit code 1;
       last 5 log lines:
       > *** cannot parse "/nix/store/znciz83i4x4nz23fw1qrkkmbyg7f5dh8-source/mars.cabal":
       > /nix/store/znciz83i4x4nz23fw1qrkkmbyg7f5dh8-source/mars.cabal:0:0: Unsupported cabal-version 3.8. See https://github.com/haskell/cabal/issues/4899.
       >
       > *** parsing cabal file: user error (*** Exiting.)
       > cabal2nix: nix-prefetch-url: createProcess: posix_spawnp: illegal operation (Inappropriate ioctl for device)
       For full logs, run 'nix log /nix/store/7hr7x6h9z814lslnvvy2piv543bzzf33-cabal2nix-mars.drv'.
(use '--show-trace' to show detailed location information)

Checking that running cabal2nix directly gives the same error:

% nix run nixpkgs#cabal2nix .
*** cannot parse "././mars.cabal":
mars.cabal:0:0: Unsupported cabal-version 3.8. See https://github.com/haskell/cabal/issues/4899.

*** parsing cabal file: user error (*** Exiting.)
cabal2nix: user error (*** Found neither a .cabal file nor package.yaml. Exiting.)

I then checked that the GHC 9.4.2 cabal2nix package works!

% nix run nixpkgs#haskell.packages.ghc942.cabal2nix .
{ mkDerivation, base, bytestring, github-webhooks, lib, servant
, servant-github-webhook, servant-server, text, wai, warp
}:
mkDerivation {
  pname = "<project name>";
  version = "0.1.0.0";
  src = ./.;
  isLibrary = false;
  isExecutable = true;
  executableHaskellDepends = [
    base bytestring github-webhooks servant servant-github-webhook
    servant-server text wai warp
  ];
  license = "unknown";
  mainProgram = "<main program>";
}

So I then set haskellPackages = pkgs.haskell.packages.ghc942 in my flake:

{
  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs/nixpkgs-unstable";
    flake-parts.url = "github:hercules-ci/flake-parts";
    haskell-flake.url = "github:srid/haskell-flake";
  };
  outputs = inputs@{ self, nixpkgs, flake-parts, ... }:
    flake-parts.lib.mkFlake { inherit self; } {
      systems = nixpkgs.lib.systems.flakeExposed;
      imports = [ inputs.haskell-flake.flakeModule ];
      perSystem = { self', pkgs, ... }: {
        haskellProjects.default = {
          name = "<project name>";  # assumes myproject.cabal
          root = ./.;
          haskellPackages = pkgs.haskell.packages.ghc942;
        };
      };
    };
}

I now expected that the build would work. However, running nix build still gives the same error about Cabal 3.8. Is my assumption that haskellPackages would override cabal2nix wrong?

[srid]

I'm not sure what's going on here. We do use callCabal2nixWithOptions on the final package set:

haskell-flake/flake-module.nix

Line 170 in 2ef06a7

package' = hp.callCabal2nixWithOptions cfg.name cfg.root "" { };

You could try #34, but that may not make any difference. If you can reproduce this problem on https://github.com/srid/haskell-multi-nix as well, I'd say this would be an issue with upstream nixpkgs.

[srid]

Wait, are you using the latest haskell-flake? We stopped using developPackage as of #23

[wolverian]

Sorry, I was just exploring there if the upstream tooling worked as I expected. I didn't mean to imply that haskell-flake calls developPackage.

[srid]

I see.

Now I'm not sure how to write in the Flake world that I want to override nixpkgs like this.

Since you are using the flake-parts framework, see the code change above srid/haskell-template#55 (comment)

[wolverian]

That was the next thing I would have tried, thank you! I'll just check it works as expected.

[wolverian]

Yep, looks like overriding _module.args.pkgs works to override the version of cabal2nix as well. 🎉