Overriding a Haskell package dependency version

[wolverian]

I'm trying to use GHC 9.4.2 to compile my project, i.e. haskellPackages = pkgs.haskell.packages.ghc942;. However, at the moment, nixpkgs is giving me the version 0.5.1 of hspec-contrib, which doesn't compile on 9.4.2, so I'd like to override its version to 0.5.1.1, which should compile. I've tried:

        haskellProjects.default = {
          root = ./.;
          haskellPackages = pkgs.haskell.packages.ghc942;
          source-overrides = {
            hspec-contrib = "0.5.1.1";
          };
        };

However, this gives me the following error:

$ nix develop
[...]
error: A definition for option `perSystem.aarch64-darwin.haskellProjects.default.source-overrides.hspec-contrib' is not of type `path'. Definition values:
       - In `<unknown-file>, via option perSystem': "0.5.1.1"
(use '--show-trace' to show detailed location information)

Any clue how to fix this? Thanks!

[wolverian]

BTW, this is particularly confusing because my cabal file doesn't depend on hspec or hspec-contrib. The only dependency is base.

[srid]

Note to self: I should explain source-overrides in https://srid.ca/haskell-flake/dependency

Unlike overrides, when using source-overrides you are required to specify a Nix path value. This could be a flake input (with flake = false;); see "ema", "heist-extra" and "tailwind" in https://github.com/EmaApps/emanote/blob/master/flake.nix as an example.

That said, if you are trying to use a specific version on Hackage, you should instead use overrides and callHackage:

haskellProjects.default = 
  overrides = self: super: {
    hspec-contrib = self.callHackage "hspec-contrib" "0.5.1.1" {};
  };
};

By the way, you can also use this overlay directly if you want GHC 9.4 support: https://horizon-haskell.net/ ... see srid/haskell-template#55

[wolverian]

Thank you, this clarifies things. I was confused by the documentation of source-overrides in Nixpkgs Haskell code I think.

[wolverian]

Unfortunately Horizon doesn't provide aarch64-darwin packages, which are what I'm after.

[srid]

cc @locallycompact