Patching in Nix

Today I wanted to move one of my Haskell projects to GHC 9.2.4 and found that envy didn't compile due to an upper bound on its dependency on bytestring, it didn't allow 0.11.*.

After creating a PR I decided I didn't want to wait for upstream so instead I started looking into options for patching the source of a derivation of a package from Hackage. In the past I've written about building Haskell packages from GitHub and an older one were I used callHackageDirect to build Haskell packages from Hackage. I wasn't sure how to patch up a package from Hackage though, but after a bit of digging through haskell-modules I found appendPatch.

The patch wasn't too hard to put together once I recalled the name of the patch queue tool I used regularly years ago, quilt. I put the resulting patch in the nix folder I already had, and the full override ended up looking like this

...
hl = haskell.lib;
hsPkgs = haskell.packages.ghc924;

extraHsPkgs = hsPkgs.override {
  overrides = self: super: {
    envy = hl.appendPatch (self.callHackageDirect {
      pkg = "envy";
      ver = "2.1.0.0";
      sha256 =
        "sha256-yk8ARRyhTf9ImFJhDnVwaDiEQi3Rp4yBvswsWVVgurg=";
    } { }) ./nix/envy-fix-deps.patch;
  };
};
...