Using the nix overrides

The most convenient way to use the nix script is to make a new top level attribute for the overriden package set.

I achieve this using an overlay.

~/.config/nixpkgs/overlays.nix contains a list of overlays which augment the main package set. I just have one which I keep in my home directory for convenience.

[ (import ~/overlay.nix) ]

Then in ~/overlay.nix, I define a new attribute which contains the patch overrides.

self: super:
{
  # An attribute which contains the head overrides.
  patches = super.callPackage <local-path-to-overrides.nix>
              { patches = <path-to-patch-folder>; };


  # A modified package set intented to be used with ghcHEAD
  ghcHEAD = super.haskell.packages.ghcHEAD.override
  { overrides = sel: sup:
                  # The patches from the directory
                  ((super.callPackage self.patches { haskellLib = haskell.lib.compose; } sel sup)
                  # Any more local overrides you want.
                  // { mkDerivation = drv: sup.mkDerivation
                        ( drv // { jailbreak = true; doHaddock = false; });
                        generic-deriving = super.haskell.lib.dontCheck sup.generic-deriving;
                       });
                   };
}

The patches attribute is the file which generate-nix-overrides.hs creates. You need to set the correct paths to overrides.nix and the patch folder.

For example, if you have cloned head.hackage into your home directory, you would set them to ~/head.hackage/scripts/overrides.nix and ~/head.hackage/patches respectively. Having a local installation is desirable as when using head.hackage you have to commonly add new patches to patches.

Once you have setup this overlay, you can test your new attribute by trying to build a package.

nix-shell -p "ghcHEAD.ghcWithPackages (ps: [ps.primitive])"

and it might work, or it might not :). If it doesn't work, add a patch to your patches directory and try again.