[Nix-dev] haskell: overriding ghc784 packages?

[William Casarin]

Hey Peter,

In the Haskell infrastructure guide[1], it describes how to override
haskellPackages to add additional packages to the package set:

    {
      packageOverrides = super: let self = super.pkgs; in
      {
        haskellPackages = super.haskellPackages.override {
          overrides = self: super: {
            foo = self.callPackage ../src/foo {};
          };
        };
      };
    }

Is there any way to do this for ghc784 packages? I tried:

    {
      packageOverrides = super: let self = super.pkgs; in
      {
        haskell.packages.ghc784 = super.haskell.packages.ghc784.override {
          overrides = self: super: {
            foo = self.callPackage ../src/foo {};
          };
        };
      };
    }

and it works when I nix-shell into my project, but it seems to break nixpkgs.
For example when I run:

    nix-env -qaP

it fails with:

    error: attribute ‘ghc7101’ missing, at
/home/jb55/nixpkgs/pkgs/top-level/all-packages.nix:3980:21

Leading me to believe I'm not doing this right. I don't get this error when
overriding haskellPackages.

Any ideas?

Cheers,
Bill

[1] http://hydra.nixos.org/build/24422016/download/1/nixpkgs/manual.html#how-to-build-projects-that-depend-on-each-other