How to use a package revision not yet in the published nixpkgs (but described in the development branches of nixpkgs)?

I'd like to use the new release of ghc (7.10, because of a new feature -- static ptr), and I'm using a single-user nix environment. I don't find the wanted new release in the published lists at ; e.g., the latest ghc's revision points to (it's pre-7.10; but what's the point in using a pre-release, if a stable 7.10 has been released already since then, 2014-12). But that's the release branch of nixpkgs (release-14.12). Looking into other branches, I can find nix expressions for newer ghc: * master branch of nixpkgs: * staging: (Both master and staging revisions of ghc/7.10.1.nix are identical.) How do I install them into my nix environment (perhaps, with re-compilation)?