To put in ~/.config/nixpkgs/

config.nix

{
  packageOverrides = ps: rec {
    haskell.compiler = ps.haskell.compiler // {
      myghc = ps.haskell.compiler.ghcHEAD.overrideDerivation (p: {
        src = fetchgit {
          url = "git://git.haskell.org/ghc.git";
          rev = "commit hash here";
          sha256 = "sha that nix computes for this";
        };
      });
    };

    haskell.packages = ps.haskell.packages // {
      myghc = ps.haskell.packages.ghcHEAD.override {
    	  ghc = haskell.compiler.myghc;
    	};
    };
  };
}