shell.nix

let
  myHaskellPackageOverlay = self: super: {
    haskellPackages = super.haskellPackages.override {
      overrides = hself: hsuper: {
        random =
          let newRandomSrc = builtins.fetchGit {
                url = "file:///Users/dom/random";
                rev = "85a6dd013283437c05ab99498fc2f7351d5c504a";
                ref = "interface-to-performance";
              };
          in
          # Since cabal2nix has a transitive dependency on random, we need to
          # get the callCabal2nix function from the normal haskellPackages that
          # is not being overridden.
          (import <nixpkgs> {}).haskell.lib.dontCheck ((import <nixpkgs> {}).haskellPackages.callCabal2nix "random" newRandomSrc { });
      };
    };
  };

  noCheckOvelay = self: super: {
    mkDerivation = args: super.mkDerivation (args // {
      doCheck = false;
    });
  };

  activeOverlays = [ noCheckOvelay ];

  foo = builtins.foldl' (pkgs: overlay: pkgs.extend overlay) nixpkgs.haskellPackages activeOverlays;

  nixpkgs = import <nixpkgs> { overlays = [ myHaskellPackageOverlay ]; };
in
# This example derivation file is assumed to be in the current directory.  "./default.nix" is
# the current file you are looking at.
import ./default.nix {
  reverseDepsOf = "random";
  inherit nixpkgs;
}