~/.nixpkgs/config.nix

pkgs : {
  cabal.libraryProfiling = true;
}

{
  packageOverrides = self : { haskellPackages_ghc741_test = {
      all = (self.haskellPackages_ghc741.override {
        extraPrefs = super : { transformers = super.transformers_0_2_2_0; };
      }).ghcWithPackages (self : [
        self.mtl
      ]);
    };
  };
}


-----------
$> nix-env -i haskellPackages_ghc741_test --show-trace
error: while evaluating the function at `/nix/store/p4lzfb8xqy260xchbynsrb10gn82p8si-nixos-0.1pre3914_bce1cdd-702854c/nixos/nixpkgs/pkgs/top-level/all-packages.nix:8:1':
while evaluating the function at `/nix/store/p4lzfb8xqy260xchbynsrb10gn82p8si-nixos-0.1pre3914_bce1cdd-702854c/nixos/nixpkgs/pkgs/top-level/all-packages.nix:108:26':
while evaluating the function at `/nix/store/p4lzfb8xqy260xchbynsrb10gn82p8si-nixos-0.1pre3914_bce1cdd-702854c/nixos/nixpkgs/pkgs/top-level/all-packages.nix:126:19':
while evaluating the function at `/nix/store/p4lzfb8xqy260xchbynsrb10gn82p8si-nixos-0.1pre3914_bce1cdd-702854c/nixos/nixpkgs/pkgs/top-level/all-packages.nix:83:25':
while evaluating the function at `/nix/store/p4lzfb8xqy260xchbynsrb10gn82p8si-nixos-0.1pre3914_bce1cdd-702854c/nixos/nixpkgs/pkgs/lib/attrsets.nix:19:35':
while evaluating the builtin function `hasAttr':
while evaluating the function at `/home/dmalikov/.nixpkgs/config.nix:1:1':
attempt to call something which is neither a function nor a primop (built-in operation) but an attribute set