Is there a better way to override a library in Haskell.nix?

default.nix

let 

    src = import ./sources.nix;

    hnSrc = src."haskell.nix";

    pinnedNixpkgsSrc = "${hnSrc}/nixpkgs";

    hnArgsOrig = import hnSrc;

    hnArgs = hnArgsOrig // { 
        nixpkgs-pin = "release-19.03"; 

	# DESIGN: exposing an alternative "cabal-doctest" library in Nixpkgs.
	#
	overlays = hnArgsOrig.overlays ++ [(self: super: {
            cabal-doctest = cabal-doctest-alt;
        })];

    };

    pkgs = import pinnedNixpkgsSrc hnArgs;

    hn = (import pinnedNixpkgsSrc hnArgs).haskell-nix;

    # DESIGN: my alternative cabal-doctest Haskell library
    #
    cabal-doctest-alt = (hn.cabalProject { 
        name = "cabal-doctest-alt";
        src = src.cabal-doctest; 
    }).cabal-doctest.components.library;

    plan = hn.importAndFilterProject 
        (hn.callCabalProjectToNix { 
            name = "exceptions-checked";
            src = ../.; 
            index-state = "2019-12-26T01:32:20Z";
            index-sha256 = "1b9956ypjb65sigpwqiy6ylbk15mngg61h5a6q3panwalycnsbsg";
        });

    # DESIGN: I'm removing the reference to "cabal-doctest" from the plan, and
    # it then finds it in the Nixpkgs overlay above.
    #
    modifiedPlan = plan.pkgs // {
        pkgs = hackage: {
            packages = builtins.removeAttrs (plan.pkgs.pkgs hackage).packages ["cabal-doctest"];
            compiler = (plan.pkgs.pkgs hackage).compiler;
        };
    }; 

    pkgSet = hn.mkCabalProjectPkgSet { 
        plan-pkgs = modifiedPlan;
        modules = [({config, ...}: { 
            reinstallableLibGhc = true; 
            packages.exceptions-checked.components.tests.doctests = {
                extraSrcFiles = [ "test" ];
                preCheck = ''
                    ghc-pkg init dist/package.conf.inplace
                '';
                preConfigure = ''
                    substituteInPlace test/doctests.hs \
                        --replace "B.flags"          "B.flags_exe_doctests" \
                        --replace "B.pkgs"           "B.pkgs_exe_doctests"  \
                        --replace "B.module_sources" "B.module_sources_exe_doctests"
                '';
            };
        })];
    };

in { inherit hn pkgs pkgSet plan; }

This is not terribly different from most Haskell.nix example usages. The main thing I'm trying to do is override a dependency with custom one I'm maintaining outside of Hackage (for a moment).

In Cabal, I'd do this with the following stanza:

source-repository-package
    type: git
    location: https://github.com/shajra/cabal-doctest.git
    tag: 94cf2a01ffd734ab89a041f1ba5073d2d3640d9a 

That's pretty convenient. But I don't think plan-to-nix knows what to do with this stanza. So I got the code above to work, but it feels like an uncomfortable hack. I feel there should a better way with Haskell.nix.

I'm modifying the auto-generated plan to not include cabal-doctest, and then when it's not found, I slip in the alternative library as a Haskell overlay. There's "DESIGN:" comments above in the critical places I do this.

I really appreciate a recommendation on a better way to go. I'd expect that for something as useful as overriding a dependency that there's a better way to do it in Haskell.nix.

I made a thing that solves the problem: https://github.com/ilyakooo0/haskell-nix-extra-hackage