Flake example

flake.nix

{
  inputs.flake-utils.url = "github:numtide/flake-utils";
  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";

  outputs = { self, nixpkgs, flake-utils }:

    let
      inherit (nixpkgs) lib;

      # Add the flake's overlay to each of the pkgs
      eachDefaultEnvironment = f: flake-utils.lib.eachDefaultSystem
        (
          system:
          f {
            inherit system;
            pkgs = (import nixpkgs { inherit system; config.allowUnfree = true; }).extend self.overlay;
          }
        );

    in
    eachDefaultEnvironment
      ({ pkgs, system }: {

        devShell = import ./shell.nix ({ inherit pkgs; } // self.packages."${system}");

        packages = rec {
          py-hello = pkgs.python3Packages.py-hello;
        };

        defaultPackage = (self.packages."${system}").py-hello;

      }) // {

        overlay = import ./overlay.nix self.inputs;
        checks = self.packages;

      };
}

overlay.nix

inputs: final: prev:
{

  # How it's done for python
  python3 = prev.python3.override {
    packageOverrides = pythonFinal: pythonSelf: {
      py-hello = final.runCommand "py-hello" { } ''
        ${final.hello}/bin/hello
        # presumably you need python for something here...
        mkdir $out
      '';
    };
  };

  # How it's done for haskell
  # haskell = prev.haskell // {
  #   packageOverrides = haskellFinal: haskellPrev: {
  #     lens = haskellFinal.lens_5_0_1;
  #     rel8 = haskellFinal.callCabal2nix "rel8" "${inputs.rel8}" { };
  #     hs-opentelemetry-api = haskellFinal.callCabal2nixWithOptions "hs-opentelemetry" "${inputs.hs-opentelemetry}" "--subpath api" { };
  #     thread-utils-context = haskellFinal.callCabal2nixWithOptions "thread-utils-context" "${inputs.thread-utils-context}" "" { };
  #     thread-utils-finalizers = haskellFinal.callCabal2nixWithOptions "thread-utils-finalizers" "${inputs.thread-utils}" "--subpath thread-utils-finalizers" { };
  #   };
  # };

}

shell.nix

{ pkgs
, py-hello
, ...
}:

pkgs.mkShell {
  buildInputs = with pkgs; [ py-hello ];
}