sample flake.nix for emacs projects

.envrc

use flake
eval "$shellHook"

flake.nix

{
  description = "codex dev environment";

  inputs = {
    nixpkgs.url = github:nixos/nixpkgs/21.11;
    flake-utils.url = github:numtide/flake-utils;

    multi-emacs.url = github:sigma/multi-emacs;
    multi-emacs.inputs.nixpkgs.follows = "nixpkgs";
    multi-emacs.inputs.flake-utils.follows = "flake-utils";
  };

  outputs = { self, nixpkgs, flake-utils, multi-emacs }:
    flake-utils.lib.eachDefaultSystem (system: {
      devShell =
        let
          pkgs = import nixpkgs {
            inherit system;
            overlays = [
              multi-emacs.overlay
            ];
          };
        in
          pkgs.devshell.mkShell {
            packages = [
              (pkgs.multiEmacs {
               wrapper = emacs: pkgs.emacsWithPackagesFromPackageRequires {
                  package = emacs;
                  packageElisp = builtins.readFile ./codex.el;
                };
                flavors = [
                  pkgs.emacs-25-1 pkgs.emacs-25-2 pkgs.emacs-25-3
                  pkgs.emacs-26-1 pkgs.emacs-26-2 pkgs.emacs-26-3
                  pkgs.emacs-27-1 pkgs.emacs-27-2
                  pkgs.emacs-snapshot
                ];
              })
            ];
          };
    });
}