{ pkgs, depot, ... }:

let
  inherit (builtins) path;
  inherit (depot.third_party.nixpkgs) emacsPackagesGen emacs27;
  inherit (pkgs) writeShellScript writeShellScriptBin;
  inherit (pkgs.lib.strings) concatStringsSep makeBinPath;

  emacsBinPath = makeBinPath (with pkgs; [
    ripgrep
    bat
    fd
    fzf
    pass
    tokei
    nmap
    tldr
    diskus
    jq
    pup
    exa
    gitAndTools.hub
    kubectl
    google-cloud-sdk
    xsv
    scrot
    clipmenu
    xorg.xset
    direnv
    nix
  ]);

  emacsWithPackages = (emacsPackagesGen emacs27).emacsWithPackages;

  wpcarrosEmacs = emacsWithPackages (epkgs:
    (with epkgs.tvlPackages; [
      tvl
    ]) ++

    (with epkgs.elpaPackages; [
      exwm
    ]) ++

    (with epkgs.melpaPackages; [
      org-bullets
      sly
      notmuch
      elm-mode
      ts
      vterm
      base16-theme
      password-store
      clipmon # TODO: Prefer an Emacs client for clipmenud.
      evil
      evil-collection
      evil-commentary
      evil-surround
      key-chord
      add-node-modules-path # TODO: Assess whether or not I need this with Nix.
      web-mode
      rjsx-mode
      tide
      prettier-js
      flycheck
      diminish
      doom-themes
      telephone-line
      which-key
      all-the-icons
      all-the-icons-ivy
      ivy
      ivy-pass
      ivy-prescient
      restclient
      package-lint
      parsec
      magit-popup
      direnv
      alert
      nix-mode
      racer
      rust-mode
      rainbow-delimiters
      racket-mode
      lispyville
      elisp-slime-nav
      py-yapf
      reason-mode
      elixir-mode
      go-mode
      company
      markdown-mode
      refine
      deferred
      magit
      request
      pcre2el
      helpful
      exec-path-from-shell # TODO: Determine if Nix solves this problem.
      yasnippet
      projectile
      deadgrep
      counsel
      counsel-projectile
      engine-mode # TODO: Learn what this is.
      eglot
      dap-mode
      lsp-ui
      suggest
      paradox
      flymake-shellcheck
      fish-mode
      tuareg
      haskell-mode
      lsp-haskell
      use-package
      general
      clojure-mode
      cider
      f
      dash
      company
      counsel
      flycheck
    ]));

  vendorDir = path {
    path = ./.emacs.d/vendor;
    name = "emacs-vendor";
  };

  # TODO: byte-compile these by packaging each as an Elisp library.
  wpcDir = path {
    path = ./.emacs.d/wpc;
    name = "emacs-libs";
  };

  wpcPackageEl = path {
    path = ./.emacs.d/wpc/wpc-package.el;
    name = "wpc-package.el";
  };

  initEl = path {
    path = ./.emacs.d/init.el;
    name = "init.el";
  };

  loadPath = concatStringsSep ":" [
    wpcDir
    vendorDir
    # TODO: Explain why the trailing ":" is needed.
    "${wpcarrosEmacs.deps}/share/emacs/site-lisp:"
  ];

  withEmacsPath = { emacsBin, briefcasePath ? "$HOME/briefcase" }:
    writeShellScriptBin "wpcarros-emacs" ''
      export XMODIFIERS=emacs
      export BRIEFCASE=${briefcasePath}
      export GOOGLE_BRIEFCASE="$HOME/google-briefcase"
      export PATH="${emacsBinPath}:$PATH"
      export EMACSLOADPATH="${loadPath}"
      exec ${emacsBin} \
        --debug-init \
        --no-init-file \
        --no-site-file \
        --no-site-lisp \
        --load ${initEl} \
        "$@"
    '';
in {
  inherit initEl withEmacsPath;

  # I need to start my Emacs from CI without the call to `--load ${initEl}`.
  runScript = { script, briefcasePath }:
    writeShellScript "run-emacs-script" ''
      export BRIEFCASE=${briefcasePath}
      export PATH="${emacsBinPath}:$PATH"
      export EMACSLOADPATH="${wpcDir}:${vendorDir}:${wpcarrosEmacs.deps}/share/emacs/site-lisp"
      exec ${wpcarrosEmacs}/bin/emacs \
        --no-site-file \
        --no-site-lisp \
        --no-init-file \
        --script ${script} \
        "$@"
    '';

  # Use `nix-env -f '<briefcase>' emacs.nixos` to install `wpcarros-emacs` on
  # NixOS machines.
  nixos = { briefcasePath ? "$HOME/briefcase" }: withEmacsPath {
    inherit briefcasePath;
    emacsBin = "${wpcarrosEmacs}/bin/emacs";
  };
}