{
  config,
  inputs,
  lib,
  localUsername ? lib.my.username,
  pkgs,
  this,
  ...
}:
with lib; let
  cfg = config.nixfiles.modules.emacs;
in {
  options.nixfiles.modules.emacs.enable = mkEnableOption "GNU Emacs";

  config = mkIf cfg.enable {
    secrets.authinfo = {
      file = "${inputs.self}/secrets/authinfo";
      owner = localUsername;
    };

    nixfiles.modules = {
      fonts.enable = true;
      git.client.enable = true;
      gnupg.enable = true;
    };

    hm = {
      xdg.configFile = {
        "doom/init.el".source = ./doom/init.el;
        "doom/packages.el".source = ./doom/packages.el;
        "doom/config.el" = {
          text = concatStringsSep "\n" [
            (let
              # NOTE gopls will require the "go" executable which must be provided
              # by the project's flake/shell.
              extraBins = with pkgs;
                [
                  (aspellWithDicts (p: with p; [en ru])) # :checkers (spell +aspell)
                  (python3.withPackages (p:
                    with p; [
                      black # :lang python :editor format
                      isort # :lang python
                      pyflakes # :lang python
                      python-lsp-server # :lang (python +lsp)
                    ]))
                  asmfmt # :editor format
                  bash-language-server # :lang (sh +lsp)
                  clang-tools # :lang (cc +lsp) :editor format
                  cmake # :term vterm
                  cmake-format # :lang cc :editor format
                  cmigemo # :lang japanese
                  css-language-server # :lang (web +lsp)
                  dhall-language-server # :lang (dhall +lsp)
                  dockerfile-language-server # :tools (docker +lsp)
                  editorconfig # :tools editorconfig
                  fd # doom!
                  gcc # :tools magit :term vterm
                  gnuplot # :lang (org +gnuplot)
                  gnutls # doom!
                  go-language-server # :lang (go +lsp)
                  gomodifytags # :lang go
                  gore # :lang go
                  gotests # :lang go
                  gotools # :lang go
                  graphviz # :lang (org +roam2) :lang plantuml
                  html-language-server # :lang (web +lsp)
                  html-tidy # :lang web
                  jre # :lang plantuml
                  json-language-server # :lang (json +lsp)
                  libtool # :term vterm
                  nix-language-server # :lang (nix +lsp)
                  nodePackages.eslint # :lang (json +lsp)
                  nodePackages.js-beautify # :lang web
                  nodePackages.prettier # :editor format
                  nodePackages.stylelint # :lang web
                  nodejs # :tools debugger
                  pandoc # :lang org markdown latex
                  perl # term vterm
                  pinentry-emacs # doom!
                  pre-commit # :tools magit
                  ripgrep # doom!
                  rust-analyzer # :lang (rust +lsp)
                  rustfmt # :lang rust
                  shellcheck # :lang sh
                  shfmt # :lang sh :editor format
                  sqlite # :lang (org +roam2) :tools lookup
                  texlab # lang (tex +lsp)
                  texlive.combined.scheme-full # :lang org tex
                  unzip # :tools debugger
                  wordnet # :tools (lookup +dictionary +offline)
                  yaml-language-server # :lang (yaml +lsp)
                  zls # :lang (zig +lsp)
                  zstd # :emacs undo
                ]
                ++ (
                  #
                  # GDB doesn't support[1] Apple Silicon on MacOS.
                  #
                  # [1]: https://inbox.sourceware.org/gdb/6b48224b-9e2e-518d-793b-df4fc5514884@arm.com/
                  if (this.system != "aarch64-darwin")
                  then [gdb] # :tools debugger
                  else [lldb] # :tools debugger
                )
                ++ optionals (!pkgs.stdenv.isDarwin)
                [
                  # NOTE Haskell is pretty much broken every couple of days on
                  # MacOS and I usually don't write anything in Haskell while
                  # I'm on my work laptop, so... ShellCheck seems to be working,
                  # though.
                  haskellPackages.ormolu # :lang haskell :editor format
                  haskellPackages.haskell-language-server # :lang (haskell +lsp)
                  haskellPackages.cabal-fmt # :lang haskell :editor format
                  haskellPackages.cabal-install # :lang haskell
                  haskellPackages.hoogle # :lang haskell
                ];
            in ''
              ;; This will integrate packages which are required by various
              ;; modules without polluting the user's profile.
              (setq exec-path (append exec-path '(${
                concatMapStringsSep " " (x: ''"${x}/bin"'') extraBins
              })))
              (setenv "PATH" (concat (getenv "PATH") ":${
                concatMapStringsSep ":" (x: "${x}/bin") extraBins
              }"))

              (appendq! auth-sources '("${config.secrets.authinfo.path}"))

              ;; Font must be set to N+2 because otherwise it looks too small.
              (setq doom-font (font-spec
                                :family "${config.fontScheme.monospaceFont.family}"
                                :size ${toString (config.fontScheme.monospaceFont.size + 2)})
                    doom-unicode-font doom-font)

              ;; :app irc
              (setq circe-default-nick "${my.username}"
                    circe-default-realname "${my.email}"
                    circe-default-user circe-default-nick)

              ;; :lang plantuml
              (setq org-plantuml-jar-path "${pkgs.plantuml}/lib/plantuml.jar")

              ;; :input japanese
              (setq migemo-dictionary "${pkgs.cmigemo}/share/migemo/utf-8/migemo-dict")

              ;; :input japanese
              (setq skk-large-jisyo "${pkgs.skk-dicts}/share/skk/SKK-JISYO.L")

              ;; :lang nix
              (setq nix-nixfmt-bin "${pkgs.writeShellScript "nixfmt" ''
                ${pkgs.alejandra}/bin/alejandra --quiet "$@"
              ''}")
            '')
            (with config.hm.accounts.email; let
              mu4eAccounts = let
                muAccounts = filter (a: a.mu.enable) (attrValues accounts);
              in
                concatMapStringsSep "\n" (a:
                  with a; let
                    personalAddresses = concatMapStringsSep " " (v: ''"${v}"'') aliases;
                  in ''
                    (set-email-account! "${name}"
                      '((user-full-name        . "${realName}")
                        (user-mail-address     . "${address}")
                        (mu4e-inbox-folder     . "/${name}/${folders.inbox}")
                        (mu4e-sent-folder      . "/${name}/${folders.sent}")
                        (mu4e-drafts-folder    . "/${name}/${folders.drafts}")
                        (mu4e-trash-folder     . "/${name}/${folders.trash}")
                        (mu4e-refile-folder    . "/${name}/Archive")
                        (+mu4e-personal-addresses (list ${personalAddresses})))
                      t)
                  '')
                muAccounts;
            in ''
              (setq mu4e-root-maildir "${maildirBasePath}")

              ${mu4eAccounts}
            '')
            (builtins.readFile ./doom/config.el)
          ];
          onChange = with config.hm.programs; ''
            export DOOMDIR="$HOME/.config/doom"
            export EMACSDIR="$HOME/.config/emacs"

            if [[ ! -d "$EMACSDIR/.git" ]]; then
              ${git.package}/bin/git clone --depth=1 --branch=master \
                "https://github.com/doomemacs/doomemacs" "$EMACSDIR"
            fi

            if [[ ! -d "$DOOMDIR" ]]; then
              mkdir -p "$DOOMDIR/snippets"
            fi

            if [[ -x "$EMACSDIR/bin/doom" ]]; then
              oldpath="$PATH"
              export PATH="''${PATH:-/bin}:${emacs.package}/bin:${git.package}/bin"

              "$EMACSDIR/bin/doom" sync -e -p --force --verbose

              export PATH="$oldpath"
              unset oldpath
            fi
          '';
        };
      };

      programs.emacs = {
        enable = true;
        package = pkgs.emacs28; # Pin to avoid surprises.
      };
    };
  };
}