depot/third_party/nixpkgs/doc/packages/emacs.section.md

Emacs

Configuring Emacs

The Emacs package comes with some extra helpers to make it easier to configure. emacs.pkgs.withPackages allows you to manage packages from ELPA. This means that you will not have to install that packages from within Emacs. For instance, if you wanted to use company counsel, flycheck, ivy, magit, projectile, and use-package you could use this as a ~/.config/nixpkgs/config.nix override:

{
  packageOverrides = pkgs: with pkgs; {
    myEmacs = emacs.pkgs.withPackages (epkgs: (with epkgs.melpaStablePackages; [
      company
      counsel
      flycheck
      ivy
      magit
      projectile
      use-package
    ]));
  };
}

You can install it like any other packages via nix-env -iA myEmacs. However, this will only install those packages. It will not configure them for us. To do this, we need to provide a configuration file. Luckily, it is possible to do this from within Nix! By modifying the above example, we can make Emacs load a custom config file. The key is to create a package that provides a default.el file in /share/emacs/site-start/. Emacs knows to load this file automatically when it starts.

{
  packageOverrides = pkgs: with pkgs; rec {
    myEmacsConfig = writeText "default.el" ''
      (eval-when-compile
        (require 'use-package))

      ;; load some packages

      (use-package company
        :bind ("<C-tab>" . company-complete)
        :diminish company-mode
        :commands (company-mode global-company-mode)
        :defer 1
        :config
        (global-company-mode))

      (use-package counsel
        :commands (counsel-descbinds)
        :bind (([remap execute-extended-command] . counsel-M-x)
               ("C-x C-f" . counsel-find-file)
               ("C-c g" . counsel-git)
               ("C-c j" . counsel-git-grep)
               ("C-c k" . counsel-ag)
               ("C-x l" . counsel-locate)
               ("M-y" . counsel-yank-pop)))

      (use-package flycheck
        :defer 2
        :config (global-flycheck-mode))

      (use-package ivy
        :defer 1
        :bind (("C-c C-r" . ivy-resume)
               ("C-x C-b" . ivy-switch-buffer)
               :map ivy-minibuffer-map
               ("C-j" . ivy-call))
        :diminish ivy-mode
        :commands ivy-mode
        :config
        (ivy-mode 1))

      (use-package magit
        :defer
        :if (executable-find "git")
        :bind (("C-x g" . magit-status)
               ("C-x G" . magit-dispatch-popup))
        :init
        (setq magit-completing-read-function 'ivy-completing-read))

      (use-package projectile
        :commands projectile-mode
        :bind-keymap ("C-c p" . projectile-command-map)
        :defer 5
        :config
        (projectile-global-mode))
    '';

    myEmacs = emacs.pkgs.withPackages (epkgs: (with epkgs.melpaStablePackages; [
      (runCommand "default.el" {} ''
         mkdir -p $out/share/emacs/site-lisp
         cp ${myEmacsConfig} $out/share/emacs/site-lisp/default.el
       '')
      company
      counsel
      flycheck
      ivy
      magit
      projectile
      use-package
    ]));
  };
}

This provides a fairly full Emacs start file. It will load in addition to the user's personal config. You can always disable it by passing -q to the Emacs command.

Sometimes emacs.pkgs.withPackages is not enough, as this package set has some priorities imposed on packages (with the lowest priority assigned to GNU-devel ELPA, and the highest for packages manually defined in pkgs/applications/editors/emacs/elisp-packages/manual-packages). But you can't control these priorities when some package is installed as a dependency. You can override it on a per-package-basis, providing all the required dependencies manually, but it's tedious and there is always a possibility that an unwanted dependency will sneak in through some other package. To completely override such a package, you can use overrideScope.

let
  overrides = self: super: rec {
    haskell-mode = self.melpaPackages.haskell-mode;
    # ...
  };
in
((emacsPackagesFor emacs).overrideScope overrides).withPackages
  (p: with p; [
    # here both these package will use haskell-mode of our own choice
    ghc-mod
    dante
  ])

}