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Spotlight: use-package, a declarative configuration tool

Have you ever heard of use-package? It’s a declarative way of expressing package configuration in Emacs but without the tears. It’s written by the inimitable John Wiegley, the new GNU Emacs lead maintainer and author of many, many cool tools like a commandline ledger, Emacs’s Eshell, and much more.

It’s fair to say most of us have declared .emacs bankruptcy at least once. It comes about when the passage of time and your ossified hacks-upon-hacks conspire against you, and a fresh Emacs instance fails to start cleanly.

Even in a post-hack world of package managers, starter kits, and sane defaults, it still sneaks up on you as you pile on more and more packages and customizations. use-package goes some way towards solving that by introducing a simple, declarative macro that – through some eldritch elisp magic – ensures your package is loaded only when it’s needed, and that its settings are loaded alongside it, in a deterministic and understandable manner.

use-package is very easy to use, and it ships with a very nice file explaining how it works.

For instance, if you regularly use smex – an excellent IDO-like interface for M-x – you can add this to your init file:

(use-package smex
  :ensure t
  :bind (("M-x" . smex))
  :config (smex-initialize))

And now Emacs ensures the package is installed (and installs it from the package archives if it is not) and it binds M-x to smex, and calls out to smex-initialize when you first press M-x.

You can combine your customizations with :config, :init and :preface. Each type works in a slightly different way.

The :config keyword executes the code after a package is loaded; the :init keyword executes the code before the package is loaded; and the :preface keyword is there so the Emacs byte compiler and the lisp evaluator know about things like function and symbol declarations.

My personal view is to put most things into :config (as you probably want to initialize your own settings after the package has loaded its own defaults) and only toy with :init and :preface if you have a good reason to.

Here’s an example where I change the default M-x re-builder syntax to string:

(use-package re-builder
  :bind (("C-c R" . re-builder))
  :config (setq reb-re-syntax 'string))

You can even migrate all your hand-crafted auto-mode-alist alterations to use-package. Here I declare that .txt, .rst and .rest files must use rst-mode:

(use-package rst
  :mode (("\\.txt$" . rst-mode)
         ("\\.rst$" . rst-mode)
         ("\\.rest$" . rst-mode)))

You can use it with built-in “packages” in Emacs also. Here I re-bind certain keys in M-x term and M-x ansi-term:

(use-package term
  (defun mp-term-custom-settings ()
    (local-set-key (kbd "M-p") 'term-send-up)
    (local-set-key (kbd "M-n") 'term-send-down))
  (add-hook 'term-load-hook 'mp-term-custom-settings)
  (define-key term-raw-map (kbd "M-o") 'other-window)
  (define-key term-raw-map (kbd "M-p") 'term-send-up)
  (define-key term-raw-map (kbd "M-n") 'term-send-down))

As you can see, it keeps everything nice and tidy. The older way of evaluating things after load, or even just require’ing all the packages you use and then applying your changes is completely unnecessary with use-package. use-package is clever enough to only autoload commands you’ve bound (or explicitly declared with :commands), meaning Emacs won’t load the package into memory until you actually need it. In practical terms, it should greatly speed up your Emacs start time.

I think the winning feature, for me, is the ability to automatically ensure the package is present before continuing.

It is also a convenient way to share snippets of code. You can copy my smex declaration above and not only will you get the package installed automatically (if it is not already), you will get my declared changes as well.