Extensions and modes

Lem contribs are not loaded at startup.

M-x load-library

and choose a library (for example, tetris). Then, you can call this contrib’s commands (in that case, M-x tetris).

A major mode is associated to a keymap and commands. It is typically used for a programming language mode, but also for non-programming modes (directory-mode, tetris-mode…), or to complement a programming mode keymap (paredit).

If you want to add support for another programming language, please see Lem’s modes/ directory (and get in touch!).

Also, remember that you don’t need to create a major mode to add custom commands to Lem. A define-command in that case is enough, please see configuring Lem.

But, how do we create a new major mode?

Create a .lisp file and create a new package. You’ll want to “use” :lem:

(defpackage :lem-awesome-mode
  (:use :cl :lem)
  (:export :awesome-command ;; <-- we'll write them
           :awesome-mode))
(in-package :lem-awesome-mode)

Name the major mode to be created and name its keymap:

(define-major-mode awesome-mode ()
    (:name "Awesome mode"
     :keymap *awesome-mode-keymap*)
  ;; you can do write more startup code here.
  )

You can also create a minor mode with lem:define-minor-mode. A minor mode will only complement a major mode and won’t overwrite all its keybindings.

It’s time to write your code and define commands.

(define-command awesome-command () ()
  (message "Hello Lem!"))

(define-key *awesome-mode-keymap* "!" 'awesome-command)

Don’t forget to export both :awesome-mode and :awesome-command in your package definition.

Now, open any file and activate your mode with M-x awesome-mode. Try to write a “!”: you should see the “Hello Lem!” message pop-up. You’ll have to re-activate the mode you were in to stop this functionnality.

You can take inspiration from all Lem modes in modes/ or src/ext/. Do a grep for “define-major-mode” or “define-minor-mode”.

• Pareto - A mode, complement to the built-in Paredit mode, similar to Emacs’ Lispy-mode. Makes Lisp code editing even more efficient!