21.5 Inheritance and Keymaps

A keymap can inherit the bindings of another keymap, which we call the parent keymap. Such a keymap looks like this:

(keymap elements… . parent-keymap)

The effect is that this keymap inherits all the bindings of parent-keymap, whatever they may be at the time a key is looked up, but can add to them or override them with elements.

If you change the bindings in parent-keymap using define-key or other key-binding functions, these changed bindings are visible in the inheriting keymap, unless shadowed by the bindings made by elements. The converse is not true: if you use define-key to change bindings in the inheriting keymap, these changes are recorded in elements, but have no effect on parent-keymap.

The proper way to construct a keymap with a parent is to use set-keymap-parent; if you have code that directly constructs a keymap with a parent, please convert the program to use set-keymap-parent instead.

Function: keymap-parent keymap

This returns the parent keymap of keymap. If keymap has no parent, keymap-parent returns nil.

Function: set-keymap-parent keymap parent

This sets the parent keymap of keymap to parent, and returns parent. If parent is nil, this function gives keymap no parent at all.

If keymap has submaps (bindings for prefix keys), they too receive new parent keymaps that reflect what parent specifies for those prefix keys.

Here is an example showing how to make a keymap that inherits from text-mode-map:

(let ((map (make-sparse-keymap)))
  (set-keymap-parent map text-mode-map)
  map)

A non-sparse keymap can have a parent too, but this is not very useful. A non-sparse keymap always specifies something as the binding for every numeric character code without modifier bits, even if it is nil, so these character’s bindings are never inherited from the parent keymap.

Sometimes you want to make a keymap that inherits from more than one map. You can use the function make-composed-keymap for this.

Function: make-composed-keymap maps &optional parent

This function returns a new keymap composed of the existing keymap(s) maps, and optionally inheriting from a parent keymap parent. maps can be a single keymap or a list of more than one. When looking up a key in the resulting new map, Emacs searches in each of the maps in turn, and then in parent, stopping at the first match. A nil binding in any one of maps overrides any binding in parent, but it does not override any non-nil binding in any other of the maps.

For example, here is how Emacs sets the parent of help-mode-map, such that it inherits from both button-buffer-map and special-mode-map:

(defvar help-mode-map
  (let ((map (make-sparse-keymap)))
    (set-keymap-parent map
      (make-composed-keymap button-buffer-map special-mode-map))
    ... map) ... )