GNU Emacs Lisp Reference Manual

19.12.1: Defining Menus

A keymap is suitable for menu use if it has an overall prompt string, which is a string that appears as an element of the keymap. (See Format of Keymaps.) The string should describe the purpose of the menu. The easiest way to construct a keymap with a prompt string is to specify the string as an argument when you call make-keymap or make-sparse-keymap (see Creating Keymaps).

The order of items in the menu is the same as the order of bindings in the keymap. Since define-key puts new bindings at the front, you should define the menu items starting at the bottom of the menu and moving to the top, if you care about the order. When you add an item to an existing menu, you can specify its position in the menu using define-key-after (see Modifying Menus).

The individual bindings in the menu keymap should have item strings; these strings become the items displayed in the menu. A binding with an item string looks like this:

(string . real-binding)

The item string for a binding should be short---one or two words. It should describe the action of the command it corresponds to.

As far as define-key is concerned, string is part of the event's binding. However, lookup-key returns just real-binding, and only real-binding is used for executing the key.

You can also supply a second string, called the help string, as follows:

(string help-string . real-binding)

Currently Emacs does not actually use help-string; it knows only how to ignore help-string in order to extract real-binding. In the future we may use help-string as extended documentation for the menu item, available on request.

If real-binding is nil, then string appears in the menu but cannot be selected.

If real-binding is a symbol and has a non-nil menu-enable property, that property is an expression that controls whether the menu item is enabled. Every time the keymap is used to display a menu, Emacs evaluates the expression, and it enables the menu item only if the expression's value is non-nil. When a menu item is disabled, it is displayed in a ``fuzzy'' fashion, and cannot be selected with the mouse.

The menu bar does not recalculate which items are enabled every time you look at a menu. This is because the X toolkit requires the whole tree of menus in advance. To force recalculation of the menu bar, call force-mode-line-update (see Mode Line Format).

Sometimes it is useful to make menu items that use the ``same'' command but with different enable conditions. You can do this by defining alias commands. Here's an example that makes two aliases for toggle-read-only and gives them different enable conditions:

(defalias 'make-read-only 'toggle-read-only)
(put 'make-read-only 'menu-enable '(not buffer-read-only))
(defalias 'make-writable 'toggle-read-only)
(put 'make-writable 'menu-enable 'buffer-read-only)

You've probably noticed that menu items show the equivalent keyboard key sequence (if any) to invoke the same command. To save time on recalculation, menu display caches this information in a sublist in the binding, like this:

(string [help-string] (key-binding-data) . real-binding)

Don't put these sublists in the menu item yourself; menu display calculates them automatically. Don't add keyboard equivalents to the item strings in a mouse menu, since that is redundant.

If an alias command has no keyboard equivalent itself, menus show the keyboard equivalent of its underlying command. In the example above, menu items defined to run make-read-only or make-writable would show the keyboard equivalents of toggle-read-only.