21.10 Key Lookup

Key lookup is the process of finding the binding of a key sequence from a given keymap. The execution or use of the binding is not part of key lookup.

Key lookup uses just the event type of each event in the key sequence; the rest of the event is ignored. In fact, a key sequence used for key lookup may designate a mouse event with just its types (a symbol) instead of the entire event (a list). See Input Events. Such a “key sequence” is insufficient for command-execute to run, but it is sufficient for looking up or rebinding a key.

When the key sequence consists of multiple events, key lookup processes the events sequentially: the binding of the first event is found, and must be a keymap; then the second event’s binding is found in that keymap, and so on until all the events in the key sequence are used up. (The binding thus found for the last event may or may not be a keymap.) Thus, the process of key lookup is defined in terms of a simpler process for looking up a single event in a keymap. How that is done depends on the type of object associated with the event in that keymap.

Let’s use the term keymap entry to describe the value found by looking up an event type in a keymap. (This doesn’t include the item string and other extra elements in a keymap element for a menu item, because lookup-key and other key lookup functions don’t include them in the returned value.) While any Lisp object may be stored in a keymap as a keymap entry, not all make sense for key lookup. Here is a table of the meaningful types of keymap entries:

nil

nil means that the events used so far in the lookup form an undefined key. When a keymap fails to mention an event type at all, and has no default binding, that is equivalent to a binding of nil for that event type.

command

The events used so far in the lookup form a complete key, and command is its binding. See What Is a Function.

array

The array (either a string or a vector) is a keyboard macro. The events used so far in the lookup form a complete key, and the array is its binding. See Keyboard Macros, for more information.

keymap

The events used so far in the lookup form a prefix key. The next event of the key sequence is looked up in keymap.

list

The meaning of a list depends on what it contains:

• If the CAR of list is the symbol keymap, then the list is a keymap, and is treated as a keymap (see above).
• If the CAR of list is lambda, then the list is a lambda expression. This is presumed to be a function, and is treated as such (see above). In order to execute properly as a key binding, this function must be a command—it must have an interactive specification. See Defining Commands.
• If the CAR of list is a keymap and the CDR is an event type, then this is an indirect entry:

  (othermap . othertype)
  

  When key lookup encounters an indirect entry, it looks up instead the binding of othertype in othermap and uses that.

  This feature permits you to define one key as an alias for another key. For example, an entry whose CAR is the keymap called esc-map and whose CDR is 32 (the code for SPC) means, “Use the global binding of Meta-SPC, whatever that may be”.

symbol

The function definition of symbol is used in place of symbol. If that too is a symbol, then this process is repeated, any number of times. Ultimately this should lead to an object that is a keymap, a command, or a keyboard macro. A list is allowed if it is a keymap or a command, but indirect entries are not understood when found via symbols.

Note that keymaps and keyboard macros (strings and vectors) are not valid functions, so a symbol with a keymap, string, or vector as its function definition is invalid as a function. It is, however, valid as a key binding. If the definition is a keyboard macro, then the symbol is also valid as an argument to command-execute (see Interactive Call).

The symbol undefined is worth special mention: it means to treat the key as undefined. Strictly speaking, the key is defined, and its binding is the command undefined; but that command does the same thing that is done automatically for an undefined key: it rings the bell (by calling ding) but does not signal an error.

undefined is used in local keymaps to override a global key binding and make the key “undefined” locally. A local binding of nil would fail to do this because it would not override the global binding.

anything else

If any other type of object is found, the events used so far in the lookup form a complete key, and the object is its binding, but the binding is not executable as a command.

In short, a keymap entry may be a keymap, a command, a keyboard macro, a symbol that leads to one of them, or an indirection or nil.