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:
nilmeans 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
nilfor that event type.
keymap, then the list is a keymap, and is treated as a keymap (see above).
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
interactivespecification. See Defining Commands.
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
(see Interactive Call).
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
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
In short, a keymap entry may be a keymap, a command, a keyboard
macro, a symbol that leads to one of them, or