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
keymap-lookup 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.
The events used so far in the lookup form a complete key, and command is its binding. See What Is a Function?.
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.
The events used so far in the lookup form a prefix key. The next event of the key sequence is looked up in keymap.
The meaning of a list depends on what it contains:
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 interactive
specification. See Defining Commands.
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.
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.
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 nil.