Yrurtj

Specifically, the keyboard event C-x, following by N z characters, will repeat the previous command N times.

I do not understand how this can work. As far as I understand, after the input event C-x z, for the behavior to be as specified, Emacs would need to somehow remap the z character to the command repeat in order for the next z to actually again execute `repeat'. Is this correct? If so, I do not understand how Emacs can change the keymap immediately after executing a command. If not, what is going on?

The code is in repeat.el. See command repeat, which is what C-x z is bound to.

The code uses the fact that Emacs records the current command and the last command in variables. The repeat code resets the variable this-command to what it records as the command that was previous to command repeat.

Note that you can define your own repeatable command by using code such as this:

(defun my-repeat-command (command)

  "Repeat COMMAND."

  (require 'repeat)

  (let ((repeat-previous-repeated-command  command)

        (repeat-message-function           #'ignore)

        (last-repeatable-command           'repeat))

    (repeat nil)))



(defun some-command (...)

  (interactive...)

  ...)



(defun some-command-repeat ()

  "Invoke `some-command' in a repeatable way."

  (interactive)

  (my-repeat-command 'some-command))

So it doesn't remap the key on the fly. Instead it just fools Emacs into thinking that the current command is whatever command was used last.

And yes, there are other ways of doing this kind of thing, such as using a transient keymap - that does more what you were guessing.

As you guessed, for this to work, the repeat command needs to be able to "change the keymap" immediately after executing a command. And indeed, repeat ends with a call to set-transient-map which makes a new keymap active (rather than modify in-place one of the currently active keymaps, it works by changing the set of active keymaps) with a binding for z which shadows the normal binding of z. This keymap needs to be short-lived, so set-transient-map internally uses pre-command-hook to deactivate that special keymap as soon as you hit something else than z.