non-blocking y-or-n-p

Question

I have a function that offers to do handy things. Right now, it's using y-or-n-p to ask if it should take action. The problem is that y-or-n-p blocks the command loop, which is inconvenient if you want to pass on the action. Instead, I'd like y to execute the action, but let everything else through.

(defun my/y-or-n-p (prompt callback)
  (message prompt)
  (set-transient-map `(keymap (121 . ,callback))))

(defun test-my/y-or-n-p ()
  (my/y-or-n-p "message?" (lambda () (interactive) (message "message hi!"))))

;; this works
(test-my/y-or-n-p)

;; this does not
(add-hook 'emacs-startup-hook #'test-my/y-or-n-p)

There are two problems with my code:

• The prompt in the echo area can be overwritten by other messages
• Doesn't seem to work in emacs-startup-hook (and maybe other hooks)

Does anyone know what's wrong with it?

Answer 1

Something like the following should work

(defun my-lenient-y-or-n-p (prompt)
  (let ((query-replace-map (copy-keymap query-replace-map)))
    (define-key query-replace-map [t] 'skip)
    (y-or-n-p prompt)))

If you look at the documentation of y-or-n-p, it mentions that it looks up the query-replace-map to decide which action to take for a given key. I am quoting it here for completeness (emphasis mine)

(...)

To be precise, this function translates user input into responses by consulting the bindings in query-replace-map; see the documentation of that variable for more information. In this case, the useful bindings are act, skip, recenter, scroll-up, scroll-down, and quit. An act response means yes, and a skip response means no. A quit response means to invoke keyboard-quit. If the user enters recenter, scroll-up, or scroll-down responses, perform the requested window recentering or scrolling and ask again.

(...)

So basically if we can invoke y-or-n-p with modified query-replace-map which defines the binding skip as the default we should get what you want.

We can achieve this by copying the query-replace-map (so that global value is unchanged) and defining the binding skip for all undefined keys. So how do we define a default binding in a keymap, for this see the documentation of define-keymap (emphasis mine)

(...) KEY is a string or a vector of symbols and characters, representing a sequence of keystrokes and events. Non-ASCII characters with codes above 127 (such as ISO Latin-1) can be represented by vectors. Two types of vector have special meanings: [remap COMMAND] remaps any key binding for COMMAND. [t] creates a default definition, which applies to any event with no other definition in KEYMAP.

So binding [t] to skip in the copy of query-replace-map should do the trick for us. Of course we let bind the new keymap around the call to y-or-no-p so that the modified keymap is used instead of the global definition