Re: Advantage using mapc over dolist

[Tomas Hlavaty <tom@logand.com>]

On Mon 02 Dec 2024 at 20:17, Heime <heimeborgia@protonmail.com> wrote:
> How can I find an example of exiting prematurely from mapc?

silly example:

(catch 'done
  (let ((z 0))
    (mapc (lambda (x)
            (if (< x 10)
                (setq z (+ z x))
              (throw 'done z)))
          '(3 5 7 13))))

You can search emacs sources for mapc.
I have a custom rg-emacs function for that:

(rg-emacs "mapc ")

which finds tempo-is-user-element function showing something
interesting:

(defun tempo-is-user-element (element)
  "Try all the user-defined element handlers in `tempo-user-elements'."
  ;; Sigh... I need (some list)
  (catch 'found
    (mapc (lambda (handler)
            (let ((result (funcall handler element)))
              (if result (throw 'found result))))
	  tempo-user-elements)
    (throw 'found nil)))

although in Common Lisp, I would avoid catch, throw and mapc and used
dolist with return or return-from instead:

(defun tempo-is-user-element (element)
  (dolist (handler tempo-user-elements)
    (let ((z (funcall handler element)))
      (when z
        (cl-return z)))))

This is more readable than the catch, mapc, throw code in Emacs.

Unfortunately, cl-return-from does not seems to be as convenient.
cl-return might do what I expect cl:return to be doing.