Emacs Lisp: Exit Loop/Function, catch/throw

By Xah Lee. Date: . Last updated: .

This page shows you how to exit a loop, or exit a function.

In many languages, there's “break” or “exit” for exiting a loop, or use “return” to exit a function.

to exit in elisp:

Exit a Function

To exit a function, just put throw 'tagname value where you want to, and, wrap your function body with a catch 'tagname.

Here's example.

(defun test-exit-f ()
  "example. using catch/throw to exit function"
  (interactive)
  (catch 'aaa
    (if (y-or-n-p "exit?")
        (progn
          (message "existing")
          (throw 'aaa 3) ; if yes, exit right away, return 3 to catch
          )
      (progn ; else, go on
        (message "went on")
        4 ; return 4
        ))))

Exit by Error

You can exit by calling error or user-error .

(defun test-exit-f ()
  "example"
  (interactive)
  (if (y-or-n-p "invoke user-error to exit?")
      (user-error "Error, because: %s" "you said so!")
    (progn ; else, go on
      (message "went on")
      )))

Exit a map

You can also use throwcatch to exit a map or loop.

(catch 'tagname body) → evaluates body and return body's last expression, but if body contains (throw …) and it is called, return the value throw passes.

(throw tagName passValue) → jump to a outer (catch 'tagName) and continue there, passing the value of passValue to it. Both tagName passValue are evaluated.

(let ((myList [0 1 2 3 4 5]))

  ;; map lambda onto a list. If value 3 is found, return 3, else nil
  (catch 'bbb
    (mapc
     (lambda (x)
       (message "%s" x)
       (when (equal x 3) (throw 'bbb x)))
     myList)
    nil
    ))

(info "(elisp) Catch and Throw")

Exit a While Loop by Flag

Here's a sample of setting flag:

(let ((myList [0 1 2 3 4 5] )
      (foundFlag-p nil )
      (i 0))

  (while (and
          (not foundFlag-p)
          (<= i (length myList)))

    ;; if found, set foundFlag-p
    (when (equal (elt myList i) 3)
      (setq foundFlag-p t ))

    (message "value: %s" i)
    (setq i (1+ i))))
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