Emacs Lisp: throw, catch, exit function

To exit a loop or exit a function:

• Use catch and throw to exit loop/function. These are like “break”, “goto” in other programing languages.
• Use error or user-error to signal a error and exit.

Exit a Function

To exit a function, first wrap the function body by (catch 'tagName body) and put (throw 'tagName value) at the place you want to exit.

(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.

(info "(elisp) Catch and Throw")

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 throw and catch to exit a map or loop.

(setq 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
  )

Exit a While Loop

;; example of exit a while loop
;; prints a random integer between 0 to 10, inclusive
;; if 5 comes up, then exit after printing it
(catch 'abc
  (while (setq xx (random 10))
    (message "%s" xx)
    (when (eq 5 xx) (throw 'abc t))))

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