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GNU Emacs Lisp Reference Manual

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20.6 Yes-or-No Queries

This section describes functions used to ask the user a yes-or-no question. The function y-or-n-p can be answered with a single character; it is useful for questions where an inadvertent wrong answer will not have serious consequences. yes-or-no-p is suitable for more momentous questions, since it requires three or four characters to answer.

If either of these functions is called in a command that was invoked using the mouse--more precisely, if last-nonmenu-event (see section 21.4 Information from the Command Loop) is either nil or a list--then it uses a dialog box or pop-up menu to ask the question. Otherwise, it uses keyboard input. You can force use of the mouse or use of keyboard input by binding last-nonmenu-event to a suitable value around the call.

Strictly speaking, yes-or-no-p uses the minibuffer and y-or-n-p does not; but it seems best to describe them together.

Function: y-or-n-p prompt
This function asks the user a question, expecting input in the echo area. It returns t if the user types y, nil if the user types n. This function also accepts SPC to mean yes and DEL to mean no. It accepts C-] to mean "quit", like C-g, because the question might look like a minibuffer and for that reason the user might try to use C-] to get out. The answer is a single character, with no RET needed to terminate it. Upper and lower case are equivalent.

"Asking the question" means printing prompt in the echo area, followed by the string `(y or n) '. If the input is not one of the expected answers (y, n, SPC, DEL, or something that quits), the function responds `Please answer y or n.', and repeats the request.

This function does not actually use the minibuffer, since it does not allow editing of the answer. It actually uses the echo area (see section 38.4 The Echo Area), which uses the same screen space as the minibuffer. The cursor moves to the echo area while the question is being asked.

The answers and their meanings, even `y' and `n', are not hardwired. The keymap query-replace-map specifies them. See section 34.5 Search and Replace.

In the following example, the user first types q, which is invalid. At the next prompt the user types y.

 
(y-or-n-p "Do you need a lift? ")

;; After evaluation of the preceding expression, 
;;   the following prompt appears in the echo area:

---------- Echo area ----------
Do you need a lift? (y or n) 
---------- Echo area ----------

;; If the user then types q, the following appears:

---------- Echo area ----------
Please answer y or n.  Do you need a lift? (y or n) 
---------- Echo area ----------

;; When the user types a valid answer,
;;   it is displayed after the question:

---------- Echo area ----------
Do you need a lift? (y or n) y
---------- Echo area ----------

We show successive lines of echo area messages, but only one actually appears on the screen at a time.

Function: y-or-n-p-with-timeout prompt seconds default-value
Like y-or-n-p, except that if the user fails to answer within seconds seconds, this function stops waiting and returns default-value. It works by setting up a timer; see 40.7 Timers for Delayed Execution. The argument seconds may be an integer or a floating point number.

Function: yes-or-no-p prompt
This function asks the user a question, expecting input in the minibuffer. It returns t if the user enters `yes', nil if the user types `no'. The user must type RET to finalize the response. Upper and lower case are equivalent.

yes-or-no-p starts by displaying prompt in the echo area, followed by `(yes or no) '. The user must type one of the expected responses; otherwise, the function responds `Please answer yes or no.', waits about two seconds and repeats the request.

yes-or-no-p requires more work from the user than y-or-n-p and is appropriate for more crucial decisions.

Here is an example:

 
(yes-or-no-p "Do you really want to remove everything? ")

;; After evaluation of the preceding expression, 
;;   the following prompt appears, 
;;   with an empty minibuffer:

---------- Buffer: minibuffer ----------
Do you really want to remove everything? (yes or no) 
---------- Buffer: minibuffer ----------

If the user first types y RET, which is invalid because this function demands the entire word `yes', it responds by displaying these prompts, with a brief pause between them:

 
---------- Buffer: minibuffer ----------
Please answer yes or no.
Do you really want to remove everything? (yes or no)
---------- Buffer: minibuffer ----------


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