Yes-or-No Queries - SXEmacs Lisp Reference Manual

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. Variations of these functions can be used to ask a yes-or-no question using a dialog box, or optionally using one.

If either of these functions is called in a command that was invoked using the mouse, then it uses a dialog box or pop-up menu to ask the question. Otherwise, it uses keyboard input.

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 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 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: 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 ----------

— Function: yes-or-no-p-dialog-box prompt

This function asks the user a “y or n” question with a popup dialog box. It returns t if the answer is “yes”. prompt is the string to display to ask the question.

The following functions ask a question either in the minibuffer or a dialog box, depending on whether the last user event (which presumably invoked this command) was a keyboard or mouse event. When SXEmacs is running on a window system, the functions y-or-n-p and yes-or-no-p are replaced with the following functions, so that menu items bring up dialog boxes instead of minibuffer questions.

24.6 Yes-or-No Queries