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38.8 Temporary Displays

Temporary displays are used by Lisp programs to put output into a buffer and then present it to the user for perusal rather than for editing. Many help commands use this feature.

— Macro: with-output-to-temp-buffer buffer-name forms...

This function executes forms while arranging to insert any output they print into the buffer named buffer-name, which is first created if necessary, and put into Help mode. Finally, the buffer is displayed in some window, but not selected. (See the similar form with-temp-buffer-window below.)

If the forms do not change the major mode in the output buffer, so that it is still Help mode at the end of their execution, then with-output-to-temp-buffer makes this buffer read-only at the end, and also scans it for function and variable names to make them into clickable cross-references. See Tips for Documentation Strings, in particular the item on hyperlinks in documentation strings, for more details.

The string buffer-name specifies the temporary buffer, which need not already exist. The argument must be a string, not a buffer. The buffer is erased initially (with no questions asked), and it is marked as unmodified after with-output-to-temp-buffer exits.

with-output-to-temp-buffer binds standard-output to the temporary buffer, then it evaluates the forms in forms. Output using the Lisp output functions within forms goes by default to that buffer (but screen display and messages in the echo area, although they are “output” in the general sense of the word, are not affected). See Output Functions.

Several hooks are available for customizing the behavior of this construct; they are listed below.

The value of the last form in forms is returned.

          ---------- Buffer: foo ----------
           This is the contents of foo.
          ---------- Buffer: foo ----------
          
          (with-output-to-temp-buffer "foo"
              (print 20)
              (print standard-output))
          ⇒ #<buffer foo>
          
          ---------- Buffer: foo ----------
          20
          
          #<buffer foo>
          
          ---------- Buffer: foo ----------
— User Option: temp-buffer-show-function

If this variable is non-nil, with-output-to-temp-buffer calls it as a function to do the job of displaying a help buffer. The function gets one argument, which is the buffer it should display.

It is a good idea for this function to run temp-buffer-show-hook just as with-output-to-temp-buffer normally would, inside of save-selected-window and with the chosen window and buffer selected.

— Variable: temp-buffer-setup-hook

This normal hook is run by with-output-to-temp-buffer before evaluating body. When the hook runs, the temporary buffer is current. This hook is normally set up with a function to put the buffer in Help mode.

— Variable: temp-buffer-show-hook

This normal hook is run by with-output-to-temp-buffer after displaying the temporary buffer. When the hook runs, the temporary buffer is current, and the window it was displayed in is selected.

— Macro: with-temp-buffer-window buffer-or-name action quit-function forms...

This macro is similar to with-output-to-temp-buffer. Like that construct, it executes forms while arranging to insert any output they print into the buffer named buffer-or-name. Finally, the buffer is displayed in some window, but not selected. Unlike with-output-to-temp-buffer, this does not switch to Help mode.

The argument buffer-or-name specifies the temporary buffer. It can be either a buffer, which must already exist, or a string, in which case a buffer of that name is created if necessary. The buffer is marked as unmodified and read-only when with-temp-buffer-window exits.

This macro does not call temp-buffer-show-function. Rather, it passes the action argument to display-buffer in order to display the buffer.

The value of the last form in forms is returned, unless the argument quit-function is specified. In that case, it is called with two arguments: the window showing the buffer and the result of forms. The final return value is then whatever quit-function returns.

This macro uses the normal hooks temp-buffer-window-setup-hook and temp-buffer-window-show-hook in place of the analogous hooks run by with-output-to-temp-buffer.

— Function: momentary-string-display string position &optional char message

This function momentarily displays string in the current buffer at position. It has no effect on the undo list or on the buffer's modification status.

The momentary display remains until the next input event. If the next input event is char, momentary-string-display ignores it and returns. Otherwise, that event remains buffered for subsequent use as input. Thus, typing char will simply remove the string from the display, while typing (say) C-f will remove the string from the display and later (presumably) move point forward. The argument char is a space by default.

The return value of momentary-string-display is not meaningful.

If the string string does not contain control characters, you can do the same job in a more general way by creating (and then subsequently deleting) an overlay with a before-string property. See Overlay Properties.

If message is non-nil, it is displayed in the echo area while string is displayed in the buffer. If it is nil, a default message says to type char to continue.

In this example, point is initially located at the beginning of the second line:

          ---------- Buffer: foo ----------
          This is the contents of foo.
          -!-Second line.
          ---------- Buffer: foo ----------
          
          (momentary-string-display
            "**** Important Message! ****"
            (point) ?\r
            "Type RET when done reading")
          ⇒ t
          
          ---------- Buffer: foo ----------
          This is the contents of foo.
          **** Important Message! ****Second line.
          ---------- Buffer: foo ----------
          
          ---------- Echo Area ----------
          Type RET when done reading
          ---------- Echo Area ----------