Function: with-buffer-unmodified-if-unchanged

with-buffer-unmodified-if-unchanged is a macro defined in compat-29.el.

Signature

(with-buffer-unmodified-if-unchanged &rest BODY)

Documentation

[Compatibility macro for with-buffer-unmodified-if-unchanged, defined in Emacs
29.1. See (compat) Emacs 29.1' for more details.]

Like progn, but change buffer-modified status only if buffer text changes. If the buffer was unmodified before execution of BODY, and buffer text after execution of BODY is identical to what it was before, ensure that buffer is still marked unmodified afterwards. For example, the following won't change the buffer's modification status:

  (with-buffer-unmodified-if-unchanged (insert "a") (delete-char -1))

Note that only changes in the raw byte sequence of the buffer text, as stored in the internal representation, are monitored for the purpose of detecting the lack of changes in buffer text. Any other changes that are normally perceived as
"buffer modifications", such as changes in text properties,
buffer-file-coding-system, buffer multibyteness, etc. -- will not be noticed, and the buffer will still be marked unmodified, effectively ignoring those changes.

Source Code

;; Defined in ~/.emacs.d/elpa/compat-30.1.0.1/compat-29.el
;;;; Defined in subr-x.el

(compat-defmacro with-buffer-unmodified-if-unchanged (&rest body) ;; <compat-tests:with-buffer-unmodified-if-unchanged>
  "Like `progn', but change buffer-modified status only if buffer text changes.
If the buffer was unmodified before execution of BODY, and
buffer text after execution of BODY is identical to what it was
before, ensure that buffer is still marked unmodified afterwards.
For example, the following won't change the buffer's modification
status:

  (with-buffer-unmodified-if-unchanged
    (insert \"a\")
    (delete-char -1))

Note that only changes in the raw byte sequence of the buffer text,
as stored in the internal representation, are monitored for the
purpose of detecting the lack of changes in buffer text.  Any other
changes that are normally perceived as \"buffer modifications\", such
as changes in text properties, `buffer-file-coding-system', buffer
multibyteness, etc. -- will not be noticed, and the buffer will still
be marked unmodified, effectively ignoring those changes."
  (declare (debug t) (indent 0))
  (let ((hash (gensym))
        (buffer (gensym)))
    `(let ((,hash (and (not (buffer-modified-p))
                       (buffer-hash)))
           (,buffer (current-buffer)))
       (prog1
           (progn
             ,@body)
         ;; If we didn't change anything in the buffer (and the buffer
         ;; was previously unmodified), then flip the modification status
         ;; back to "unchanged".
         (when (and ,hash (buffer-live-p ,buffer))
           (with-current-buffer ,buffer
             (when (and (buffer-modified-p)
                        (equal ,hash (buffer-hash)))
               (restore-buffer-modified-p nil))))))))