* lisp-mode.el (lisp-fill-paragraph): New function.

(shared-lisp-mode-map): Bind M-q to lisp-fill-paragraph.

lisp/emacs-lisp/lisp-mode.el

@@ -102,6 +102,7 @@
     ()
    (setq shared-lisp-mode-map (make-sparse-keymap))
    (define-key shared-lisp-mode-map "\e\C-q" 'indent-sexp)
+   (define-key shared-lisp-mode-map "\M-q" 'lisp-fill-paragraph)
    (define-key shared-lisp-mode-map "\177" 'backward-delete-char-untabify)
    (define-key shared-lisp-mode-map "\t" 'lisp-indent-line))
 
@@ -593,6 +594,75 @@ ENDPOS is encountered."
       (indent-sexp endmark)
       (set-marker endmark nil))))
 
+;;;; Lisp paragraph filling commands.
+
+(defun lisp-fill-paragraph (&optional justify)
+  "Like \\[fill-paragraph], but handle Emacs Lisp comments.
+If any of the current line is a comment, fill the comment or the
+paragraph of it that point is in, preserving the comment's indentation
+and initial semicolons."
+  (interactive "P")
+  (let (
+	;; Non-nil if the current line contains a comment.
+	has-comment
+
+	;; If has-comment, the appropriate fill-prefix for the comment.
+	comment-fill-prefix
+	)
+
+    ;; Figure out what kind of comment we are looking at.
+    (save-excursion
+      (beginning-of-line)
+      (cond
+
+       ;; A line with nothing but a comment on it?
+       ((looking-at "[ \t]*;[; \t]*")
+	(setq has-comment t
+	      comment-fill-prefix (buffer-substring (match-beginning 0)
+						    (match-end 0))))
+
+       ;; A line with some code, followed by a comment?  Remember that the
+       ;; semi which starts the comment shouldn't be part of a string or
+       ;; character.
+       ((progn
+	  (while (not (looking-at ";\\|$"))
+	    (skip-chars-forward "^;\n\"\\\\?")
+	    (cond
+	     ((eq (char-after (point)) ?\\) (forward-char 2))
+	     ((memq (char-after (point)) '(?\" ??)) (forward-sexp 1))))
+	  (looking-at ";+[\t ]*"))
+	(setq has-comment t)
+	(setq comment-fill-prefix
+	      (concat (make-string (current-column) ? )
+		      (buffer-substring (match-beginning 0) (match-end 0)))))))
+
+    (if (not has-comment)
+	(fill-paragraph justify)
+
+      ;; Narrow to include only the comment, and then fill the region.
+      (save-restriction
+	(narrow-to-region
+	 ;; Find the first line we should include in the region to fill.
+	 (save-excursion
+	   (while (and (zerop (forward-line -1))
+		       (looking-at "^[ \t]*;")))
+	   ;; We may have gone to far.  Go forward again.
+	   (or (looking-at "^[ \t]*;")
+	       (forward-line 1))
+	   (point))
+	 ;; Find the beginning of the first line past the region to fill.
+	 (save-excursion
+	   (while (progn (forward-line 1)
+			 (looking-at "^[ \t]*;")))
+	   (point)))
+
+	;; Lines with only semicolons on them can be paragraph boundaries.
+	(let ((paragraph-start (concat paragraph-start "\\|^[ \t;]*$"))
+	      (paragraph-separate (concat paragraph-start "\\|^[ \t;]*$"))
+	      (fill-prefix comment-fill-prefix))
+	  (fill-paragraph justify))))))
+
+
 (defun indent-code-rigidly (start end arg &optional nochange-regexp)
   "Indent all lines of code, starting in the region, sideways by ARG columns.
 Does not affect lines starting inside comments or strings, assuming that