[[de:Bangla]]
If you keep hitting ##M-q## to fill your paragraphs (see FillParagraph), then ##auto-fill-mode## is for you. It doesn't get rid of ##M-q## altogether (use RefillMode (which replaces ManiacMode) for that), but it is a very good approximation:
Type ##M-x auto-fill-mode## to activate the MinorMode for the current buffer, or put the following in your <tt>[InitFile .emacs]</tt> to activate it for all text mode buffers:

    (add-hook 'text-mode-hook 'turn-on-auto-fill)

If you want Emacs to ''ask'' you whether to use Auto Fill Mode when opening a text file, you can do:

    (add-hook 'text-mode-hook
              (lambda ()
                (when (y-or-n-p "Auto Fill mode? ")
                  (turn-on-auto-fill))))

This is what I have in my ##.emacs## file to quickly ''toggle'' auto fill mode:

    (global-set-key (kbd "C-c q") 'auto-fill-mode)

Similarly, if you want to activate it for a particular mode, proceed as follows:

* Determine the MajorMode. Switch to a buffer with the appropriate mode and type ##M-: major-mode RET##. This might return something like ##change-log-mode##, or whatever the MajorMode is.

* Guess the name of the mode hook. Just append "##-hook##" and hope for the best. This gets me ##change-log-mode-hook##. If you want to verify the existence of such a hook variable, you can try to read its documentation: ##C-h v change-log-mode-hook RET##. This is not guaranteed to work, however, because hooks need not be defined. Therefore, just use the guessed name anyway.

* Add the following to your ##.emacs## file:

    (add-hook 'change-log-mode-hook 'turn-on-auto-fill)

If you would like to enable auto fill for all major modes, you can add this single line to your configuration:

     (setq-default auto-fill-function 'do-auto-fill)

To automatically fill comments but not code in ProgrammingModes, something like this can be used.

    (add-hook 'c-mode-common-hook
              (lambda ()
                (auto-fill-mode 1)
                (set (make-local-variable 'fill-nobreak-predicate)
                     (lambda ()
                       (not (eq (get-text-property (point) 'face)
                                'font-lock-comment-face))))))

Or you could define this and call it in your programming mode hooks as needed, which seems to work more reliably for me:

    (defun comment-auto-fill ()
      (setq-local comment-auto-fill-only-comments t)
      (auto-fill-mode 1))

FillingComments mentions this approach and should probably be merged with this page.

----

The variable ##fill-no-break-predicate## holds a list of functions that can be used 
to compute when a particular point is line-breaked on.  This can be used to write modes 
that have unusual breaking behaviour, such as Wiki types modes.

[[https://github.com/nicferrier/creole-mode/blob/e3a2b15b228c9c1df7560ec390424040d69b8bb7/creole-mode.el#L70|Here's an example]]
from a WikiCreole mode where auto-fill is useful but shouldn't affect text inside links. 

==== auto-fill and org-mode ====
There is an (apparent) incompatibility between (i) auto-fill-mode, (ii) org-mode, and (iii) ##comment-auto-fill-only-comments##.  See https://github.com/syl20bnr/spacemacs/issues/11749#issuecomment-526757521.

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CategoryComments
CategoryFilling 
CategoryModes 
CategoryDotEmacs