Stefan Monnier <monnier@iro.umontreal.ca> writes:

>> Given that this is a first version, I have not added any reference to
>> the manuals.  If you think it makes sense, please let me know and I'll
>> modify the patch accordingly.
>
> Maybe a short version of the explanation you give below would be good to
> have in the manual (tho Eli suggests a URL instead, so maybe that's
> good enough?).
>
>> +(defun fill-paragraph-semlf (&optional justify)
>> +  "Fill paragraph at or after point using semantic linefeeds.
>> +
>> +This function ensures that a newline character follows every
>> +sentence, as punctuated by a period (.), exclamation mark (!), or
>> +question mark (?).
>
> This seems inaccurate: it just uses whichever definition of sentence is
> used by `forward-sentence`, so it may ignore some of those chars or pay
> attention to others.

I have updated the patch with a more precise definition.  Also, I added
links to the sources I referenced for semantic linefeeds.

>> +If JUSTIFY is non-nil (interactively, with prefix argument), justify as
>> +well.  If `sentence-end-double-space' is non-nil, then period followed
>> +by one space does not end a sentence, so don't break a line there.  The
>> +variable `fill-column' controls the width for filling."
>
> I'd move the "The" to the last line.  🙂

Fixed :)

>> +  (interactive "P")
>> +  (save-excursion
>> +    (let ((end (progn
>> +		 (fill-forward-paragraph 1)
>> +		 (backward-word)
>> +		 (end-of-line)
>> +		 (point)))
>> +	  (start (progn
>> +		   (fill-forward-paragraph -1)
>> +		   (forward-word)
>> +		   (beginning-of-line)
>> +		   (point)))
>> +	  pfx)
>> +      (with-restriction start end
>> +	(let ((fill-column (point-max)))
>> +	  (setq pfx (or (fill-region-as-paragraph (point-min) (point-max)) "")))
>> +	(goto-char (point-min))
>> +	(while (not (eobp))
>> +	  (let ((fill-prefix pfx))
>> +	    (fill-region-as-paragraph (point)
>> +				      (progn (forward-sentence) (point))
>> +				      justify))
>> +	  (when (and (> (point) (line-beginning-position))
>> +		     (< (point) (line-end-position)))
>> +	    (delete-horizontal-space)
>> +	    (newline)
>> +	    (insert pfx))))))
>> +  t)
>
> Please try and separate it into a `fill-region-semlf` function and then
> another one which applies it to a paragraph, so that it can also be used
> to fill a specific user-specified region (or the whole buffer).

I'm not sure about this one.  The idea is that `fill-paragraph-semlf'
can be assigned as a value for `fill-paragraph-function' (I also
included a mention to this in the doc string) or it ca be called
directly.  The thing is that `fill-paragraph' docs say:

  The REGION argument is non-nil if called interactively; in that
  case, if Transient Mark mode is enabled and the mark is active,
  call `fill-region' to fill each of the paragraphs in the active
  region, instead of just filling the current paragraph.

And `fill-paragraph-function' docs say:

  Note: This only affects ‘fill-paragraph’ and not ‘fill-region’
  nor ‘auto-fill-mode’

So, if I'm not wrong, filling regions and paragraphs is different in the
current design.

I agree that it would be useful to apply `fill-paragraph-semlf' on a
region or the whole buffer.  But, the same could be said about any other
`fill-paragraph-function'.  So, do we really want a specific
`fill-region-semlf' function?

I attach a new version of the patch.

Thanks!