Re: "Font-lock is limited to text matching" is a myth

[Daniel Colascione]

On Aug 11, 2009, at 10:37 AM, Miles Bader wrote:
> Daniel Colascione <address@hidden> writes:
> > > Note: diagnostic messages in js2-mode are highlighted using  
> > > overlays.
> > > I tried using overlays for all highlighting but it was unacceptably
> > > slow and had a tendency to crash Emacs.
> >
> > I've had the same thought. Just as in aside, overlays seem like a  
> > much
> > better conceptual fit for fontification than text properties do.
>
> Why?

AIUI, text properties are considered *part* of the document being  
edited, which is why modifying text properties modifies the buffer  
modified flag, why they appear in the undo list, and so on, whereas  
overlays are for transient, regenerable information *about* the text  
in the document. Fontification is definitely regenerable, transient  
information, and so should use overlays. I'm also not particularly  
fond of fontification being carried around when I kill and yank.

But like you said, these are minor issues and can be worked around.  
It's no big deal, and text properties perform much better on GNU  
Emacs. (IIRC, each overlay actually uses a pair of markers. It's no  
wonder that using hundreds or thousands of them is slow.)

> [Aside from the "changing text-properties affect buffer-modified  
> status"
> silliness, which I think is probably a mistake, but that's easy enough
> to work around.]

Don't forget the undo entries too. Actually, the full list comes from  
font-lock.el:

(defmacro save-buffer-state (varlist &rest body)
    "Bind variables according to VARLIST and eval BODY restoring  
buffer state."
    (declare (indent 1) (debug let))
    (let ((modified (make-symbol "modified")))
      `(let* ,(append varlist
                      `((,modified (buffer-modified-p))
                        (buffer-undo-list t)
                        (inhibit-read-only t)
                        (inhibit-point-motion-hooks t)
                        (inhibit-modification-hooks t)
                        deactivate-mark
                        buffer-file-name
                        buffer-file-truename))
         (progn
           ,@body)
         (unless ,modified
           (restore-buffer-modified-p nil)))))