Re: save-excursion and the mark

[Ivan Shmakov]

>>>>> Andreas Röhler <address@hidden> writes:
>>>>> On 25.02.2015 03:59, Stefan Monnier wrote:

 >>> See for example indent-code-ridigly.

 >> Can you give more details about how this requires save&restore of
 >> the mark (or its "active" status)?

 > Think a shifting code by TAB in python-mode.

        FWIW, python.el doesn’t seem to use the mark.

 > Mark might exist somewhere in line. Shifting will use its own region
 > – but original region should be restored – if any.  Shifting by
 > itself should not create a region nor clutter the mark-ring.

        There seem to be a misunderstanding; the notion of /Emacs Lisp/
        region is different to that of /Emacs/ region, the former being
        (more or less) a way of saying “from here to there”, and
        does /not/ (generally) involve either mark or point.

        For instance, indent-code-ridigly can be used from Lisp like:

   (indent-code-ridigly 13 37)

        Or perhaps:

   (when-let ((here  (re-search-backward "START" nil t))
              (there (re-search-forward  "END"   nil t)))
     (indent-code-ridigly here there))

        With the common idiom being:

   (let ((save (point)))
     ; Move point forward, by some amount.
     (indent-code-ridigly save (point)))

        As should be obvious, the mark is /not/ involved in either case.
        And, arguably, it /should not/ be.

[…]

 > commit b1d6ddd44614c84746f5ee494e1f29cd9be8a2d8

 > Notable changes to goto-line, i. e. calling push-mark.

        That makes a point, indeed: there’s a class of Emacs commands
        intended first and foremost for interactive use.  Naturally,
        goto-line is one of them; to quote its docstring:

This function is for interactive use only;
in Lisp code use `forward-line' instead.

        The proposed change to save-excursion would expose the bugs in
        the code which uses such commands inside of save-excursion,
        which now masks the unfortunate side-effects of such (mis)use.

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