save-restriction, save-excursion

save-restriction, save-excursion

[pokerface]

I am beginning to study Emacs Lisp and I have the following question:

Why is it wrong to write (save-restriction
                           (save-excursion
                               ....))

and it is recommended instead to write
                         (save-excursion
                           (save-restriction
                               ....))

Can someone please give an example of the difference?

Thank you.

Re: save-restriction, save-excursion

[Ryan Yeske]

pokerface <pokerface@use.net> writes:

> I am beginning to study Emacs Lisp and I have the following question:
> 
> Why is it wrong to write (save-restriction
>                            (save-excursion
>                                ....))
> 
> and it is recommended instead to write
>                          (save-excursion
>                            (save-restriction
>                                ....))

From the node "Narrowing" in the elisp manual:

     `save-restriction' does _not_ restore point and the mark; use
     `save-excursion' for that.  If you use both `save-restriction' and
     `save-excursion' together, `save-excursion' should come first (on
     the outside).  Otherwise, the old point value would be restored
     with temporary narrowing still in effect.  If the old point value
     were outside the limits of the temporary narrowing, this would
     fail to restore it accurately.

> Can someone please give an example of the difference?

(defun do-excursion-restriction ()
  (save-excursion
    (save-restriction
      (do-it))))

(defun do-restriction-excursion ()
  (save-restriction
    (save-excursion
      (do-it))))

(defun do-it ()
  (goto-char (point-min))
  (narrow-to-region (point) (+ 1 (point))))
    
;; this one restores point properly:
(do-excursion-restriction)
;; this one doesn't:
(do-restriction-excursion)

;; Hope that helps.
;; Ryan

Re: save-restriction, save-excursion

[David Kastrup]

Ryan Yeske <rcyeske+spam@vcn.bc.ca> writes:

> pokerface <pokerface@use.net> writes:
> 
> > I am beginning to study Emacs Lisp and I have the following question:
> > 
> > Why is it wrong to write (save-restriction
> >                            (save-excursion
> >                                ....))
> > 
> > and it is recommended instead to write
> >                          (save-excursion
> >                            (save-restriction
> >                                ....))
> 
> From the node "Narrowing" in the elisp manual:
> 
>      `save-restriction' does _not_ restore point and the mark; use
>      `save-excursion' for that.  If you use both `save-restriction' and
>      `save-excursion' together, `save-excursion' should come first (on
>      the outside).  Otherwise, the old point value would be restored
>      with temporary narrowing still in effect.  If the old point value
>      were outside the limits of the temporary narrowing, this would
>      fail to restore it accurately.

Hmmm.  What if I have a restriction active and write
(save-excursion
  (save-restriction
    (widen)
    (goto (point-min))))

When the restriction gets restored, point will lie outside of the
restored restriction.  Wouldn't that cause trouble?

Should one rather have
(save-excursion
  (save-restriction

When the change of restriction is to something narrower, but

(save-restriction
  (save-excursion

when the change is to something wider?

Or is this nonsense for some reason?

-- 
David Kastrup, Kriemhildstr. 15, 44793 Bochum
Email: David.Kastrup@t-online.de

Re: save-restriction, save-excursion

[Ryan Yeske]

David Kastrup <David.Kastrup@t-online.de> writes:

> Ryan Yeske <rcyeske+spam@vcn.bc.ca> writes:
> 
> > From the node "Narrowing" in the elisp manual:
> > 
> >      `save-restriction' does _not_ restore point and the mark; use
> >      `save-excursion' for that.  If you use both `save-restriction' and
> >      `save-excursion' together, `save-excursion' should come first (on
> >      the outside).  Otherwise, the old point value would be restored
> >      with temporary narrowing still in effect.  If the old point value
> >      were outside the limits of the temporary narrowing, this would
> >      fail to restore it accurately.
> 
> Hmmm.  What if I have a restriction active and write
> (save-excursion
>   (save-restriction
>     (widen)
>     (goto (point-min))))
> 
> When the restriction gets restored, point will lie outside of the
> restored restriction.  Wouldn't that cause trouble?

Try it :)

It helps me to think about it like this:

1) save the point
2) save the restriction
3) frob the restriction (narrow *or* widen)
4) frob the point
5) restore the old restriction
6) restore the old point
 
It doesn't matter whether you are widening or narrowing.  When the
restriction is restored (step 5), the point is adjusted to be within
the restriction (the point can never be outside the current
restriction).  It doesn't really matter though, since the point is
then immediatly restored to the initial value within the initial
restriction at step 6.

Ryan

Re: save-restriction, save-excursion

[pokerface]

Thank you gentlemen for the helpful replies!

Ryan Yeske <rcyeske@vcn.bc.ca> wrote in
news:87fzw9ceye.fsf@cut.hotdog.tmp: 

> David Kastrup <David.Kastrup@t-online.de> writes:
> 
>> Ryan Yeske <rcyeske+spam@vcn.bc.ca> writes:
>> 
>> > From the node "Narrowing" in the elisp manual:
>> > 
>> >      `save-restriction' does _not_ restore point and the mark; use
>> >      `save-excursion' for that.  If you use both `save-restriction'
>> >      and `save-excursion' together, `save-excursion' should come
>> >      first (on the outside).  Otherwise, the old point value would
>> >      be restored with temporary narrowing still in effect.  If the
>> >      old point value were outside the limits of the temporary
>> >      narrowing, this would fail to restore it accurately.
>> 
>> Hmmm.  What if I have a restriction active and write
>> (save-excursion
>>   (save-restriction
>>     (widen)
>>     (goto (point-min))))
>> 
>> When the restriction gets restored, point will lie outside of the
>> restored restriction.  Wouldn't that cause trouble?
> 
> Try it :)
> 
> It helps me to think about it like this:
> 
> 1) save the point
> 2) save the restriction
> 3) frob the restriction (narrow *or* widen)
> 4) frob the point
> 5) restore the old restriction
> 6) restore the old point
>  
> It doesn't matter whether you are widening or narrowing.  When the
> restriction is restored (step 5), the point is adjusted to be within
> the restriction (the point can never be outside the current
> restriction).  It doesn't really matter though, since the point is
> then immediatly restored to the initial value within the initial
> restriction at step 6.
> 
> Ryan
>