Re: find-file-noselect needs save-match-data

[martin rudalics <rudalics@gmx.at>]

 > AFAIK save-match-data is only needed in 2 circumstances:
 >
 > 1 - when implementing a function that needs to leave the
 >     match-data unaffected.  This is typically only the case for functions
 >     that are placed on hooks.  In most cases such save-match-data are
 >     necessary because the function you define does touch the match-data
 >     (otherwise you wouldn't have used save-match-data) and the function that
 >     runs the hook will sooner or later need the match-data to
 >     stay unaffected.

Exact.

 > 2 - around code that needs to be run after string-match but before match-end

I fail to understand this.

 >     and of which we do not know that it leaves the match-data unaffected.
 >     This may be unnecessary because the code may in fact preserve the
 >     match-data even though you don't know it.
 >
 > So although number 1 may be an unnecessary use of save-match-data, I find
 > it's usually useful/necessary.  Number 2 OTOH can often turn out to be
 > unnecessary.  Luckily number 2 is very rarely needed because in most cases
 > it's easier to restructure your code such that save-match-data is not needed
 > any more.

Saving match-data is needed if the index for accessing match-data is
known only after performing a second match operation like in

(let (index)
   (when (re-search-forward ...)
     (save-match-data
       (if (string-match ...)
	  (setq index ...)))
     (match-beginning index)))

I think we agree that this is rarely used.

 > So I don't see current usage as problematic w.r.t efficiency.

The info.el examples in my earlier mail seem to contradict this.

 > The only real problem with it is that people tend to not know/understand it
 > and are thus tempted to complain that most functions need to be wrapped in
 > save-match-data, which then leads to long discussions such as this one.
 > Maybe we should make the convention more prominent by adding it to the
 > docstring of save-match-data?

We probably should remove bad uses of `save-match-data' from Emacs code
in the first place.