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

From: martin rudalics
Subject: Re: find-file-noselect needs save-match-data
Date: Thu, 14 Jun 2007 08:57:40 +0200
User-agent: Mozilla Thunderbird 1.0 (Windows/20041206)

> 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.


> 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 ...)
      (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.

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