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Re: Why looking-at-p works?

From: John Mastro
Subject: Re: Why looking-at-p works?
Date: Tue, 6 Mar 2018 10:53:15 -0800

Marcin Borkowski <> wrote:
> Hi all,
> (defsubst looking-at-p (regexp)
>   "\
> Same as `looking-at' except this function does not change the match data."
>   (let ((inhibit-changing-match-data t))
>     (looking-at regexp)))
> What happens is that if I make looking-at in the above code fail (e.g.,
> by saying (looking-at-p 123)), inhibit-changing-match-data remains nil,
> even though there is no unwind-protect here.  Why does it work like
> this?

If I understand your question correctly, it's nothing specific to
looking-at-p. Let-bindings are protected by a sort of implicit
unwind-protect so that they're always "un-done" upon exiting the scope,
even in case of non-local exits like an error. Otherwise errors could
leave global variables in unpredictable states.


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