GNU bug report logs - #45619
No warning when pcase-let is binding dynamic variable

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From: Stefan Monnier <monnier <at> iro.umontreal.ca>
To: Michael Heerdegen <michael_heerdegen <at> web.de>
Cc: jixiuf <jixiuf <at> qq.com>, 45619 <at> debbugs.gnu.org
Subject: bug#45619: 28.0.50; pcase-let on MacOS doesn't work
Date: Mon, 04 Jan 2021 21:06:27 -0500

>> It's messier than that: the issue is that it doesn't bind variables
>> which aren't used lexically (this is needed to avoid spurious warnings
>> about unused vars when the var is used in on place but not in another).
> BTW, a similar issue (pitfall) I faced was that I sometimes expect that
> when a variable is already bound, already the first appearance of the
> symbol would be turned into an equality test.

Hmm... so-called "non-linear patterns".  We should emit a warning when the
same var is used twice in a pattern, indeed, to avoid surprises.

We could also do as you describe but I'm not completely sure it's a good
idea (it begs questions about which equality test to use and when it
should be tested).

> I wonder if pcase variable bindings could be hygienic in the sense that
> they would use fresh symbols internally.  That would maybe make the
> semantics clearer.

I don't know what you mean by that.  Can you clarify?


        Stefan

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