From: Michael Heerdegen <michael_heerdegen@web.de>
To: Drew Adams <drew.adams@oracle.com>
Cc: Lars Ingebrigtsen <larsi@gnus.org>, 31350@debbugs.gnu.org
Subject: bug#31350: 27.0; `pcase' message: "Redundant pcase pattern"
Date: Sun, 04 Oct 2020 03:37:38 +0200 [thread overview]
Message-ID: <87sgau93fh.fsf@web.de> (raw)
In-Reply-To: <8bcc3be5-c9db-4015-8f0f-4efe77ff27ee@default> (Drew Adams's message of "Sat, 3 Oct 2020 15:58:34 -0700 (PDT)")
Drew Adams <drew.adams@oracle.com> writes:
> I'll repeat the request, which is what I think a user
> would like to know:
>
> Can the messaging at least tell you:
> (1) all of a set of clauses that are mutually redundant and
> (2) which one of them will actually be used by the compiled
> code, the others presumably having been pruned?
>
> IOW, what's the actual effect, for users? How does pcase
> deal with the redundancy? Can that at least be documented
> somewhere (maybe it is already)?
I think you make an error in reasoning: we don't have mutual redundancy
here. A case can be redundant when it will never match because whenever
would match, a previous case in the cases list always matches, so it is
effectively shadowed. But this is not symmetric (cases are always tried
from first to last (!), and e.g. a `_' catchall pattern in one case will
make all following cases redundant, but not the other way round).
I think the message is more like a warning that a case can never match,
and in all cases where this happened to me, as also in your case, the
reason was a very obvious editing mistake. I don't think there is much
to say here, Emacs just tells you: this case here will never be used,
look what you have done wrong. There is a problem with your code
whenever you see that message. And there is nothing to say about the
semantics as well.
BTW, I think the implementation only covers the most obvious and
simplistic cases, like those involving catchall patterns or duplicated
patterns.
Michael.
next prev parent reply other threads:[~2020-10-04 1:37 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-05-03 0:48 bug#31350: 27.0; `pcase' message: "Redundant pcase pattern" Drew Adams
2018-05-03 1:16 ` Michael Heerdegen
2018-05-03 18:18 ` Drew Adams
2020-08-20 15:55 ` Lars Ingebrigtsen
2020-10-03 22:52 ` Michael Heerdegen
2020-10-03 22:58 ` Drew Adams
2020-10-04 1:37 ` Michael Heerdegen [this message]
2020-10-04 2:06 ` Drew Adams
2020-10-04 14:03 ` Lars Ingebrigtsen
2020-10-04 17:45 ` Drew Adams
2020-10-04 23:55 ` Michael Heerdegen
2020-10-04 23:54 ` Michael Heerdegen
2020-12-12 13:24 ` Lars Ingebrigtsen
2020-12-12 17:00 ` Drew Adams
2020-12-12 19:01 ` Eli Zaretskii
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