GNU bug report logs - #31350
27.0; `pcase' message: "Redundant pcase pattern"

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From: Michael Heerdegen <michael_heerdegen <at> web.de>
To: Drew Adams <drew.adams <at> oracle.com>
Cc: Lars Ingebrigtsen <larsi <at> gnus.org>, 31350 <at> debbugs.gnu.org
Subject: bug#31350: 27.0; `pcase' message: "Redundant pcase pattern"
Date: Sun, 04 Oct 2020 03:37:38 +0200

Drew Adams <drew.adams <at> 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.

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