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bug#31350: 27.0; `pcase' message: "Redundant pcase pattern"
From: |
Drew Adams |
Subject: |
bug#31350: 27.0; `pcase' message: "Redundant pcase pattern" |
Date: |
Sat, 3 Oct 2020 19:06:46 -0700 (PDT) |
> > 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.
If what you say is the case then the help should tell
users that. To me it's not obvious.
And even if a user (somehow) understands that later
cases are made redundant by earlier ones, how to tell
which earlier ones are implicated?