From: hokomo <hokomo@airmail.cc>
To: Michael Heerdegen <michael_heerdegen@web.de>
Cc: 59887-done@debbugs.gnu.org
Subject: bug#59887: pcase vs. pcase-let: Underscore in backquote-style patterns
Date: Tue, 13 Dec 2022 03:26:14 +0100 [thread overview]
Message-ID: <87h6y081hs.fsf@airmail.cc> (raw)
In-Reply-To: <87a63s2gcm.fsf@web.de>
> My question is that when we make the text even longer, would
> that help
> people that don't read carefully (because we don't need to
> address
> others) at all?
I believe it would. Even though I should've been more careful with
reading the whole page, one's first instinct (at least mine) when
reading a reference manual is to jump directly to the operator in
question and expect to find all of the necessary and essential
information there, whether it is a detailed explanation or just a
hint or short remark mentioning some concepts that were introduced
more thoroughly earlier in the manual.
As an example, the beginning of the Handling Errors page [1]
describes, among other things, the meaning of the `debug' symbol
within a condition-case handler's condition list. However, the
description of condition-case specifically also includes the short
remark "which can include debug to allow the debugger to run
before the handler" which is useful to point the reader to the
description at the beginning (all it takes is searching for
"debug" on the same page after reading the remark).
[1]
<https://www.gnu.org/software/emacs/manual/html_node/elisp/Handling-Errors.html>
> My second question is if that would have helped you at all,
> because your
> crucial misunderstanding was about the meaning of `_`. Using
> patterns
> in `pcase-let' that don't match generally doesn't make much
> sense, it's
> totally unclear what would happen in this case. That's another
> reason
> why I don't want to over-emphasize this case.
>
> Maybe saying that `_` is not special when used as a QPAT would
> make
> sense, in (info "(elisp) Backquote Patterns"). I mean in this
> paragraph:
>
> | ‘SYMBOL’
> | ‘KEYWORD’
> | ‘NUMBER’
> | ‘STRING’
> | Matches if the corresponding element of EXPVAL is ‘equal’
> to the
> | specified literal object.
>
> We could add that `_` is not special (no symbol is special as a
> qpat,
> actually). Would that give a useful hint? It seems that some
> people
> seem to expect that `_` is special everywhere in pcase.
That is indeed the core of the issue and I definitely think it
would be a good idea to have an explicit statement that the
underscore symbol is not special as a QPAT. You can sort of infer
it from the specification, but given the unspecified behavior of
pcase-let in the case of a non-match, making it explicit would be
nice.
I think I would've ended up poking around pcase-let in any case
after being puzzled about its behavior, just out of curiosity.
Having a short remark about "structural compatibility" in the
documentation of the specific operator would then help me quickly
narrow down to what I need.
hokomo
prev parent reply other threads:[~2022-12-13 2:26 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-12-07 16:28 bug#59887: pcase vs. pcase-let: Underscore in backquote-style patterns hokomo
2022-12-09 2:57 ` Michael Heerdegen
2022-12-12 2:50 ` Michael Heerdegen
2022-12-12 18:26 ` hokomo
2022-12-13 1:17 ` Michael Heerdegen
2022-12-13 1:19 ` hokomo
2022-12-13 2:21 ` Michael Heerdegen
2022-12-13 2:26 ` hokomo [this message]
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