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Subject: |
pcase vs. pcase-let: Underscore in backquote-style patterns |
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Date: |
Wed, 07 Dec 2022 17:28:57 +0100 |
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User-agent: |
mu4e 1.8.9; emacs 28.2 |
Hello,
How exactly is the underscore symbol treated in pcase's
backquote-style patterns? Seems like at least pcase and pcase-let
treat it inconsistently (I haven't checked the other pcase
operators).
pcase treats the underscore as a literal symbol to match, hence
this fails:
(pcase '(1 2 3)
(`(1 _ ,x)
x))
;; => nil
Adding the missing comma in front of the underscore gives us the
expected behavior:
(pcase '(1 2 3)
(`(1 ,_ ,x)
x))
;; => 3
However, pcase-let is less strict about this, producing the same
result with or without the comma:
(pcase-let ((`(1 _ ,x) '(1 2 3)))
x)
;; => 3
(pcase-let ((`(1 ,_ ,x) '(1 2 3)))
x)
;; => 3
Additionally, I would think one would still be able to match a
literal underscore symbol even with pcase-let, but the following
still ends up matching:
(pcase-let ((`(1 ,'_ ,x) '(1 2 3)))
x)
;; => 3
I think that matching a literal underscore symbol is rare enough
that the ideal behavior would probably be for an underscore within
a backquote template to be treated as a wildcard whenever it
appears literally (e.g., `(1 _)) or unquoted (e.g., `(1 ,_)).
However, as soon as explicitly quoted (e.g., `(1 ,'_)), it should
be treated as a match for a literal underscore symbol. In other
words, I would expect the following would be different from the
above:
(pcase '(1 2 3)
(`(1 _ ,x)
x))
;; => 3 (instead of nil)
(pcase-let ((`(1 ,'_ ,x) '(1 2 3)))
x)
;; => nil (instead of 3)
I'm not 100% sure if these requirements would cause any
backwards-incompatible changes or inconsistencies with the other
pcase operators though. I'm also assuming that `(1 _) and `(1 ,'_)
can be distinguished, but maybe this is not true?
hokomo
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--- Begin Message ---
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Subject: |
Re: bug#59887: pcase vs. pcase-let: Underscore in backquote-style patterns |
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Date: |
Mon, 12 Dec 2022 03:50:41 +0100 |
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User-agent: |
Gnus/5.13 (Gnus v5.13) |
hokomo <hokomo@airmail.cc> writes:
> How exactly is the underscore symbol treated in pcase's
> backquote-style patterns?
I think the current behavior can be understood and explained from the
documentation quite well. If you can point to something concrete
missing, please elaborate, and we can reopen this report.
For now I'm closing it: everything works as documented, and we had
decided not to complicate the semantics of `_`, so as far as I see it
nothing is to be done here.
Thanks,
Michael.
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