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bug#59887: pcase vs. pcase-let: Underscore in backquote-style patterns
From: |
hokomo |
Subject: |
bug#59887: pcase vs. pcase-let: Underscore in backquote-style patterns |
Date: |
Wed, 07 Dec 2022 17:28:57 +0100 |
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
- bug#59887: pcase vs. pcase-let: Underscore in backquote-style patterns,
hokomo <=