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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: |
Mon, 12 Dec 2022 19:26:28 +0100 |
User-agent: |
mu4e 1.8.9; emacs 28.2 |
Note this part of the `pcase-let' documentation string:
| Each EXP should match (i.e. be of compatible structure) to its
| respective PATTERN; a mismatch may signal an error or may go
| undetected, binding variables to arbitrary values, such as
nil.
Your first case is invalid because the pattern doesn't match the
value.
Here it goes undetected and bindings get established.
I see! That clarifies the behavior I was seeing.
This behavior is not perfect, but AFAIR it has been preferred
over the
less efficient code that better checks would mean. So it's the
programmer's task to use only matching patterns. This is not
really a
restriction because `pcase-let' is intended to create bindings,
not for
testing whether a pattern matches some value.
I suppose that's a fair tradeoff.
This idea had been discussed in the past. It had some votes but
it had
been decided not to implement such a feature because it would
not really
fit into the existing semantics, just for the sake of leaving
out one
",". So I'm afraid I don't think we will change this.
Right, it doesn't seem like such a huge win now that I understand
that the behavior of pcase-let was according to its specification
and there was no inconsistency to begin with. It would maybe make
the code a little bit easier to read in certain cases, but I can
see your point.
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.
Your quote above made everything clear, but I completely missed it
since I was reading the Emacs Lisp manual's explanation [1] rather
than pcase-let's docstring. Maybe it would be beneficial to
include the above quote in the manual as well.
[1]
<https://www.gnu.org/software/emacs/manual/html_node/elisp/Destructuring-with-pcase-Patterns.html>
Thanks!
hokomo