GNU bug report logs - #71503
30.0.50; pcase-lambda (with "and" and "guard") does not work as expected

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From: Eli Zaretskii <eliz <at> gnu.org>
To: Philip Kaludercic <philipk <at> posteo.net>
Cc: mekeor <at> posteo.de, monnier <at> iro.umontreal.ca, 71503 <at> debbugs.gnu.org
Subject: bug#71503: 30.0.50; pcase-lambda (with "and" and "guard") does not work as expected
Date: Sat, 20 Jul 2024 13:46:45 +0300

> From: Philip Kaludercic <philipk <at> posteo.net>
> Cc: mekeor <at> posteo.de,  monnier <at> iro.umontreal.ca,  71503 <at> debbugs.gnu.org
> Date: Sat, 20 Jul 2024 10:29:55 +0000
> 
> >> > > The difference is that pcase-lambda doesn't do case-distinction, but
> >> > > just pattern matching/destruncting.  So if the pattern-matching fails,
> >> > > then the variable is just not bound, instead of the entire expression
> >> > > falling back to returning no value/nil.  I am guessing you wanted to
> >> > > have something like Scheme's `case-lambda'[0]?  Or we could clarify this
> >> > > point in the docstring.
> >> > 
> >> > Would you mind suggesting a clarification for the doc string (and the
> >> > ELisp manual as well)?
> 
> Sorry, I didn't realise you were pining me.  I was thinking of something
> like
> 
> diff --git a/lisp/emacs-lisp/pcase.el b/lisp/emacs-lisp/pcase.el
> index 5a7f3995311..f546ba34c7c 100644
> --- a/lisp/emacs-lisp/pcase.el
> +++ b/lisp/emacs-lisp/pcase.el
> @@ -241,9 +241,10 @@ pcase-exhaustive
>  ;;;###autoload
>  (defmacro pcase-lambda (lambda-list &rest body)
>    "Like `lambda' but allow each argument to be a pattern.
> -I.e. accepts the usual &optional and &rest keywords, but every
> -formal argument can be any pattern accepted by `pcase' (a mere
> -variable name being but a special case of it)."
> +I.e. accepts the usual &optional and &rest keywords, but every formal
> +argument can be any pattern accepted by `pcase' (a mere variable name
> +being but a special case of it).  Keep in mind that BODY is always
> +evaluated, regardless of whether the argument-patterns match or not."
>    (declare (doc-string 2) (indent defun)
>             (debug (&define (&rest pcase-PAT) lambda-doc def-body)))
>    (let* ((bindings ())
> 
> but now I notice that `pcase-let*' documents that all expressions should
> match,
> 
>   Each EXP should match its respective PATTERN (i.e. be of structure
>   compatible to PATTERN); a mismatch may signal an error or may go
>   undetected, binding variables to arbitrary values, such as nil.
> 
> Since `pcase-lambda' inherits these semantics we would have to propagate
> this promise -- or be more specific about what happens, e.g. binding
> values to nil.

Thanks.  If this still can be fixed by some documentation changes, I'd
prefer to do that.

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