Subject: bug#69108: false-positive warning "variable ‘_’ not left unused" in if-let* and if-let

From: Konstantin Kharlamov <[email-masked]>
Date: Sat, 17 Feb 2024 12:20:16 +0300

On Sat, 2024-02-17 at 10:04 +0200, Eli Zaretskii wrote:
> > Cc: [email-masked]
> > Date: Sat, 17 Feb 2024 01:28:55 +0100
> > From:  Michael Heerdegen via "Bug reports for GNU Emacs,
> >  the Swiss army knife of text editors" <[email-masked]>
> > 
> > --- a/lisp/subr.el
> > +++ b/lisp/subr.el
> > @@ -2575,12 +2575,12 @@ delay-mode-hooks
> >  (defun internal--build-binding (binding prev-var)
> >    "Check and build a single BINDING with PREV-VAR."
> >    (setq binding
> > -        (cond
> > -         ((symbolp binding)
> > +        (pcase binding
> > +         ((pred symbolp)
> >            (list binding binding))
> > -         ((null (cdr binding))
> > -          (list (make-symbol "s") (car binding)))
> > -         (t binding)))
> > +         ((or `(,test) `(_ ,test))
> > +          (list (make-symbol "s") test))
> > +         (_ binding)))
> 
> Thanks, but can we please leave this as 'cond', instead of converting
> it to a 'pcase'?  It doesn't seem to be justified here, and even less
> so since you need to rewrite all the existing conditions.

Just a side note, from my experience pcase is very slow¹, so if a
function supposed to be called often, I presume it's better to avoid
`pcase`. Although, Idk how it compares to `cond`. But judging from the
fact `cond` is implemented in C, it is likely faster.

1:
https://github.com/ankurdave/color-identifiers-mode/commit/bc566bcdbd79f230b35eafd2b6c4f8428402ec09