bug#72328: [PATCH] Nested backquote in pcase

[Eli Zaretskii <eliz@gnu.org>]

> Cc: 72328@debbugs.gnu.org
> Date: Mon, 29 Jul 2024 18:03:59 +0200
> From:  Michael Heerdegen via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> Thuna <thuna.cing@gmail.com> writes:
> 
> > > This would be a backward-incompatible change, breaking existing code.
> >
> > Yes, I forgot to mention this in my original report, but given how
> > unintuitive (IMO) the current behavior is, I expect that very few people
> > will be using patterns which would be effected by this change.
> 
> We have _dozens_ of such patterns in the Emacs Elisp sources that would
> break.  Your expectation is wrong.
> 
> 
> > > And this change would break the symmetry between `backquote' the macro
> > > and backquote patterns in `pcase'.  This is an important design idea.
> >
> > I am not quite sure why you think that this breaks symmetry with the
> > backquote macro;
> 
> We want that something like this works as expected:
> 
> #+begin_src emacs-lisp
> (let ((a 1) (b 2))
>   (pcase `(69 foo (97 ,a ((,b))))
>     (`(69 foo (97 ,a ((,b))))
>     (list a b)))) ==> (1 2)
> #+end_src
> 
> I think it's obvious what I mean with symmetry between backquote and
> pcase backquote without giving a formal definition.
> 
> 
> > the purpose of this patch was to establish a symmetry
> > in the first place.  Would you consider the examples provided as
> > unnatural?
> 
> I'm more concerned about what we would loose.
> 
> 
> > AFAICT right now the only way to match an unevaluated (or quoted) `,foo
> > is with `(,'\` (,'\, foo)) which is quite unideal.
> 
> If `foo` is the literal symbol you can simply match using a quote pattern
> like in
> 
>   (pcase '`,foo
>     ('`,foo t))
>  ==> t
> 
> 
> > It is not often that you need this but having this not be
> > representable in this way is not particularly useful.
> 
> It indeed gets ugly when one wants to match a non-constant
> backquote expression using a backquote pcase pattern with partial
> unquotes (the "mixed case").
> 
> But making pcase backquote patterns less expressive just to make this
> special case simpler doesn't make sense to me.  OTOH I agree that having
> a convenient solution for this kind of problem would be nice.

Stefan, any comments?