bug#72328: [PATCH] Nested backquote in pcase

[Eli Zaretskii <eliz@gnu.org>]

> From: Thuna <thuna.cing@gmail.com>
> Cc: michael_heerdegen@web.de, monnier@iro.umontreal.ca, 72328@debbugs.gnu.org
> Date: Mon, 05 Aug 2024 21:32:43 +0200
> 
> >> Note that I do not believe that there are no people who would be
> >> effected by this, positively or negatively.
> >
> > We've learned from bitter experience that such arguments are usually
> > false.  IOW, we don't really know enough to make such assertions.
> 
> I think there is a misunderstanding.  I am not saying that there isn't
> anyone who would be effected by this, it is the opposite.  I understand
> that this will effect people, and I agree that at minimum there needs to
> be a decent period where the current behavior is maintained but marked
> as obsolete.

How do you envision making such a behavior change in a way that will
leave the current behavior still maintained (and obsolete)?

> An indefinite feature-freeze is where I have a problem.

Disagreeing with a specific change is not tantamount to an indefinite
feature-freeze.  It is quite possible that someone will come up with a
different idea of a change, which we will be able to reconcile easier
with the previous behavior.

> >> (defun macroexp-null (exp)
> >>   "Return non-nil if EXP will always evaluate to nil.
> >> This form does not take non-local exits or side-effects into account."
> >>   (pcase exp
> >>     ((or 'nil ''nil '#'nil '`nil ``,,(pred macroexp-null))
> >>      t)))
> >> 
> >> which without this change would read as:
> >> 
> >> (defun macroexp-null (exp)
> >>   "Return non-nil if EXP will always evaluate to nil.
> >> This form does not take non-local exits or side-effects into account."
> >>   (pcase exp
> >>     ((or 'nil ''nil '#'nil '`nil
> >>          `(,'\` (,'\, ,(pred macroexp-null))))
> >>      t)))
> >
> > Thanks, now you just need to explain why you needed this code and what
> > did its caller do to require this.
> 
> I do not understand what you are asking for.  Whether `macroexp-null'
> should exist or not, what it is trying to do should be fairly clear, so
> should the way in which it benefits from the changed behavior.

I asked to explain _why_ you need this.  Risking to say the obvious, a
program exists to do some job, and a function like the above is
therefore part of some larger job.  We are asking you to describe the
higher-level context, which we could then use to try to decide whether
the need is important enough to justify the backward-incompatible
change.

> I also cannot provide any justification for this patch above and beyond
> what I have already mentioned in my initial message: This patch
> establishes a symmetry between pcase's backquote pattern and quasiquote,
> which allows trivially matching against the result of a quasiquote form.

We would like to hear reasons for wanting this.

> I would appreciate it if you would state your opinion on this patch,
> putting aside concerns of backwards compatibility for a moment.  I am
> working under the assumption that this is an improvement and is
> desirable, yet I have not yet heard from you or Stefan as to whether you
> see it that way or not.

I believe Stefan did say that.  Me, my only stake here is the concern
of backwards compatibility, which is why I'm talking only about that.

Thanks.