bug#66750: Unhelpful text in C-h v for variables with a lambda form as value

[Alan Mackenzie]

Hello, Eli.

On Sat, Oct 28, 2023 at 21:38:36 +0300, Eli Zaretskii wrote:
> > Cc: 66750@debbugs.gnu.org, Andrea Corallo <acorallo@gnu.org>,
> >  Stefan Kangas <stefankangas@gmail.com>, acm@muc.de
> > Date: Sat, 28 Oct 2023 18:17:04 +0000
> > From: Alan Mackenzie <acm@muc.de>

> > > but that's 127kB, so ... could [you] briefly describe the overall design
> > > (IOW, how it's seen by ELisp programmers, byte-compiler hackers, and
> > > ELisp users)?

> > Certainly.  Each lambda expression has (usually) a defun within which it
> > is defined.  Sometimes it's in a defvar, or defcustom.  That
> > @dfn{defining symbol} is recorded in the lambda form in one of three
> > ways:
> > (i) For a cons form, it's (cadr form), a new field inserted between the
> >   symbol `lambda' and the argument list.
> > (ii) For a byte-compiled form, it's (aref form 5), this new field going
> >   after the doc string and before any interactive form in the compiled
> >   form.
> > (iii) For a native-compiled subr it's (subr-native-defining-symbol
> > subr), a function defined in data.c.  It accesses a new field in struct
> > Lisp_Subr called defining_symbol.

> > There are lots of detailed changes in eval.c and bytecomp.el (and
> > friends).  Also the macro `lambda' in subr.el has been amended to insert
> > the current global defining-symbol if there isn't already a non-nil
> > symbol in that position.  cl-print-object/compiled-function has been
> > amended to print the defining-symbol, and there is a new
> > cl-print-object/subr which does the same.

> > The intention is that compiled objects from earlier Emacsen can still be
> > loaded and run by feature/named-lambdas, just without the defining
> > symbols (which will appear to be nil).

> > > Also, what other approaches have you considered/tried and what were the
> > > problems you've encountered, if any?

> > feature/named-lambdas was originally intended for use in backtraces.

> > For the current bug, I've considered individually replacing each lambda
> > with a named defun, so that C-h v will show that name rather than an
> > unhelpful byte/native compiled anonymous function.  That would be a lot
> > of work - my scripting found 63 defcustoms set to lambdas, 29 uses in
> > doc strings, and 215 suspicious occurrences with ordinary variables
> > (quite a few of which will be harmless).  Amending all these (I guess
> > around 200 lambdas) would probably be too much work.

> Thanks.  However, now I'm confused: what exactly does this feature
> give us, if it doesn't handle all the lambdas in the Emacs tree and
> (AFAIU) will not affect lambdas in third-party packages?  Which
> lambdas will now have a defining symbol, and which will not?

It will handle all the lambdas in the Emacs tree.  Each of them will get
a defining-symbol set.  Also third-party packages with lambdas will also
get defining-symbols, provided they are compiled with
feature/named-lambdas.  It's just that .elc's/.eln's built with, say,
Emacs-29 will continue to be loadable and runnable, but obviously without
the defining symbol facility for those lambdas.

-- 
Alan Mackenzie (Nuremberg, Germany).