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

[Alan Mackenzie]

Hello, Stefan.

On Sat, Oct 28, 2023 at 13:17:13 -0400, Stefan Monnier wrote:
> >> > Currently feature/named-lambdas is mostly working, not fully working.

> Didn't know about this feature.

It's been hanging around the savannah repository since July.

> I like the idea of keeping better track of the origin of lambda
> expressions (tho, admittedly, the same problem occurs with other kinds
> of data: I have several times been faced with a keymap or a char-table,
> wondering where the heel it came from).

Maybe something similar might be possible for those type of objects.

> I took a look at

>     git log -p main..origin/feature/named-lambdas

> 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.

>         Stefan

-- 
Alan Mackenzie (Nuremberg, Germany).