Re: Correct line/column numbers in byte compiler messages [Was: GNU is looking for Google Summer of Code Projects]

[Stefan Monnier]

>> I think fat-cons cells are cheap to implement (with (hopefully) no
>> performance impact when not used .....
> They may be cheap to implement in themselves, but adapting the entire
> byte compiler and all our macros to the heavily restricted semantics
> they would impose would be an enormous job.

The idea is that you want to make it work acceptably even if only some
of the cons-cells are fat.  This way, as you adapt the existing code to
pay attention/preserve fat-cons-cells, your location information gets
more and more precise, but even before you've done this enormous job,
you already get some of the benefit.

> I've tried something similar, and gave up in exhaustion.

If you want "exact" results, then you'll get tired long before getting
there, yes.  But it's not needed.

> Where does this 99.9% come from?  How is this cons tracking you're
> proposing supposed to work, when there are an infinite number of
> occurrences of the likes of
>
>     (cons (car form) (cdr form))
>
> in our code?

This still preserves info inside the fat-cons-cells contained in (car
form) and (cdr form), so it's not as bad as it looks.

Of course, when such code is applied recursively on all sub-expressions
(i.e. in a code-walker such as macroexpand-all, cconv, and byte-opt)
then we lose all the info, so we do need to change those before we can
benefit, but AFAICT those 3 are the only crucial ones (there are a few
other code-walkers around, such as generator.el) and hopefully some of
that rewrite can be made fairly mechanically.

> Are you saying that this is how other Lisp compilers deal with source
> code positions?  How do they deal with the difficult problem of user
> macros?

Not sure about Common-Lisp, but Scheme systems deal with it by
distinguishing "sexp" from "syntax objects" where syntax objects are
basically sexps wrapped (recursively) within location wrappers.

> I think there's quite a bit of doubt as to whether this could work
> effectively in Emacs.

I have no doubt that it can work.

I am not sure it'll be acceptable, OTOH, because it will depend on the
overhead it will impose on the execution of the byte-compiler.

> The way to dispel this doubt is for Somebody (tm) to implement it.

Exactly.

> To which the answer is to install the working solution pending the
> implementation of something better, after which it can be superseded.

Ever heard of temporary hacks that end up permanent?

Take for example the issue of .... oh, I don't know ... line numbers in
error messages?  ;-)

To a large extent the reason we don't have better line-numbers right now
is because of the hack we accepted some years ago, so now instead of
working on "giving line-numbers in error messages", we're reduced to
"improve the precision of line-numbers in error messages" which is not
nearly as pressing an issue.


        Stefan