bug#24102: Use guile variable objects as SRFI-111 boxes.

[Andy Wingo <wingo@pobox.com>]

On Wed 31 Aug 2016 11:03, Andy Wingo <wingo@pobox.com> writes:

> On Thu 18 Aug 2016 18:14, Mark H Weaver <mhw@netris.org> writes:
>
>> As I wrote above, the current guile compiler can already do this kind of
>> type inference, although it does not currently do this for boxes.
>> we can already anticipate having native code generation in the
>> next couple of years, and we must keep boxes semantically simple so that
>> our future compiler will be able to generate good code for this very
>> important fundamental type.
>
> For what it's worth, I don't see the optimization argument as weighing
> very heavily on this discussion.  I would rather have fewer fundamental
> data types rather than more, in the next two years or so.  I see the
> mid-term result here being that SRFI-111 boxes are much slower than
> variables.
>
> The highest performance compilation tier we can imagine would include
> adaptive optimization, and when it runs you can know that the variables
> that a bit of code uses are bound or not.  Also in that case we can
> reasonably make any call to variable-unset! deoptimize any code that
> uses variables, forcing it to reoptimize later.  Since variable-unset!
> is quite rare this is no big deal I think.

Following up here :)  So again I think the optimization argument is not
so important; if that were the only consideration then IMO the balance
of things would be that we should apply Glenn's patch.

There is a semantic consideration as well -- box-ref on a box created by
make-box should never throw an exception, and code that uses the
SRFI-111 should be able to rely on this.  We should probably not
introduce a gratuitous incompatibility here.  I propose to add a flag to
variables indicating that certain variables may not be unset.  We can
also consider reversing this, in that only variables with the flag can
be unset; my understanding is that the only user of variable-unset! is
the Elisp language on variables that it creates, so that would be
acceptable too.

Andy