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

[Andy Wingo <wingo@pobox.com>]

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.

Andy