John, Shiro, thank you for your comments.

I see two ways how to implement what you are asking for. The obvious
way would be to specify procedures like

(unbox-value <box> <index>)

and

(box-values-length <box>)

that retrieve a single value or return the number of values store, respectively.

There is a second way, however, which can also be applied in other
situations where multiple values occur. What I am thinking of are
special forms not restricted to boxes (but could be applied to, say,
the Justs and Rights of SRFI 189 likewise) as follows:

(values->vector <expr>)
(values-length <expr>)
(values->list <expr>)
(values-ref <expr> <index>)

These are expressions defined as syntax (important because Scheme's
applicative syntax doesn't allow multiple values). When they are
evaluated, <expr> is evaluated and expected to return values, which
are then converted to a vector, a list, etc. Moreover, as this is
syntax, an implementation can expand these forms into the most
efficient ones.

Applied to the case of boxes, instead of "(box-values-length <box>)",
we would then have (values-length (unbox box)). Again, as this is
syntax, the implementation's expander can expand "(values-length
(unbox box))" into some specialized code.

In case, you also want to set a single value, the way to go in the
second approach is as in SRFI 17: (values-set! <expr> <index> <value>)
is a special form that works in a context like "(values-set! (unbox
<box>) <index> <value>)".

Am Mo., 4. Mai 2020 um 23:22 Uhr schrieb John Cowan <xxxxxx@ccil.org>:
>
> And also, I think, to retrieve the nth value.
>
> On Mon, May 4, 2020 at 5:21 PM Shiro Kawai <xxxxxx@gmail.com> wrote:
>>
>> Can it also have an interface to query how many values a given box is holding?
>> Without that, any generic code to deal with boxes must receive the unboxed values
>> as a list.  Although clever compilers may eliminate construction of intermediate lists
>> in some cases, it'll be easier if one can get the information (e.g. one can have fast-path
>> for single-valued box, or check the box argument and reject early if the passed box
>> has different values than expected.)
>>