As SRFI 195 may not need any additional introspection procedures when
we have a multiple-values SRFI, I am waiting with finishing SRFI 195
until I have submitted my multiple-values SRFI, which should happen
soon. Thanks to all who have already submitted their ideas to me
through this mailing list.

Marc

Am Di., 5. Mai 2020 um 08:12 Uhr schrieb Marc Nieper-Wißkirchen
<xxxxxx@nieper-wisskirchen.de>:
>
> 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.)
> >>