[Rd] [External] Questions regarding ALTREP_SET_ELT APIs

[Tierney, Luke]

On Tue, 30 Jul 2019, Wang Jiefei wrote:

> Hi all,
>
> I'm wondering if there is any way to define a `SET_ELT` function for an
> ALTREP class? I see there are ` ALTINTEGER_SET_ELT` etc. functions exported
> in Rinternal.h, but there is no corresponding ALTREP APIs to define them.
> The only way to set the value of an ALTREP is through a pointer, which will
> require that the ALTREP data is in memory. Is it on purpose?

For now, yes. We do support a Set_elt method for ALTSTRING classes but
not yet for others. I seem to recall that there are some issues with
going there for others, but we'll probably take a closer look later
this year.

One thing to keep in mind is that the R pass-by-value semantics
require that C code duplicate an object for which MAYBE_REFERENCED is
true, and the assumption in existing code is that duplicate returns an
object that can safely be mutated. That places a lot of limitations on
what can be done. You can see some notes on the issues in the
README.md and the vignette in
https://github.com/ALTREP-examples/Rpkg-mutable.

Best,

luke

> Will there be
> any plan to develop these ALTREP set element APIs?
>
> Best,
> Jiefei
>
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>
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-- 
Luke Tierney
Ralph E. Wareham Professor of Mathematical Sciences
University of Iowa                  Phone:             319-335-3386
Department of Statistics and        Fax:               319-335-3017
    Actuarial Science
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