[Rd] API for converting LANGSXP to LISTSXP?

[Kevin Ushey]

In this case, Rcpp was internally converting (already-existing)
LISTSXPs to LANGSXPs using SET_TYPEOF in some places; the goal was to
allow Rcpp to continue doing this without using SET_TYPEOF just to
preserve existing behavior in an API-compliant way. I ended up doing
exactly what you suggested; thanks.

On Sat, Jul 6, 2024 at 8:09 PM Ivan Krylov <ikrylov using disroot.org> wrote:
>
> On Fri, 5 Jul 2024 15:27:50 +0800
> Kevin Ushey <kevinushey using gmail.com> wrote:
>
> > A common idiom in the R sources is to convert objects between LANGSXP
> > and LISTSXP by using SET_TYPEOF. However, this is soon going to be
> > disallowed in packages.
>
> Would you mind providing an example where a package needs to take an
> existing LISTSXP and convert it to a LANGSXP (or vice versa)? I think
> that Luke Tierney intended to replace the uses of
> SET_TYPEOF(allocList(...), LANGSXP) with allocLang(...).
>
> At least it's easy to manually convert between the two by replacing the
> head of the list using LCONS(CAR(list), CDR(list)) or CONS(CAR(lang),
> CDR(lang)): in a call, the rest of the arguments are ordinary LISTSXPs.
>
> --
> Best regards,
> Ivan