Re: [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  wrote:
>
> On Fri, 5 Jul 2024 15:27:50 +0800
> Kevin Ushey  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

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Re: [Rd] API for converting LANGSXP to LISTSXP?

[Ivan Krylov via R-devel]

On Fri, 5 Jul 2024 15:27:50 +0800
Kevin Ushey  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

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[Rd] API for converting LANGSXP to LISTSXP?

[Kevin Ushey]

Hi,

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. From what I can see, there isn't currently a
direct way to convert between these two object types using the
available API. At the R level, one can convert calls to pairlists
with:

> as.call(pairlist(as.symbol("rnorm"), 42))
rnorm(42)

However, the reverse is not possible:

> as.pairlist(call("rnorm", 42))
Error in as.pairlist(call("rnorm", 42)) :
  'language' object cannot be coerced to type 'pairlist'

One can do such a conversion via conversion to e.g. an intermediate R
list (VECSXP), but that seems wasteful. Would it make sense to permit
this coercion? Or, is there some other relevant API I'm missing?

For completeness, Rf_coerceVector() also emits the same error above
since it uses the same code path.

Thanks,
Kevin

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