[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