some questions about R internal SEXP types

The general principle is that R packages are only allowed to use what
is
documented in the R help (? command) and in Writing R Extensions. The
former covers what is allowed from R code in extensions, the latter
mostly what is allowed from C code in extensions (with some
references
to Fortran).

If you are implementing a Go interface for writing R packages, such
Go
interface should thus only use what is in the R help and in Writing R
Extensions. Otherwise, packages would not be able to use such
interface.

What is described in R Internals is for understanding the internal
structure of R implementation itself, so for development of R itself,
it
could help indeed also debugging of R itself and in some cases
debugging
or performance analysis of extensions. R Internals can help in giving
an
intuition, but when people are implementing R itself, they also need
to
check the code. R Internals does not describe any interface for
external
code, if it states any constraints about say pairlists, etc, take it
as
an intuition for what has been intended and probably holds or held at
some level of abstraction, but you need to check the source code for
the
details, anyway (e.g., at some very low level CAR and CDR can be any
SEXP or R_NilValue, locally in some functions even C NULL).
Internally,
some C code uses C NULL SEXPs, but it is rare and local, and again,
only
the interface described in Writing R Extensions is for external use.

WRE speaks about "R NULL", "R NULL object" or "C NULL" in some cases
to
avoid confusion, e.g. for values types as "void *". SEXPs that
packages
obtain using the interface in WRE should not be C NULL, only R NULL
(R_NilValue). External pointers can become C NULL and this is
documented
in WRE 5.13.

Best
Tomas

On 9/6/20 3:44 AM, Dan Kortschak via R-devel wrote:

Hello,

I am writing an R/Go interoperability tool[1] that work similarly
to
Rcpp; the tool takes packages written in Go and performs the
necessary
Go type analysis to wrap the Go code with C and R shims that allow
the
Go code to then be called from R. The system is largely complete
(with
the exception of having a clean approach to handling generalised
attributes in the easy case[2] - the less hand holding case does
handle
these). Testing of some of the code is unfortunately lacking
because of
the difficulties of testing across environments.

To make the system flexible I have provided an (intentionally
incomplete) Go API into the R internals which allows reasonably Go
type-safe interaction with SEXP values (Go does not have unions, so
this is uglier than it might be otherwise and unions are faked with
Go
interface values). For efficiency reasons I've avoided using R
internal
calls where possible (accessors are done with Go code directly, but
allocations are done in R's C code to avoid having to duplicate the
garbage collection mechanics in Go with the obvious risks of error
and
possible behaviour skew in the future).

In doing this work I have some questions that I have not been able
to
find answers for in the R-ints doc or hadley/r-internals.

    1. In R-ints, the LISTSXP SEXP type CDR is said to hold
"usually"
       LISTSXP or NULL. What does the "usually" mean here? Is it
possible
       for the CDR to hold values other than LISTSXP or NULL, and
is
       this NULL NILSXP or C NULL? I assume that the CAR can hold
any type
       of SEXP, is this correct?
    2. The LANGSXP and DOTSXP types are lists, but the R-ints
comments on
       them do not say whether the CDR of one of these lists is the
same at
       the head of the list of devolves to a LISTSXP. Looking
through the
       code suggests to me that functions that allocate these two
types
       allocate a LISTSXP and then change only the head of the list
to be
       the LANGSXP or DOTSXP that's required, meaning that the tail
of the
       list is all LISTSXP. Is this correct?

The last question is more a question of interest in design
strategy,
and the answer may have been lost to time. In order to reduce the
need
to go through Go's interface assertions in a number of cases I have
decided to reinterpret R_NilValue to an untyped Go nil (this is
important for example in list traversal where the CDR can
(hopefully)
be only one of two types LISTSXP or NILSXP; in Go this would
require a
generalised SEXP return, but by doing this reinterpretation I can
return a *List pointer which may be nil, greatly simplifying the
code
and improving the performance). My question her is why a singleton
null
value was chosen to be represented as a fully allocated SEXP value
rather than just a C NULL. Also, whether C NULL is used to any
great
extent within the internal code. Note that the Go API provides a
mechanism to easily reconvert the nil's used back to a R_NilValue
when
returning from a Go function[3].

thanks
Dan Kortschak

[1]https://github.com/rgonomic/rgo
[2]https://github.com/rgonomic/rgo/issues/1
[3]