[Rd] Rf_defineVar(symbol, R_UnboundValue, environment) questions

William Dunlap wdun|@p @end|ng |rom t|bco@com
Thu Aug 15 19:12:46 CEST 2019

While poking around the C++ code in the dplyr package I ran across the idiom
   Rf_defineVar(symbol, R_UnboundValue, environment)
to [sort of] remove 'symbol' from 'environment'

Using it makes the R-level functions objects(), exists(), and get()
somewhat inconsistent and I was wondering if that was intended.  E.g.,  use
SHLIB to make something from the following C code that dyn.load can load
into R

% cat defineVarAsUnboundValue.c
#include <R.h>
#include <Rinternals.h>

SEXP defineVarAsUnboundValue(SEXP name, SEXP envir)
    Rf_defineVar(name, R_UnboundValue, envir);
    return R_NilValue;
erratic:bill:292% R-3.6.1 CMD SHLIB defineVarAsUnboundValue.c
gcc -std=gnu99 -I"/home/R/R-3.6.1/lib64/R/include" -DNDEBUG
-I/usr/local/include  -fpic  -g -O2  -c defineVarAsUnboundValue.c -o
gcc -std=gnu99 -shared -L/home/R/R-3.6.1/lib64/R/lib -L/usr/local/lib64 -o
defineVarAsUnboundValue.so defineVarAsUnboundValue.o
-L/home/R/R-3.6.1/lib64/R/lib -lR
erratic:bill:293% R-3.6.1 --quiet --vanilla
> dyn.load("defineVarAsUnboundValue.so")
> envir <- list2env(list(One=1, Two=2))
> objects(envir)
[1] "One" "Two"
> .Call("defineVarAsUnboundValue", quote(Two), envir)
> objects(envir)
[1] "One"
> objects(envir, all.names=TRUE) # is "Two" a 'hidden' object?
[1] "One" "Two"
> exists("Two", envir=envir, inherits=FALSE)
[1] TRUE
> get("Two", envir=envir, inherits=FALSE) # get fails when exists says ok
Error in get("Two", envir = envir, inherits = FALSE) :
  object 'Two' not found

Should Rf_defineVar(sym, R_UnboundValue, envir) remove sym from envir?

Bill Dunlap
TIBCO Software
wdunlap tibco.com

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