[Rcpp-devel] using an unloaded package's lazy-load data in Rcpp function

Thanks for the reply. The solution of requiring the namespace from within
Rcpp hadn't occurred to me.

I had used Environment::namespace_env("pkg_name") before, but I think the
problem here is that lazy-loaded (and maybe other elements we expect in R)
are not available if this is done from Rcpp. E.g.

# (using R --vanilla)
Rcpp::cppFunction('Rcpp::Environment getUnloaded2() { return
Rcpp::Environment::namespace_env("nycflights13"); }')
ls(envir = getUnloaded2())
# empty, whereas the following:

nycflights13::flights
# pulls the data, with the package not being loaded or attached.

To close this thread, I think what the latter command does differently to
Rcpp is to load the package, because sessionInfo() after running the second
command, includes nycflights13 in the section "loaded via a namespace (and
not attached):" This is not the case after pulling the environment into an
Rcpp variable.

Thanks again for steering me,
Jack



On 04/15/2018 10:18 PM, Kevin Ushey wrote:

The environments called 'package:foo' are associated with attached

packages; that is, they're located on the search path. You likely want to
find the actual package namespace.

I think we have a static function Environment::namespace_env() for this.

Or you can just call back to R with requireNamespace().

Kevin

On Fri, Apr 13, 2018 at 10:10 PM, Wasey, Jack O <jack at jackwasey.com

<mailto:jack at jackwasey.com>> wrote:

    Dear all,

    It seems I can reference lazy-loaded data from an unloaded,

unattached package from R, but not using the equivalent Rcpp as described
in the quickref document. I checked out: http://dirk.eddelbuettel.com/
code/rcpp/html/Environment_8h_source.html <http://dirk.eddelbuettel.com/
code/rcpp/html/Environment_8h_source.html> and even the scant references
to environments in the C API section of "Writing R Extensions" and doesn't
see what I'm missing. I suspect this is a limitation of R's C interface,
not necessarily Rcpp itself.

    I use the package nycflights13 as an example. Running

`sessionInfo()` shows the package nycflights13 is not loaded, then I run:

    R --vanilla

    > a <- nycflights13::flights

    Which completes without error. In Rcpp, I cannot even get a handle

on the package namespace if it is not loaded.

    Rcpp::cppFunction('Rcpp::Environment getUnloaded() {

Rcpp::Environment env("package:nycflights13"); return env;}')

    getUnloaded()
    Error in getUnloaded() :
      Cannot convert object to an environment: [type=character;

target=ENVSXP].

    library(nycflights13)
    Rcpp::cppFunction('Rcpp::Environment getUnloaded() {

Rcpp::Environment env("package:nycflights13"); return env;}')

    getUnloaded()
    <environment: package:nycflights13>
    attr(,"name")
    [1] "package:nycflights13"
    attr(,"path")
    [1] "/Library/Frameworks/R.framework/Versions/3.4/Resources/library/

nycflights13"

    >

    I also found that simple using `loadNamespace` was insufficient for

the Rcpp approach to work, but after `library` or `attachNamespace` the
above function does work.

    Am I treading in another rare code path? All I want to do is use a

lazy-loaded data set in an Rcpp function when the package may not be loaded
when called. Should I just be passing it as a parameter to the function?
This seems ugly.

    Using Rcpp 0.12.16, R 3.4.3 on Mac.

    Any advice appreciated. Thanks,

    Jack

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