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

[Kevin Ushey]

Sorry, you're correct -- lazy-loaded data gets extracted not from the
package namespace but from the 'lazydata' portion of the namespace info.

Ultimately, I think you'll just want to call back to R to get what you
want, e.g.

    getExportedValue("nycflights13", "flights")

On Mon, Apr 16, 2018 at 12:48 PM, Jack Wasey <jack at jackwasey.com> wrote:

> 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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> >
> >
>
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