How to call R function (which should not be exported) from Rcpp?

Question

I want to call an R function of my package from an Rcpp function. Both R and Rcpp functions are part of the package rminizinc.

Currently, I am using the approach shown below:

    void fun(){
        Environment rmzn_env("package:rminizinc");
        Function retModel = rmzn_env["getRModel"];
        retModel(some_object);
   }

But, this approach only works when I export the getRModel() function. Is there any way I could call getRModel() without exporting it as I want it to be a helper function which should not be exposed to the user?

Answer 1

You have to get a bit meta here. It is possible to get an environment containing all the unexported functions in a package using the base R function asNamespace. This function itself can be used inside Rcpp. You then create a new Environment from the output of that function, from which you can harvest the unexported function.

As an example, let's get the unexported function ggplot2:::as_lower_ascii to do some work on a string we pass to an Rcpp function:

#include <Rcpp.h>
using namespace Rcpp;

// [[Rcpp::export]]
CharacterVector fun(CharacterVector input){
  Function asNamespace("asNamespace");
  Environment ggplot_env = asNamespace("ggplot2");
  Function to_lower_ascii = ggplot_env["to_lower_ascii"];
  return to_lower_ascii(input);
}

So if we source this, then back in R we can do:

fun("HELLO WORLD")
#> [1] "hello world"