RcppParallel: trying to implement an approach of lapply but in Parallel - rcpp

Question

I'm trying to implement an approach of lapply , like R, but in parallel using Rcpp and RcppParallel in C++. I want to apply a function to every element in a List in PARALLEL and I have some problems with the function inside 'transform' where I want to pass a function which will be applied, but that function comes from R and I get an error when I try to compile the program.
But making it in serial avoiding loops, using transform too and passing a ' Function f ' it compiles and works perfectly, that is the code:
List lapplyCppSerial(List input, Function f){
std::transform(input.begin(), input.end(), output.begin(), f);
return output;
}
When I try to make the same in the parallel way, using parallelFor() I have to create a Worker class that can "iterate" over all elements. In operator() I want to use transform() again, but I got an error passing the function as parameter like before.
I think it's something missing but I can't find it. I hope someone could help me.
Here is the code:
#include <Rcpp.h>
#include <RcppParallel.h>
// [[Rcpp::depends(RcppParallel)]]
using namespace RcppParallel;
using namespace Rcpp;
struct Fun : public Worker
{
// source vector
const RVector<double> input;
Rcpp::Function f;
// output values
RVector<double> output;
Fun(const NumericVector input,NumericVector output, Function fun)
: input(input), output(output), f(fun) {}
void operator()(std::size_t begin, std::size_t end, Rcpp::Function f) {
std::transform(input.begin() + begin, input.end() + end,
output.begin() + begin, as<double>(f)); //Here I have PROBLEMS!!!
}
};
// [[Rcpp::export]]
List lapplyCppParallel(List input, Function f) {
List output(input.size());
Fun fun(input, output, f);
parallelFor(0, input.size(), fun);
return output;
}