An attempt to construct a functional-style function value-returning function from a mutating function, using Phoenix

gistfile1.cpp

// test transforming a mutating function into a value-returning function via Phoenix
// i.e., void f(T& a) -> T f(T const& a) or T F(T a)

#define BOOST_SPIRIT_USE_PHOENIX_V3

#include <iostream>
#include <string>
#include <boost/spirit/include/phoenix.hpp>
#include <boost/algorithm/string/case_conv.hpp>

void my_fn(std::string& s) {
    boost::algorithm::to_lower(s);
}

int main() {

    namespace phx = boost::phoenix;
    using phx::local_names::_a;
    using phx::arg_names::arg1;
    using phx::placeholders::_1;
    using phx::let;
    using phx::bind;

    std::string test("Foo");

    // Attempt to make a function that copies its input, runs the mutator,
    // and returns the result:
    std::string result = let(_a = arg1)[bind(&my_fn, phx::ref(_a))](test);

    // Unfortunately let doesn't have a return value...

    std::cout << "result = " << test << std::endl;

}