work in progress async replacement

#include <future>
#include <thread>
#include <iostream>
#include <string>
#include <vector>

void thFun(std::promise<std::string>& prms) {
    try {
        std::string val = "val";
        throw std::exception("bang!");
        prms.set_value(val);
    } catch(...) {
        prms.set_exception(std::current_exception());
    }
}

std::future<std::string> myasync(std::promise<std::string>& prms) {
//std::future<std::string> myasync() {
    //std::promise<std::string> prms; //needs to outlive thread. How?

    std::future<std::string> fut = prms.get_future();
    std::thread th([&](){
        //std::promise<std::string> prms; //need to return a future before...
        thFun(std::ref(prms));
     });

    th.detach();
    return fut;
}

 int main() {

    std::promise<std::string> prms; //I really want the promise hidden iway in the myasync func and not live here in caller code but the promise needs to outlive myasync and live as long as the thread. How do I do this?
    auto fut = myasync(prms);

    //auto fut = myasync(); //Exception: future already retrieved

    try {
        auto res = fut.get();
        std::cout << "Result: " << res << std::endl;

    } catch(const std::exception& exc) {
        std::cout << "Exception: " << exc.what() << std::endl;
    }

 }