Promise.cpp

#include <stdio.h>
#include <typeinfo>
#include <functional>
#include <string>
#include <type_traits>
#include <vector>

struct Unit {};

template<typename Result, typename Error>
struct Promise {
    template<typename AcceptCB>
    auto then(const AcceptCB& accept) -> Promise<typename std::result_of<AcceptCB(Result)>::type, Error> {
        acceptCBs.emplace_back(accept);
        return Promise<typename std::result_of<AcceptCB(Result)>::type, Error>();
    }

    template<typename AcceptCB, typename RejectCB>
    auto then(const AcceptCB& accept, const RejectCB& reject) ->
        Promise<
            typename std::result_of<AcceptCB(Result)>::type,
            typename std::result_of<RejectCB(Error)>::type
        >
    {
        acceptCBs.emplace_back(accept);
        rejectCBs.emplace_back(reject);
        return Promise<
            typename std::result_of<AcceptCB(Result)>::type,
            typename std::result_of<RejectCB(Error)>::type
        >();
    }

    std::vector<std::function<void(Result)>> acceptCBs;
    std::vector<std::function<void(Error)>> rejectCBs;

    // optional<either<Result, Error>>
    bool resolved = false;
    bool success = false;
    Result result;
    Error error;
};

int main() {
    Promise<int, std::string> p;
    auto pr = p.then(
        [](int) {
            return 15.0;
        }
    ).then(
        [](double) {
            return std::string("what");
        },
        [](const std::string&) {
            return 5;
        }
    );
    printf("%s\n", typeid(pr).name());
}