GitHub - basicpp/result17: A rust like Result type for modern C++

template<typename V, typename E>
struct Result;

std::variant< ok<V>, error<E> >

auto fn() -> Result<std::ifstream, string>;
auto splitLines(std::stringstream&) -> std::vector<string>;

int main() {
    auto f = openFile("hello.txt")
        .orMap([](auto&&) { return openFile("fallback.txt"); }) // invoked only on error
        .andMap([](auto&& in) -> Result<std::stringstream, string> { // invoked only if successful
            std::stringstream out;
            out << in.rdbuf();
            if (out.str().empty()) return error("empty file"s);
            return ok(move(out));
        })
        .andMap(&splitLines) // adds no errors
        .orMap([](auto) -> Result<std::vector<string>, string> {
            return ok(std::vector<string>{});
        });
    for (auto &l : f.unwrap()) std::cout << l << '\n'; // we are sure we have a value
}