// Async dispatcher (where defered result is materialized)
Defer<Response> asyncOp(Request req) {
    Promise<Response> promise;
    auto reqId = server.makeRequest(req);
    server.handleRequest(reqId, [promise, reqId](){ promise.resolve(server.retrieveResult(reqId)); });
    return promise.defer();
}

// Async API sequential composition
Defer<Handle> openFile(string name);
Defer<Bytes> readFile(Handle file);
Defer<JSON> parse(Bytes buffer);

openFile("config.json")
    .then(readFile)
    .then(parse)
    .success([](JSON json) {
        config = json;
    })
    .fail([]() {
        die();
    });