Async I/O pattern using JS generators and promises

async-driver.js

// Pseudo code for a driver that enables doing asynchronous I/O using generators and promises.
// See the following URLs for actual code:
// https://github.com/hns/stick/blob/master/lib/stick/middleware/continuation.js
// https://github.com/hns/stick/blob/master/examples/continuation/app.js

// If app is a generator and yields promises, the result of those promises 
// will be passed back into the generator as soon as they're resolved.

function handle(app, request) {

    var response = app(request);

    // check if response is a generator
    if (response && typeof response.next === "function") {

        response = response.next();

        // check if generator yielded a promise
        // see http://wiki.commonjs.org/wiki/Promises/A
        if (response && typeof response.then === "function") {

            var deferred = defer();
            var handlePromise = function(p) {
                p.then(
                    function(value) {
                        try {
                            var v = generator.send(value);
                            if (v && typeof v.then === "function") {
                                // generator returned a promise again
                                handlePromise(v);
                            } else {
                                // generator returned final value
                                deferred.resolve(v);
                            }
                        } catch (error) {
                            // generator threw an error
                            deferred.resolve(error, true);
                        }
                    },
                    function(error) {
                        // promise resolved to error
                        deferred.resolve(error, true);
                        generator.throw(error);
                    }
                );
            };
 
            handlePromise(response);
            return deferred.promise;
        }

        // todo: handle non-promise generator
    }

    return response;
}

generator-app.js

// an application that does async I/O by yielding promises

function app(request) {
    // yield a promise
    var data = yield getDataAsPromise(request.someId);
    // get more data yielding another promise
    var moreData = yield getMoreDataAsPromise(data);
    // return the final response
    yield renderResponse(data, moreData);
}