slides.md

Is this right?

var input = //...
try {
	var result = computeResult(input);
	displayResult(result);
} catch(e) {
	displayError(e);
}

---

Let's talk about

http://www.flickr.com/photos/zanehollingsworth/5701745474

---

Is this right?

computeResult(input, function(e, result) {
	if(e) {
		displayError(e);
	} else {
		displayResult(result);
	}
});

---

Fixed

computeResult(input, function(e, result) {
	if(e) {
		displayError(e);
	} else {
		try {
			displayResult(result);
		} catch(e) {
			displayError(e);
		}
	}
});

---

Is this right?

function getTheResult() {
    try {
        return thisMightFail();
    } catch(e) {
        return recoverFromFailure(e);
    } finally {
        alwaysCleanup();
    }
}

---

Is this right?

function getTheResult(nodeCallback) {
    thisMightFail(function(error, result) {
        if(error) {
            try {
                result = recoverFromFailure(error);
                error = null;
            } catch(e) {
                error = e;
            }
        }

        alwaysCleanup();

        // If error is still falsey, we've succeeded
        nodeCallback(error, result);
    });
}

---

Is this right?

function getTheResult(onSuccess, onFailure) {
    thisMightFail(
        function(result) {
            try {
                onSuccess(result);
            } catch(e) {
                onFailure(e);
            } finally {
            	alwaysCleanup();
        	}
        },
        function(error) {
            var recoveryResult;
            try {
                recoveryResult = recoverFromFailure(error);
                error = null;
            } catch(e) {
                error = e;
            }

            try {
                alwaysCleanup();
            } catch(e) {
                error = e;
            }

            if(error) {
                onFailure(error);
            } else {
                onSuccess(recoveryResult);
            }
        }
    );
}

---

http://www.flickr.com/photos/timsnell/7986996932

---

The problem

• Impossible to reason about and maintain
• Recreate flow control and error handling machinery

---

Promises: The solution

Promises give you async versions of return and throw.

They let you take back the call stack.

---

Promises

computeResult(input)
	.then(displayResult)
	.otherwise(displayError);

http://know.cujojs.com/tutorials/async/mastering-async-error-handling-with-promises

---

Promises

function getTheResult() {
    return thisMightFail()
        .otherwise(recoverFromFailure);
        .ensure(alwaysCleanup);
}

http://know.cujojs.com/tutorials/async/mastering-async-error-handling-with-promises

---

Promises

• MultiLisp, Act 1, Prolog concurrent logic variables, Joule and E promises, java.util.concurrent.Future, Python Twisted deferred, Dojo Deferred, F# Async<T>, .NET Task<T>, C++11 std::future and std::promise, Dart Future<T>, Scala Future
• Javascript Promises/A+

---

Let's talk about

http://www.flickr.com/photos/rooners/4415074931

---

var when = require('when');

when.reduce(
	when.map($.get('/users'), getTags), groupTags)
	.then(displayTags);

---

var nodefn = require('when/node/function');
var fs = require('fs');
var readFile = nodefn.lift(fs.readFile);
var writeFile = nodefn.lift(fs.writeFile);

// This all happens async:
writeFile('out.txt', readFile('in.txt'));

---

Let's talk about

The future

---

function* someNumbers() {
  yield 1;
  yield 2;
  yield 3;
}

var iterator = someNumbers();

console.log(iterator.next()); // { value: 1, done: false }
console.log(iterator.next()); // { value: 2, done: false }
console.log(iterator.next()); // { value: 3, done: false }
console.log(iterator.next()); // { done: true }

---

// thisMightFail, recoverFromFailure, and
// alwaysCleanup are ASYNC!
var zomg = promisedGenerator(function* () {
	var result;

    try {
        result = yield thisMightFail();
    } catch(e) {
        result = yield recoverFromFailure(e);
    } finally {
        yield alwaysCleanup();
    }

    yield result;
});

var result = zomg();

---

// ES5 (and ES3)
Users.get('brian')
	.then(function(user) {
		return user.fetchBlogPosts();
	})
	.then(function(posts) {
		return posts.toHtml();
	})
	.then(function(html) {
		container.innerHTML = html;
	});

// ES6
container.innerHTML = yield Users.brian.posts.toHtml();

---

Let's talk about

http://www.flickr.com/photos/rooners/4652345494

---

Promise aren't perfect

• VMs/platforms aren't promise-aware
• disjoint or missing stack traces
• when.js and Q can trap and report unhandled rejections
  • and stitch stack traces

---

Common mistakes

• return something!!
• Don't break the chain
• rejection handlers work like catch, if you don't "rethrow", the error is considered "handled" and won't propagate
  • This consistency is a good thing

---

Code smells

// You sly dog:
// If doSomething fails, no one will ever know
promise.then(doSomething);
return promise;

// You probably meant:
return promise.then(doSomething);

---

Code smells

// Oops, you returned `undefined` which
// turns this rejection into a success
return promise.otherwise(function(e) {
	logError(e);
});

// Preferably, let the error be handled higher
// in the call stack, like exceptions!
return promise;

// But if you must:
return promise.otherwise(function(e) {
	logError(e);
	throw e;
});

---

Let's talk about

http://www.flickr.com/photos/pelegrino/6884873348

---

Aristotle: Poetics

• Exposition
• Inciting incident
• Rising action
• Climax

---

The Promises/A+ Drama

---

Exposition

http://wiki.commonjs.org/wiki/Promises/A

---

Inciting incident

Deceit!

https://github.com/jquery/jquery/commit/a41f2406748e3113751ab1e5b5d990d9144123fc

---

Rising action

Complications!

emberjs/ember.js#1406

---

Rising action

Conflict!

https://gist.github.com/domenic/3889970

---

Rising action

Heroics!

https://github.com/domenic/promise-tests

---

Rising action

Action!

https://gist.github.com/briancavalier/eb5fc157825a170c9957

---

Boom!

Explosions!

http://promisesaplus.com

---

Promises/A+ test suite

https://github.com/promises-aplus/promises-tests

---

Promises/A+ stats

• 33 verified implementations, more every week
• Several "inspired by Promises/A+" in other languages

---

DOM Promises

http://dom.spec.whatwg.org/#promises

---

ES7

Slated to have Promises as a language feature

---

Wait, what?!?

How did all this happen in about 6 months

---

Aristotle: Poetics

• Plot
• Character
• Theme
• Diction
• Music & spectacle (setting)

---

Plot

Sane JavaScript asynchrony

---

Character

Strong and cooperative community of implementors and users

---

Theme

Extend the web forward

http://yehudakatz.com/2013/05/21/extend-the-web-forward/

---

Diction

Minimal spec for sanity and interoperability

---

Setting

Github

---

Promises/A+

An open standard for sound, interoperable JavaScript promises—by implementers, for implementers.

---

Thanks!

@briancavalier

SpringJS @ Pivotal

cujoJS co-founder

promisesaplus.com co-editor

@promisesaplus

---

Credits

• Promises/A+ contributors and community
• Boom, Promises/A+ was born Domenic Denicola @ JSConf US 2013