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REDEMPTION
FROM
CALLBACK HELL
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ASYNCHRONOUS
LET’S TALK ABOUT
JAVASCRIPT APIs
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function getUser(name) {
var sql = 'SELECT * FROM users WHERE name=?';
var user = query(sql, name); // <- blocking
if (!user) throw new Error('no user!');
return user;
}
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function getUser(name, callback) {
var sql = 'SELECT * FROM users WHERE name=?';
query(sql, name, function (error, user) {
if (error) {
callback(error);
} else if (!user) {
callback(new Error('no user!'));
} else {
callback(null, user);
}
});
}
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function getUser(name) {
var sql = 'SELECT * FROM users WHERE name=?';
var user = query(sql, name); // <- blocking
if (!user) throw new Error('no user!');
return user;
}
function getUser(name, callback) {
var sql = 'SELECT * FROM users WHERE name=?';
query(sql, name, function (error, user) {
if (error) {
callback(error);
} else if (!user) {
callback(new Error('no user!'));
} else {
callback(null, user);
}
});
}
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NO RETURN
IN A CALLBACK, THERE IS
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function getUser(name, callback) {
var sql = 'SELECT * FROM users WHERE name=?';
query(sql, name, function (error, user) {
// Nobody gets this return value!
return user;
});
}
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NO THROW
IN A CALLBACK, THERE IS
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function getUser(name, callback) {
var sql = 'SELECT * FROM users WHERE name=?';
query(sql, name, function (error, user) {
// Nobody can catch this error!
if (error) throw error;
});
}
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function getUser(name, callback) {
var sql = 'SELECT * FROM users WHERE name=?';
query(sql, name, function (error, user) {
// Nobody can catch this error!
if (error) throw error; // <- BAD
});
}
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• NO RETURN
• NO THROW
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function getUser(name) {
var sql = 'SELECT * FROM users WHERE name=?';
var user = query(sql, name); // <- blocking
if (!user) throw new Error('no user!');
return user;
}
function getUser(name, callback) {
var sql = 'SELECT * FROM users WHERE name=?';
query(sql, name, function (error, user) {
if (error) {
callback(error);
} else if (!user) {
callback(new Error('no user!'));
} else {
callback(null, user);
}
});
}
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NO STACK
IN A CALLBACK, THERE IS
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NO GUARANTEES
USING CALLBACKS, THERE ARE
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function getUsers(names, callback) {
var sql = 'SELECT * FROM users WHERE name=?';
var users = [];
names.forEach(function (name) {
query(sql, name, function (error, user) {
if (error) {
callback(error);
} else if (!user) {
callback(new Error('no user!'));
} else {
users.push(user);
if (users.length === names.length) {
callback(null, users);
}
}
});
});
}
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• NO STACK (NO RETURN/THROW)
• NO GUARANTEES
• BUT AT LEAST WE DON’T BLOCK!?
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function getUser(name) {
var sql = 'SELECT * FROM users WHERE name=?';
var user = query(sql, name); // <- blocking
if (!user) throw new Error('no user!');
return user;
}
function getUser(name, callback) {
var sql = 'SELECT * FROM users WHERE name=?';
query(sql, name, function (error, user) {
if (error) {
callback(error);
} else if (!user) {
callback(new Error('no user!'));
} else {
callback(null, user);
}
});
}
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ES6 GENERATORS
THE FUTURE IS
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function fibonacci() {
var i = 0, j = 1;
while (true) {
yield i;
var t = i;
i = j;
j += t;
}
}
var generator = fibonacci();
for (var i = 0; i < 12; i++) {
print(generator.next());
}
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function fibonacci() {
var i = 0, j = 1;
while (true) {
yield i; // <- awesome
var t = i;
i = j;
j += t;
}
}
var generator = fibonacci();
for (var i = 0; i < 12; i++) {
print(generator.next());
}
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• FIRST-CLASS COROUTINES
• SUSPEND EXECUTION CONTEXT
• SMALL API
• NOT AVAILABLE YET
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PROMISES/A+
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• DISCOVERED CIRCA 1989
• INSPIRED BY E
• WIDELY USED OUTSIDE JS
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Horrible Lies!!!
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https://github.com/promises-aplus/promises-spec
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• Q
• RSVP
• when
https://github.com/promises-aplus/promises-spec/blob/master/implementations.md
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ASYNCHRONOUS
A PROMISE IS AN
VALUE
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getUser('mjackson', function (error, user) {
// ...
});
// becomes
getUser('mjackson').then(function (user) {
// ...
}, function (error) {
// ...
});
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TRANSFORMS
THERE ARE 4 BASIC SYNC TO ASYNC
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var user = getUser('mjackson');
var name = user.name;
// becomes
getUser('mjackson').then(function (user) {
return user.name;
});
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var user = getUser('mjackson');
if (!user) throw new Error('no user!');
var name = user.name;
// becomes
getUser('mjackson').then(function (user) {
if (!timeline) throw new Error('no user!');
return user.name;
});
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try {
deliverTweetTo(tweet, 'mjackson');
} catch (error) {
handleError(error);
}
// becomes
deliverTweetTo(tweet, 'mjackson')
.then(undefined, handleError);
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try {
var user = getUser('mjackson');
} catch (error) {
throw new Error('ERROR: ' + error.message);
}
// becomes
getUser('mjackson').then(undefined, function (error) {
throw new Error('ERROR: ' + error.message);
});
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try {
var user = getUser('mjackson');
} catch (error) {
throw new Error('ERROR: ' + error.message);
}
// becomes
getUser('mjackson').then(undefined, function (error) {
throw new Error('ERROR: ' + error.message);
});
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•STACK SEMANTICS
•GUARANTEES
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SEQUENCE
OPERATIONS IN
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var user = getUser('mjackson');
var tweets = getNewTweets(user);
updateTimeline(tweets);
// using callbacks
getUser('mjackson', function (user) {
getNewTweets(user, function (tweets) {
updateTimeline(tweets);
});
});
// using promises
getUser('mjackson')
.then(getNewTweets)
.then(updateTimeline);
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EXCEPTIONS
HANDLING
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try {
var user = getUser('mjackson');
var tweets = getNewTweets(user);
updateTimeline(tweets);
} catch (error) {
handleError(error);
}
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getUser('mjackson', function (error, user) {
if (error) {
handleError(error);
} else {
getNewTweets(user, function (error, tweets) {
if (error) {
handleError(error);
} else {
updateTimeline(tweets, function (error) {
if (error) handleError(error);
});
}
});
}
});
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// Same example, complete with node.js time bombs!
getUser('mjackson', function (error, user) {
if (error) throw error;
getNewTweets(user, function (error, tweets) {
if (error) throw error;
updateTimeline(tweets, function (error) {
if (error) throw error;
});
});
});
process.on('uncaughtException', handleError); // LOL
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try {
var user = getUser('mjackson');
var tweets = getNewTweets(user);
updateTimeline(tweets);
} catch (error) {
handleError(error);
}
// becomes
getUser('mjackson')
.then(getNewTweets)
.then(updateTimeline)
.then(undefined, handleError);
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FUD
DISPEL THE
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var fs = require('fs');
var q = require('q');
var readFile = q.denodeify(fs.readFile);
var promise = readFile(__filename, 'utf8');
promise.then(console.log, console.error);
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function createUser(userName, userData, callback) {
return database.ensureUserNameNotTaken(userName)
.then(function () {
database.saveUserData(userName, userData);
})
.nodeify(callback);
}
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•PROMISES COMING TO THE DOM
•GENERATORS COMING TO V8
•PROMISES ARE FUTURE-PROOF
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var q = require('q');
var getUser = q.async(function * (name) {
var sql = 'SELECT * FROM users WHERE name=?';
var user = yield query(sql, name);
if (!user) throw new Error('no user!');
return user;
});
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PARALLEL
EXECUTE OPERATIONS IN
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function getNewTweetsForUsers(users) {
return users.map(getNewTweets);
}
// becomes
var q = require('q');
function getNewTweetsForUsers(users) {
var promises = users.map(getNewTweets);
return q.all(promises);
}
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SERVERS
BUILD FAULT-TOLERANT
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var http = require('http');
var q = require('q');
var server = http.createServer(function (req, res) {
handleRequest(req).then(function (response) {
res.writeHead(response.status, response.headers);
res.end(response.content);
}, function (error) {
console.error(error);
res.writeHead(500, { 'Content-Type': 'text/plain' });
res.end('Internal Server Error');
});
});
server.listen(3000);
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https://github.com/machjs/mach
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THANKS!