Jscex: Write Sexy JavaScript

Jscex
Write Sexy JavaScript
Zhao Jie - SNDA - May 2011

About Me
• / / Jeffrey Zhao /

• Programmer
• Blogger - http://blog.zhaojie.me/
• Twitter: @jeffz_cn
• F#, JavaScript, Scala, C#, Python, .NET, Mono...
• Java (as a language) hater

What’s Jscex

• JavaScript Computation EXpression
• A JavaScript language extension to help
programming in common scenarios
• A port of F# Computation Expression
• Inspired by the idea
• Design for JavaScript

What’s NOT Jscex

• Another language
• Jscex is 100% JavaScript

• A framework
• It’s a library, works with any libraries / frameworks

• A JavaScript engine / runtime
• Execute in any ECMAScript 3 engines

Bubble Sort
var compare = function (x, y) {
return x - y;
}

var swap = function (a, i, j) {
var t = a[x]; a[x] = a[y]; a[y] = t;
}

var bubbleSort = function (array) {
for (var x = 0; x < array.length; x++) {
for (var y = 0; y < array.length - x; y++) {
if (compare(array[y], array[y + 1]) > 0) {
swap(array, y, y + 1);
}
}
}
}

Animate it!
var compare = function (x, y, callback) { var innerLoop = function (array, x, y, callback) {
setTimeout(10, function () { if (y < array.length - x) {
callback(x - y); compare(array[y], array[y + 1], function (r) {
}); if (r > 0) {
} swap(array, y, y + 1, function () {
innerLoop(array, x, y + 1, callback);
var swap = function (a, i, j, callback) { });
var t = a[x]; a[x] = a[y]; a[y] = t; } else {
repaint(a); innerLoop(array, x, y + 1, callback);
}
setTimeout(20, callback); });
} } else {
callback();
var outerLoop = function (array, x, callback) { }
if (x < array) { }
innerLoop(array, x, 0, function () {
outerLoop(array, x + 1, callback); outerLoop(array, 0, function () {
}); console.log("done!");
} else { });
callback();
}
}

Animate it!
at?
var compare = function (x, y, callback) {

th
var innerLoop = function (array, x, y, callback) {

is
setTimeout(10, function () { if (y < array.length - x) {
callback(x - y); compare(array[y], array[y + 1], function (r) {

ll
}); if (r > 0) {
} swap(array, y, y + 1, function () {

e
innerLoop(array, x, y + 1, callback);

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var swap = function (a, i, j, callback) { });
var t = a[x]; a[x] = a[y]; a[y] = t; } else {

e
repaint(a); innerLoop(array, x, y + 1, callback);

h
}

t
setTimeout(20, callback); });

t
} } else {

a
callback();

h
var outerLoop = function (array, x, callback) { }
if (x < array) { }
innerLoop(array, x, 0, function () {

W
outerLoop(array, x + 1, callback); outerLoop(array, 0, function () {
}); console.log("done!");
} else { });
callback();
}
}

Bubble Sort Animation
var compareAsync = eval(Jscex.compile("async", function (x, y) {
$await(Jscex.Async.sleep(10)); // each "compare" takes 10 ms.
return x - y;
}));

var swapAsync = eval(Jscex.compile("async", function (a, x, y) {
var t = a[x]; a[x] = a[y]; a[y] = t; // swap
repaint(a); // repaint after each swap
$await(Jscex.Async.sleep(20)); // each "swap" takes 20 ms.
}));

var bubbleSortAsync = eval(Jscex.compile("async", function (array) {
for (var x = 0; x < array.length; x++) {
for (var y = 0; y < array.length - x; y++) {
var r = $await(compareAsync(array[y], array[y + 1]));
if (r > 0) $await(swapAsync(array, y, y + 1));
}
}
}));

Keep Everything for
JavaScript Programmer

• Language features
• Language semantics
• Programming experiences

Language Features
• Support almost all JavaScript constructs
• Loops: while / for / for...in / do
• Conditions: if / switch
• Error handling: try...catch...ﬁnally
• Others: return / break / continue / throw

• Not supported
• with block
• break / continue to label
• conditional break in switch block

• Nested functions

Language Semantics

• Keep all semantics as JavaScript
• Explicit “bind” operations (like $await)
• The only extension to the language
• Act like a method call
• Make clear which operation is “special”

Programming
Experiences

• Write, execute, debug as JavaScript
• Modify and take effects immediately
• No extra compilation process
• Code generation by JIT compiler as code runs

Async Programming is
Difﬁcult

Code Locality is
Broken
• Used to expressing algorithms linearly
• Async requires logical division of algorithms
• Very difﬁcult to
• Combine multiple asynchronous operations
• Deal with exceptions and cancellation

Asynchronous Function
// use async builder to execute the compiled code
var somethingAsync = eval(Jscex.compile("async",
function (...) {
// implementation
}
));

React!!!
function () {
var res = $await(<async work>);
}

React!!!
function () {
var res = $await(<async work>);
}

an HTTP Request
an UI Event
a Timer Callback
a Query Response
a Web Service Response
an Agent Message

function () {
var img = $await(readAsync("http://..."));
console.log("loaded!");
$await(writeAsync("./files/..."));
console.log("saved!");
}

=
(function () {
var _b_ = Jscex.builders["async"];
return _b_.Start(this,
_b_.Delay(function () {
_b_.Bind(readAsync(...), function (img) {
console.log("loaded!");
return _b_.Bind(writeAsync(...), function () {
console.log("saved!");
return _b_.Normal();
});
});
})
);
})

Work with Express
var app = express.createServer();

app.get('/', function (req, res) {
/**
* Question: How to do async work here? e.g.:
*
* 1. Get multiple keys from database.
* 2. For each key, try get data from cache.
* If cache missed, get data from database.
* 3. Write a list of result to the response.
*
* Note: all "get" operations are asynchronous.
**/
});

app.listen(3000);

app.getAsync('/', eval(Jscex.compile("async", function (req, res) {

var keys = $await(db.getKeysAsync(...));

var results = [];
for (var i = 0; i < keys.length; i++) {
var r = $await(cache.getAsync(keys[i]));
if (!r) {
r = $await(db.getAsync(keys[i]));
}

results.push(r);
}

res.send(generateList(results));
})));

I/O Parallelism

• Software is often I/O bound
• Leveraging web services
• Working with data on disk

• Network and disk speeds increasing slower
• I/O resources are inherently parallel
• Huge opportunity for performance

Make Things Parallel
var getDataAsync = eval(Jscex.compile("async", function (key) {
var res = $await(cache.getAsync(key));
if (res) return res;
return $await(db.getAsync(key));
}));

app.getAsync('/', eval(Jscex.compile("async", function (req, res) {

var keys = $await(db.getKeysAsync(...));

// get tasks of “get data” (not started yet)
var tasks = keys.map(function (key) {
return getDataAsync(key);
});

// make requests in parallel
var results = $await(Jscex.Async.parallel(tasks));

res.send(generateList(results));
})));

Task Model
• Async library use a simple task model
• Easy to write bindings for async operations
• Parallel: $await(Jscex.Async.parallel(taskA, taskB))

• Series: $await(taskA.continueWith(taskB))

• The semantics of $await: wait a task to
complete
• Start the task if it’s not running
• Return immediately if it’s already completed

• We can start a task manually (if necessary)
• E.g. taskA.start(); $await(taskB); $await(taskA);

The Limitation of
Callback-based Model
// if the sequence of processing is important, how to keep it?
var i = 1;
conn.onAsync("data", eval(Jscex.compile("async", function () {
var id = i++;

$await(step1); console.log("step 1 - request " + id);

/**
* A possible sequence (which is unacceptable):
* step 1 - request 1
**/
})));

Erlang-like Agent
var i = 0;
var agent = Agent.start(eval(Jscex.compile("async", function (mailbox) {
var id = i++;

var msg = $await(mailbox.receive());
})));

conn.on("data", function (data) {
// data would be queued in mailbox
agent.send(data);
});

Language & Libraries
• Compiler (language extension)
• JIT: generate code at runtime (development)
• AOT: generate code before runtime (production)

• Builders (libraries)
• Async: simplﬁed async programming
• Sequense: generator for lazy iterator (Py, C#, JS 1.7)
• more...

AOT Compiler
// before AOT compilation
Agent.start(eval(Jscex.compile("async", function (mailbox) {
...
})));

// after AOT compilation
// independent of compiler scripts
// need a tiny library only (3kb when gzipped)
Agent.start((function (mailbox) {
var _b_ = Jscex.builders["async"];
return _b_.Start(this,
...
);
}));

Jscex Builders

• Compiler generates code into standard
patterns with a builder (name)
• A builder deﬁnes its name of the “bind” operation

• Execute with the standard methods
implemented in the builder

Not only for Async
// infinite fibonacci sequence
var fib = eval(Jscex.compile("seq", function () {
var i = 0, j = 1;
while (true) {
$yield(i); // the bind operation

var t = i;
i = j;
j += t;
}
}));

var iter = fib().skip(10).take(10);
while (iter.moveNext()) {
console.log(iter.current);
}

... and Maybe Monad
var maybeFunc = function () {
var maybeA = getA();
if (maybeA == Maybe.None) return Maybe.None;

var maybeB = getB();
if (maybeB == Maybe.None) return Maybe.None;

return maybeA.value + maybeB.value;
}

// simplified version
var maybeFunc = eval(Jscex.compile("maybe", function () {
var a = $try(getA());
var b = $try(getB());
return a + b;
}));

Performance
• Function has “linear” and “async” parts
• Linear part: compiler keeps linear codes as much as
possible
• Async part: at the same level of hand-writing, callback-
based implementation

• An async operation always takes much
more time than a bunch of normal codes
• Jscex never becomes a performance issue / bottleneck

Code Generation
Patterns
• Code generation is direct and simple
• Map to original code easily
• Easy to debug: use “debugger” statement to pause or
add breakpoints in debuggers

• Based on standard methods
• Start, Delay, Combine
• Loop, Try
• Normal, Return, Break, Continue, Throw
• Bind

Try it NOW

• Release under BSD license
• https://github.com/JeffreyZhao/jscex

Jscex: Write Sexy JavaScript

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