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CSP-style tasks and channels for JS using a sweetjs macro.

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task - a sweetjs macro for CSP in Javascript

The task macro, in conjunction with Channel objects lets you write CSP-ish code in Javascript that can interop with Node.js's callback mechanism. This came out of a need for a better way to deal with async activities than offered by Promises/A+ or even generators.

Apart from tasks and channels, cspjs attempts at a saner and more expressive error handling mechanism than the traditional try-catch-finally model. See blog post and follow up describing the error management scheme in detail.


  1. Installation
  2. Using the task macro
  3. Guarantees provided by tasks
  4. Sample code illustrating various features
  5. Error tracing
  6. Performance
    1. doxbee-sequential
    2. doxbee-sequential-errors
    3. madeup-parallel
  7. History


  1. You need to have sweetjs installed with
    npm install -g [email protected]
  2. Install cspjs using npm like this -
    npm install cspjs
    to get it into your
  3. To compile a

    file that uses the
    macro, do -
    sjs -m cspjs my-task-source.sjs > my-task-source.js
  4. To use the

    module, require it like this -
    var Channel = require('cspjs/channel');
  5. Or if you want to use channels with nodejs stream support, like this -

    // WARNING: EXPERIMENTAL. Interface may change.
    var Channel = require('cspjs/stream');

For complete documentation, see the docco generated docs in


Using the task macro

provides a single macro called
that is similar to
in form, but interprets certain statements as asynchronous operations. Different tasks may communicate with each other via
objects provided by

Any NodeJS style async operation with a

function (err, result) {..}
callback as its last argument can be conveniently used within
, which itself compiles into a function of that form.

Below is a simple hello world -

task greet(name) {
    console.log("Hello", name);
    return 42;

The above task is equivalent to the following and compiles into a function with exactly the same signature as the below function -

function greet(name, callback) {
    console.log("Hello", name);
    callback(null, 42);

.. except that upon calling,

will execute on the next IO turn instead of i]mediately. If
did a
throw 42;
instead of
return 42;
, then the callback's first "error" argument will be the one set to

Guarantees provided by tasks

  1. A

    , after compilation by cspjs, becomes an ordinary function which accepts an extra final argument that is expected to be a callback following the NodeJS convention of
    function (err, result) {...}
  2. A task will communicate normal or error return only via the

    argument. In particular, it is guaranteed to never throw .. in the normal Javascript sense.
  3. When a task function is called, it will begin executing only on the next IO turn.

  4. A task will always call the passed callback once and once only.

Sample code illustrating various features

task sampleTask(x, y, z) {
    // "sampleTask" will compile into a function with the signature -
    //    function sampleTask(x, y, z, callback) { ... }

var dist = Math.sqrt(x * x + y * y + z * z);
// Regular state variable declarations. Note that uninitialized var statements
// are illegal.

console.log("hello from cspjs!");
// Normal JS statements ending with a semicolon are treated as synchronous.
// Note that the semicolon is not optional.


Error tracing

If an error is raised deep within an async sequence of operations and the error is allowed to bubble up to one of the originating tasks, then the error object will contain a

property which will contain a trace of all the async steps that led to the error ... much like a stack trace.

Note that this tracing is always turned ON in the system and isn't optional, since there is no penalty for normal operation when such an error doesn't occur.


The macro and libraries are not feature complete and, especially I'd like to add more tracing. However, it mostly works and seems to marginally beat bluebird in performance while having the same degree of brevity as the generator based code. The caveat is that the code is evolving and performance may fluctuate a bit as some features are added. (I'll try my best to not compromise.)

Here are some sample results (as of 7 Feb 2014, on my MacBook Air 1.7GHz Core i5, 4GB RAM, node v0.11.10) -


Using doxbee-sequential.sjs.

results for 10000 parallel executions, 1 ms per I/O op

file time(ms) memory(MB) callbacks-baseline.js 385 38.61 sweetjs-task.js 672 46.71 promises-bluebird-generator.js 734 38.81 promises-bluebird.js 744 51.07 callbacks-caolan-async-waterfall.js 1211 75.30 promises-obvious-kew.js 1547 115.41 promises-tildeio-rsvp.js 2280 111.19 promises-medikoo-deferred.js 4084 311.98 promises-dfilatov-vow.js 4655 243.75 promises-cujojs-when.js 7899 263.96 promises-calvinmetcalf-liar.js 9655 237.90 promises-kriskowal-q.js 47652 700.61


Using doxbee-sequential-errors.sjs.

results for 10000 parallel executions, 1 ms per I/O op
Likelihood of rejection: 0.1

file time(ms) memory(MB) callbacks-baseline.js 490 39.61 sweetjs-task.js 690 57.55 promises-bluebird-generator.js 861 41.52 promises-bluebird.js 985 66.33 callbacks-caolan-async-waterfall.js 1278 76.50 promises-obvious-kew.js 1690 138.42 promises-tildeio-rsvp.js 2579 179.89 promises-dfilatov-vow.js 5249 345.24 promises-cujojs-when.js 8938 421.38 promises-calvinmetcalf-liar.js 9228 299.89 promises-kriskowal-q.js 48887 705.21 promises-medikoo-deferred.js OOM OOM


Using madeup-parallel.sjs.

Some libraries were disabled for this benchmark because I didn't have the patience to wait for them to complete ;P

file                                time(ms)  memory(MB)
callbacks-baseline.js                    641       46.52
sweetjs-task.js                         1930      140.13
promises-bluebird.js                    2207      167.87
promises-bluebird-generator.js          2301      170.73
callbacks-caolan-async-parallel.js      4214      216.52
promises-obvious-kew.js                 5611      739.51
promises-tildeio-rsvp.js                8857      872.50


Note: I'd placed this part at the top initially because I first wrote cspjs out of a desperate need to find a way to work with async code that was compatible with my brain. Now that cspjs has had some time in my projects, this can take a back seat.

My brain doesn't think well with promises. Despite that, bluebird is a fantastic implementation of the Promises/A+ spec and then some, that many in the community are switching to promises wholesale.

So what does my brain think well with? The kind of "communicating sequential processes" model used in Haskell, Erlang and Go works very well with my brain. Also clojure's core.async module uses this approach. Given this prominence of the CSP model, I'm quite sure there are many like me who want to use the CSP model with Javascript without having to switch to another language entirely.

So, what did I do? I wrote a sweetjs macro named task and a support library for channels that provides this facility using as close to JS syntax as possible. It compiles CSP-style code into a pure-JS (ES5) state machine. The code looks similar to generators and when generator support is ubiquitous the macro can easily be implemented to write code using them. However, for the moment, generators are not ubiquitous on the browser side and it helps to have good async facilities there too.

No additional wrappers are needed to work with NodeJS-style callbacks since a "task" compiles down to a pure-JS function which takes a NodeJS-style callback as the final argument.

Show me the code already!

  1. Compare task/doxbee-sequential and bluebird/doxbee-sequential for the
  2. Compare task/doxbee-sequential-errors and bluebird/doxbee-sequential-errors for the
  3. Compare task/madeup-parallel and bluebird/madeup-parallel for the

So what's different from ES6 generators?

There are a lot of similarities with generators, but some significant differences exist too.

In two words, the difference is "error management". I think the traditional

try {} catch (e) {} finally {}
blocks promote sloppy thinking about error conditions. I want to place function-scoped
clauses up front or anywhere I want, near the code where I should be thinking about error conditions. Also "throw-ing" an error should not mean "dropping" it to catch/finally clauses below, should it? ;)

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