continuation.js

continuation.js

CPS transformer with trampoline technique for Node.js

JavaScript is a nice programming language, but compared to Scheme, it lacks the tail call optimization. Node.js is often used with Callback Passing Style programming without tail call optimization.

This module allows to transform native JavaScript code into Continuation Passing Style (CPS) code in a best effort manner. It utilizes so-called trampoline technique to avoid the stack overflow error. Transforming all function into CPS is not very easy (and sometimes not very efficient), hence it has a fallback mechanism, that is, only supported function style is transformed into CPS and other functions are called in an original style. Because of the fallback mechanism, mixing CPS code and non-CPS code is possible.

It is only tested with Node.js, but might be usable with other JavaScript engines.

Comparison

Here is the table showing related projects.

NAME	continuation.js	Continuation	Brushtail	Cinch
CPS transformation	Mostly	Callback style	No	Callback style
Tail Call Optimization	Yes	No	Yes	No
Native JavaScript	Yes	No	Yes	Almost
require() integration	Yes	Yes	No	Yes

How to use

GitHub

% git clone https://github.com/dai-shi/continuation.js.git
% cd continuation.js
% ./bin/continuation-compile sample/fact.js > cps_fact.js

NPM

% npm install continuation.js

and add the following:

require('continuation.js').enable_on_require();

which transforms all following .js files by require.

Examples

Simple factorial function:

% cat sample/fact.js
function fact(x) {
  function fact_tail(x, r) {
    if (x === 0) {
      return r;
    } else {
      return fact_tail(x - 1, x * r);
    }
  }
  return fact_tail(x, 1);
}

exports.fact = fact;

% node -e "console.log(require('./sample/fact.js').fact(100000))"

.../continuation.js/sample/fact.js:2
  function fact_tail(x, r) {
                        ^
RangeError: Maximum call stack size exceeded

% node -e "require('./lib/continuation.js').enable_on_require();console.log(require('./sample/fact.js').fact(100000))"
Infinity

Mutual recursion example:

% cat sample/mutual.js
function isEven(x) {
  if (x === 0) {
    return true;
  } else {
    return isOdd(x - 1);
  }
}

function isOdd(x) {
  if (x === 0) {
    return false;
  } else {
    return isEven(x - 1);
  }
}

exports.isEven = isEven;
exports.isOdd = isOdd;

% node -e "console.log(require('./sample/mutual.js').isOdd(1234567))"

.../sample/mutual.js:1
tion isEven(x) {
       ^
RangeError: Maximum call stack size exceeded
% node -e "require('./lib/continuation.js').enable_on_require();console.log(require('./sample/mutual.js').isOdd(1234567))"
true

How it works

• 4 classes are defined in the global scope.
  • CpsFunction
  • CpsContinuation
  • CpsResult
  • CpsRun
• CPS enabled functions have the CpsEnabled=true property.
• Traversing AST to transform into CPS, only when possible!
• Keeping original code so that non-CPS is always possible.

Benchmark results

The following is the results of Octane benchmark suites (except for one).

Suite name	Original	CPS transformed	Improved?
Richards	370 ops/sec	16.90 ops/sec	No
DeltaBlue	178 ops/sec	6.24 ops/sec	No
Encrypt	171 ops/sec	79.93 ops/sec	No
Decrypt	9.23 ops/sec	3.65 ops/sec	No
RayTrace	19.82 ops/sec	2.23 ops/sec	No
Earley	268 ops/sec	20.77 ops/sec	No
Boyer	19.01 ops/sec	2.14 ops/sec	No
RegExp	7.20 ops/sec	5.00 ops/sec	No
Splay	116 ops/sec	69.90 ops/sec	No
NavierStokes	2.57 ops/sec	2.43 ops/sec	No
PdfJS	2.67 ops/sec	2.49 ops/sec	No
Gameboy	0.99 ops/sec	0.28 ops/sec	No
CodeLoadClosure	343 ops/sec	392 ops/sec	Yes
CodeLoadJQuery	10.23 ops/sec	10.36 ops/sec	Yes
Box2D	2.28 ops/sec	2.48 ops/sec	Yes

Limitations

• Not all calls are transformed into CPS.
• new Function is not supported.
• First-class continuation is not supported.

TODOs

• Work with try...catch and throw.
• Transform non-tail calls into CPS.