engen

The generator engine.

Async control flow using pure ES6 generators.

Requires ES6 generator support. Tested on Node 0.12 with the --harmony flag.

Async code based on yield

With generators, calling an asynchronous function and waiting for its result is as simple as calling yield f(). Parameters and return values work exactly as if the code were synchronous.

var g = require('engen');
 
function *c(x) {
  yield g.wait(1000);
  return x;
}
 
function *b() {
  return yield c('b'); // takes 1000ms
}
 
function *a() {
  var b = yield b();
  return 'a' + b;
}
 
g.run(a(), function(err, res) {
  console.log(err); // null
  console.log(res); // 'ab'
});

Yielding an array of generators will run them in parallel and return an array of ordered results.

var g = require('engen');
 
function *a() {
  yield g.wait(1000);
  return 'a';
}
 
function *b() {
  yield g.wait(2000);
  return 'b';
}
 
function *f() {
  return yield [a(), b()]; // takes 2000ms
}
 
g.run(f(), function(err, res) {
  console.log(err); // null
  console.log(res); // ['a', 'b']
});

Yielding an object of generators behaves identically to yielding an array, but the resulting value is an object.

var g = require('engen');
 
function *a() {
  yield g.wait(1000);
  return 'a';
}
 
function *b() {
  yield g.wait(2000);
  return 'b';
}
 
function *f() {
  return yield {a: a(), b: b()}; // takes 2000ms
}
 
g.run(f(), function(err, res) {
  console.log(err); // null
  console.log(res); // {a: 'a', b: 'b'}
});

Inside collections, mixing generators and simple values is allowed. Generators will be waited for, anything else will be passed through untouched.

Note: this includes objects and arrays: be careful not to nest collections of generators.

var g = require('engen');
 
function *a() {
  yield g.wait(1000);
  return 'a';
}
 
function *f() {
  return yield [a(), false, null, {}];
}
 
g.run(f(), function(err, res) {
  console.log(err); // null
  console.log(res); // ['a', false, null, {}];
});

Exceptions also behave as if the code were synchronous:

var g = require('engen');
 
function *b() {
  yield g.wait(1000);
  throw Error('oops');
}
 
function *a() {
  try {
    yield b(); // takes 1000ms
  } catch(err) {
    console.log(err) // 'Error: oops'
  }
  return 'a';
}
 
g.run(a(), function(err, res) {
  console.log(err); // null
});
 
g.run(b(), function(err, res) {
  console.log(err); // 'Error: oops'
});

API

engen.run()

Run generator based code from callback-land.

var g = require('engen');
 
function *f() {
  yield g.wait(2000);
  return 'done';
}
 
g.run(f(), function(err, result) {
  console.log(err); // null
  console.log(result); // 'done'
});
 

engen.wrap()

Wrap callback-style code and call it from generator-land.

var g = require('engen');
var readFile = g.wrap(require('fs').readFile);
 
function *f() {
  yield g.wait(1000);
  return yield readFile('test.js');
}
 
g.run(f(), function(err, res) {
  console.log(err); // null
  console.log(res); // <Buffer ...>
});

By default, wrap() expects callbacks to have the standard Node signature:

function(error, result) {}

To wrap an unusual callback, you can pass in a function as a second parameter to wrap() to perform custom handling of the callback parameters.

To handle multiple return values and return them as an array:

var g = require('engen');
 
var f = g.wrap(function(cb) {
  cb(err, 12, 34, 56);
}, function(err, a, b, c) {
  if (err) throw err;
  return [a, b, c];
});

To handle callbacks without an err parameter:

var g = require('engen');
 
var f = g.wrap(function(cb) {
  cb(12, 34, 56);
}, function(a, b, c) {
  return [a, b, c];
});

For your convenience, these two handlers are built in, as g.multipleReturnCallback and g.noErrorCallback.

engen.wait()

Generator-based version of setTimeout, provided for convenience.

var g = require('engen');
 
function *f() {
  yield g.wait(1000); // pauses for 1000ms
  return 'done';
}
 
g.run(f());

License

MIT