Co

Generator based flow-control goodness for nodejs (and soon the browser), using thunks or promises, letting you write non-blocking code in a nice-ish way.

Currently you must use the --harmony-generators flag when running node 0.11.x to get access to generators.

Co is careful to relay any errors that occur back to the generator, including those within the thunk, or from the thunk's callback. "Uncaught" exceptions in the generator are then either passed co()'s thunk or thrown.

Make sure to view the examples.

Installation

$ npm install co

Associated libraries

co-fs - core fs function wrappers
co-exec - core exec function wrapper
co-prompt - terminal user input utilities
co-express - express wrapper that enables generators to be used as middlewares
level-co - levelup wrapper
mysql-co - mysql wrapper
co-each - parallel foreach
co-wait - setTimeout wrapper
co-read - consume readable streams
co-render - template rendering built on consolidate.js
co-views - higher level template rendering based on co-render

Example

var co = require('co');

co(function *(){
  var a = yield get('http://google.com');
  var b = yield get('http://yahoo.com');
  var c = yield get('http://cloudup.com');
  console.log(a.status);
  console.log(b.status);
  console.log(c.status);
})

co(function *(){
  var a = get('http://google.com');
  var b = get('http://yahoo.com');
  var c = get('http://cloudup.com');
  var res = yield [a, b, c];
  console.log(res);
})

Yieldables

The "yieldable" objects currently supported are:

promises
thunks (functions)
array (parallel execution)
generators (delegation)
generator functions (delegation)

Thunks vs promises

While co supports promises, you may return "thunks" from your functions, which otherwise behaves just like the traditional node-style callback with a signature of: (err, result).

For example take fs.readFile, we all know the signature is:

fs.readFile(path, encoding, function(err, result){

});

To work with Co we need a function to return another function of the same signature:

fs.readFile(path, encoding)(function(err, result){

});

Which basically looks like this:

function read(path, encoding) {
  return function(cb){
    fs.readFile(path, encoding, cb);
  }
}

This is what the co.wrap(fn) utility function does for you.

Receiver propagation

When co is invoked with a receiver it will propagate to most yieldables, allowing you to alter this.

var ctx = {};

function foo() {
  assert(this == ctx);
}

co.call(ctx, function *(){
  assert(this == ctx);
  yield foo;
});

API

co(fn)

Pass a generator fn which is immediately invoked. Any yield expressions within must return a "thunk", at which point co() will defer execution.

var co = require('co');
var fs = require('fs');

function read(file) {
  return function(fn){
    fs.readFile(file, 'utf8', fn);
  }
}

co(function *(){
  var a = yield read('.gitignore');
  var b = yield read('Makefile');
  var c = yield read('package.json');
  console.log(a);
  console.log(b);
  console.log(c);
});

You may also yield Generator objects to support nesting:

var co = require('co');
var fs = require('fs');

function size(file) {
  return function(fn){
    fs.stat(file, function(err, stat){
      if (err) return fn(err);
      fn(null, stat.size);
    });
  }
}

function *foo(){
  var a = yield size('.gitignore');
  var b = yield size('Makefile');
  var c = yield size('package.json');
  return [a, b, c];
}

function *bar(){
  var a = yield size('examples/parallel.js');
  var b = yield size('examples/nested.js');
  var c = yield size('examples/simple.js');
  return [a, b, c];
}

co(function *(){
  var a = yield foo();
  var b = yield bar();
  console.log(a);
  console.log(b);
});

Or if the generator functions do not require arguments, simply yield the function:

var request = require('superagent');

var get = co.wrap(request.get);

function *results() {
  var a = yield get('http://google.com')
  var b = yield get('http://yahoo.com')
  var c = yield get('http://ign.com')
  return [a.status, b.status, c.status]
}

co(function *(){
  // 3 concurrent requests at a time
  var a = yield results;
  var b = yield results;
  var c = yield results;
  console.log(a, b, c);

  // 9 concurrent requests
  console.log(yield [results, results, results]);
});

co() return values

Since co() returns a thunk, you may pass a function to this thunk to receive the return values from the generator. Any error that occurs is passed to this (sizes) function.

var co = require('co');
var fs = require('fs');

var read = co.wrap(fs.readFile);

var sizes = co(function *(){
  var a = yield read('.gitignore');
  var b = yield read('Makefile');
  var c = yield read('package.json');
  return [a.length, b.length, c.length];
});

sizes(function(err, res){
  console.log(res);
});

co.wrap(fn, [ctx])

The co.wrap() utility simply wraps a node-style function to return a thunk.

var co = require('co');
var fs = require('fs');

var read = co.wrap(fs.readFile);

co(function *(){
  var a = yield read('.gitignore');
  var b = yield read('Makefile', 'ascii');
  var c = yield read('package.json', 'utf8');
  console.log(a);
  console.log(b);
  console.log(c);
});

An example with redis:

var co = require('co')
var redis = require('redis')
var db = redis.createClient()

db.set = co.wrap(db.set)
db.get = co.wrap(db.get)

co(function *(){
  yield db.set('foo', 'bar')
  yield db.set('bar', 'baz')

  var res = yield db.get('foo')
  console.log('foo -> %s', res);

  var res = yield db.get('bar')
  console.log('bar -> %s', res);
})

co.join(fn...)

The co.join() utility function allows you to pass multiple thunks, or an array of thunks and "join" them all into a single thunk which executes them all concurrently, instead of in sequence. Note that the resulting array ordering is retained.

var co = require('co');
var join = co.join;
var fs = require('fs');

function size(file) {
  return function(fn){
    fs.stat(file, function(err, stat){
      if (err) return fn(err);
      fn(null, stat.size);
    });
  }
}

co(function *(){
  var a = size('.gitignore');
  var b = size('index.js');
  var c = size('Makefile');
  var res = yield join(a, b, c);
  console.log(res);
  // => [ 13, 1687, 129 ]
});

As an alias of join(array) you may simply yield an array:

co(function *(){
  var a = size('.gitignore');
  var b = size('index.js');
  var c = size('Makefile');
  var res = yield [a, b, c];
  console.log(res);
  // => [ 13, 1687, 129 ]
});

Nested joins may also be expressed as simple nested arrays:

var a = [
  get('http://google.com'),
  get('http://yahoo.com'),
  get('http://ign.com')
];

var b = [
  get('http://google.com'),
  get('http://yahoo.com'),
  get('http://ign.com')
];

console.log(yield [a, b]);

Performance

On my machine 30,000 sequential stat()s takes an avg of 570ms, while the same number of sequential stat()s with co() takes 610ms, aka the overhead introduced by generators is extremely negligable.

License

MIT

benjamn/co

Co