co/Readme.md

# 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](https://github.com/visionmedia/co/tree/master/examples).

## Installation

```
$ npm install co
```

## Associated libraries

  - [co-fs](https://github.com/visionmedia/co-fs) - core `fs` function wrappers
  - [co-exec](https://github.com/visionmedia/co-exec) - core `exec` function wrapper
  - [co-prompt](https://github.com/visionmedia/co-prompt) - terminal user input utilities
  - [co-express](https://github.com/mciparelli/co-express) - [express](https://github.com/visionmedia/express) wrapper that enables generators to be used as middlewares
  - [level-co](https://github.com/juliangruber/level-co) - levelup wrapper

## Example

```js
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:

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

});
```

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

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

});
```

  Which basically looks like this:

```js
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`.

```js
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.

```js
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:


```js
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:

```js
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.

```js

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.

```js

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);
});
```

Optionally you may pass the `fn`'s receiver as the `ctx` as shown here:

```js

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

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

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.

```js

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:

```js
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:

```js
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