Thunks and co with ES6 generators

A thunk is the output of a transformed node.js-style asynchronous function. The transformed function (aka the thunkified function) is meant to work with a generator wrapper. Here is the generator wrapper I am talking about:

co(function*(){
  ...alot of synchronous looking code...
})()

That generator wrapper takes the output of a thunkified async function call in a yield context and passes the ultimate results of the async call back to the original yield location; so code like this works (within the generator wrapper):

output = yield thunkfiedAsyncFn(args...)

Where node.js asynchronous functions are called with a few arguments plus one callback function to receive the output of the async operation, a thunkified node.js function call takes the original arguments MINUS the callbcak function and outputs a thunk function. In the above example what you are yielding to the generator wrapper is the thunk function. Here is a full example of reading a list of files in series:

	#!/usr/bin/env node --harmony-generators
	var fs = require('fs')
	, thunkify = require('thunkify')
	, co = require('co')
	, readFile = thunkify(fs.readFile)
	, fns = ["file0.txt", "file1.txt"
	, "file2.txt", "file3.txt"]
	co(function*(){
	var contents = '', i
	for (i=0; i<fns.length; i+=1) {
	contents += yield readFile(fns[i])
	}
	console.log(contents)
	})()

view raw co-generator-ex0.js hosted with ❤ by GitHub

As you can see this is very different from how you would need to do it with the async module. For example, you can directly use the array subscript i of the fns array (normally you have to worry about the closure over i in a callback always being equal to 4.

To do the same async work in parallel you collect an array of thunk functions and yield that array. Example:

	#!/usr/bin/env node --harmony-generators
	var fs = require('fs')
	, thunkify = require('thunkify')
	, co = require('co')
	, readFile = thunkify(fs.readFile)
	, fns = ["file0.txt", "file1.txt"
	, "file2.txt", "file3.txt"]
	co(function*(){
	var contents, thunks
	thunks = fns.map(function(fn){
	return readFile(fn)
	})
	contents = (yield thunks).join('')
	console.log(contents)
	})()

view raw co-generator-ex1.js hosted with ❤ by GitHub