Made a lengthier example on Koa's flow and how it uses generators. This is just demonstrating how the middleware stacks and uses yield opposed to using callbacks like in ExpressJS. https://github.com/koajs/koa

koa-example.js

//Koajs example

var koa = require('koa');
var app = koa();

//Middleware cascading
app.use(function * (next) {
	//Some stuff will happen here first
	console.log('FIRST execution')

	yield next; //goes to next generator downstream

	//this is the final function-remains to be executed
	//this will be the last piece of code to be executed since no more generators are up stream
	console.log('SIXTH execution (continuing remaining of FIRST)');

	//kills the server since we're done (no need to kill, this is just for demo purposes)
	process.exit("status")
});


app.use(function * (next) {
	//do more stuff here
	console.log('SECOND execution')

	yield next; //next generator downstream..

	//coming back up stream... finishing this function
	console.log('FIFTH execution (continuing remaining of SECOND)')
});


app.use(function * (next) {
	//now some stuff will happen here SECOND
	console.log('THIRD execution')

	yield next; //goes to the last generator downstream

	//remaining of this function will execute then go back upstream
	console.log('FORTH execution (continuing remaining of THIRD)')
});


// response
app.use(function * () {//notice how no next param is passed
	//since this is the last generator, finishes this function and goes upstream
	console.log('Response... and going back upstream')
	// this.body = 'Hello World';
});


app.use(function * () {
	//this isn't executed, but why??
	//the previous didn't "yield next"
	
	console.log('another response?')
	this.body = 'Bye bye world :(';
});


app.use(function * (next) {
	//this isn't executed either, but why??
	//the stream already went back up
	
	yield next
	console.log('I dont get to run either')
});


app.listen(3000);
console.log('========== starting 3000 ==========');