两者无论写法，还是执行顺序，也没啥区别啊，区别是啥？？？

var app = require('express')();
app.use(function myMiddleware(req, res, next) {  
  res.write('Hello world.');
  next();
});
app.use(function mySecondMiddleware(req, res) {  
  res.write('\nI am a second middleware.');
  res.end();
});

var app = require('koa')();
app.use(function* myMiddleware(next) {  
  this.response.body = 'Hello world.';
  yield next;
});

app.use(function* mySecondMiddleware() {  
  this.response.body += '\nI am a second middleware.';
});