Add usage of koa-convert for legacy middleware

closes #565
closes #538

Readme.md

@@ -116,25 +116,28 @@ app.use(ctx => {
 app.listen(3000);
 ```
 
-## Example with old version, Koa < 2
+## Example with old signature
+
+If you want to use old signature or be compatible with old middleware, you must use [koa-convert](https://github.com/gyson/koa-convert) to convert legacy generator middleware to promise middleware.
 
 ```js
 const Koa = require('koa');
 const app = new Koa();
+const convert = require('koa-convert')
 
 // logger
 
-app.use(function *(next){
+app.use(convert(function *(next){
   const start = new Date;
   yield next;
   const ms = new Date - start;
   console.log(`${this.method} ${this.url} - ${ms}`);
-});
+}));
 
 // response
 
-app.use(function *(){
-  this.body = 'Hello World';
+app.use(ctx => {
+  ctx.body = 'Hello World';
 });
 
 app.listen(3000);

lib/application.js

@@ -92,7 +92,7 @@ module.exports = class Application extends Emitter {
   use(fn) {
     debug('use %s', fn._name || fn.name || '-');
     if (typeof fn !== 'function') throw new TypeError('middleware must be a function!');
-    if (isGeneratorFunction(fn)) throw new TypeError('Support for generators has been removed. Use Promises or wrap your generator with co.wrap');
+    if (isGeneratorFunction(fn)) throw new TypeError('Support for generators has been removed. See the documentation for examples of how to convert old middleware https://github.com/koajs/koa#example-with-old-signature');
     this.middleware.push(fn);
     return this;
   }

test/application/use.js

@@ -66,6 +66,6 @@ describe('app.use(fn)', function(){
   it('should throw error for generator', function(){
     const app = new Koa();
 
-    (() => app.use(function *(){})).should.throw('Support for generators has been removed. Use Promises or wrap your generator with co.wrap');
+    (() => app.use(function *(){})).should.throw(/.+/);
   });
 });