docs: create v2 Migration document (#931)

* Give v2 migration documentation its own document. Incorporate docs from #533

* Fix mis-capitalization of Koa

* Remove unnecessary Dependency section

* Hint at koa-convert enabled compatibility

* Add section on constructing with new

* Clarify es6 constructors are used

* Fix varying capitalization

* Restore mistakenly removed Dependency changes section

* v1.x should not receive feature updates

* Add next() to signature, add missing backticks

* docs++

Readme.md

@@ -44,11 +44,11 @@ app.listen(3000);
 
 
 ## Middleware
+
 Koa is a middleware framework that can take 3 different kinds of functions as middleware:
 
   * common function
   * async function
-  * generator function
 
 Here is an example of logger middleware with each of the different functions:
 
@@ -78,69 +78,46 @@ app.use((ctx, next) => {
 });
 ```
 
-### GeneratorFunction
-
-To use generator functions, you must use a wrapper such as [co](https://github.com/tj/co) that is no longer supplied with Koa.
-
-```js
-app.use(co.wrap(function *(ctx, next) {
-  const start = new Date();
-  yield next();
-  const ms = new Date() - start;
-  console.log(`${ctx.method} ${ctx.url} - ${ms}ms`);
-}));
-```
-
-### Old signature middleware (v1.x) - Deprecated
-
-**Old signature middleware (v1.x) support will be removed in v3**
-
-Koa v2.x will try to convert legacy signature, generator middleware on `app.use`, using [koa-convert](https://github.com/koajs/convert).
-It is however recommended that you choose to migrate all v1.x middleware as soon as possible.
-
-```js
-// Koa will convert
-app.use(function *(next) {
-  const start = new Date();
-  yield next;
-  const ms = new Date() - start;
-  console.log(`${this.method} ${this.url} - ${ms}ms`);
-});
-```
+### Koa v1.x Middleware Signature
 
-You could do it manually as well, in which case Koa will not convert.
+The middleware signature changed between v1.x and v2.x.  The older signature is deprecated.
 
-```js
-const convert = require('koa-convert');
+**Old signature middleware support will be removed in v3**
 
-app.use(convert(function *(next) {
-  const start = new Date();
-  yield next;
-  const ms = new Date() - start;
-  console.log(`${this.method} ${this.url} - ${ms}ms`);
-}));
-```
+Please see the [Migration Guide](docs/migration.md) for more information on upgrading from v1.x and
+using v1.x middleware with v2.x.
 
 ## Babel setup
 
-For Node 4.0+ and Babel 6.0 you can setup like this:
+If you're not using `node v7.6+`, we recommend setting up `babel` with [`babel-preset-env`](https://github.com/babel/babel-preset-env):
 
 ```bash
-$ npm install babel-register babel-plugin-transform-async-to-generator --save
+$ npm install babel-register babel-preset-env --save
 ```
 
+Setup `babel-register` in your entry file:
+
 ```js
-// set babel in entry file
-require('babel-register')({
-  plugins: ['transform-async-to-generator']
-});
+require('babel-register');
 ```
 
-Check out an example in koa's [test](test/babel/index.js).
+And have your `.babelrc` setup:
+
+```json
+{
+  "presets": [
+    ["env", {
+      "targets": {
+        "node": true
+      }
+    }]
+  ]
+}
+```
 
 ## Troubleshooting
 
-Check the [Troubleshooting Guide](docs/troubleshooting.md) or [Debugging Koa](docs/guide.md#debugging-koa) in 
+Check the [Troubleshooting Guide](docs/troubleshooting.md) or [Debugging Koa](docs/guide.md#debugging-koa) in
 the general Koa guide.
 
 ## Running tests

docs/migration.md

@@ -0,0 +1,133 @@
+# Migrating from Koa v1.x to v2.x
+
+## New middleware signature 
+
+Koa v2 introduces a new signature for middleware.
+
+**Old signature middleware (v1.x) support will be removed in v3**
+
+The new middleware signature is:
+
+```js
+// uses async arrow functions
+app.use(async (ctx, next) => {
+  try {
+    await next() // next is now a function
+  } catch (err) {
+    ctx.body = { message: err.message }
+    ctx.status = err.status || 500
+  }
+})
+
+app.use(async ctx => {
+  const user = await User.getById(this.session.userid) // await instead of yield
+  ctx.body = user // ctx instead of this
+})
+```
+
+You don't have to use asynchronous functions - you just have to pass a function that returns a promise. 
+A regular function that returns a promise works too!
+
+The signature has changed to pass `Context` via an explicit parameter, `ctx` above, instead of via
+`this`.  The context passing change makes koa more compatible with es6 arrow functions, which capture `this`.
+
+## Using v1.x Middleware in v2.x
+
+Koa v2.x will try to convert legacy signature, generator middleware on `app.use`, using [koa-convert](https://github.com/koajs/convert).
+It is however recommended that you choose to migrate all v1.x middleware as soon as possible.
+
+```js
+// Koa will convert
+app.use(function *(next) {
+  const start = new Date();
+  yield next;
+  const ms = new Date() - start;
+  console.log(`${this.method} ${this.url} - ${ms}ms`);
+});
+```
+
+You could do it manually as well, in which case Koa will not convert.
+
+```js
+const convert = require('koa-convert');
+
+app.use(convert(function *(next) {
+  const start = new Date();
+  yield next;
+  const ms = new Date() - start;
+  console.log(`${this.method} ${this.url} - ${ms}ms`);
+}));
+```
+
+## Upgrading middleware
+
+You will have to convert your generators to async functions with the new middleware signature:
+
+```js
+app.use(async ctx => {
+  const user = await Users.getById(this.session.user_id);
+  await next();
+  ctx.body = { message: 'some message' };
+})
+```
+
+Upgrading your middleware may require some work. One migration path is to update them one-by-one.
+
+1. Wrap all your current middleware in `koa-convert`
+2. Test
+3. `npm outdated` to see which koa middleware is outdated
+4. Update one outdated middleware, remove using `koa-convert`
+5. Test
+6. Repeat steps 3-5 until you're done
+
+
+## Updating your code
+
+You should start refactoring your code now to ease migrating to Koa v2:
+
+- Return promises everywhere!
+- Do not use `yield*`
+- Do not use `yield {}` or `yield []`.
+  - Convert `yield []` into `yield Promise.all([])`
+  - Convert `yield {}` into `yield Bluebird.props({})`
+
+You could also refactor your logic outside of Koa middleware functions. Create functions like 
+`function* someLogic(ctx) {}` and call it in your middleware as 
+`const result = yield someLogic(this)`.
+Not using `this` will help migrations to the new middleware signature, which does not use `this`.
+
+## Application object constructor requires new 
+
+In v1.x, the Application constructor function could be called directly, without `new` to 
+instantiate an instance of an application.  For example:
+
+```js
+var koa = require('koa');
+var app = module.exports = koa();
+```
+
+v2.x uses es6 classes which require the `new` keyword to be used.
+
+```js
+var koa = require('koa');
+var app = module.exports = new koa();
+```
+
+## ENV specific logging behavior removed
+
+An explicit check for the `test` environment was removed from error handling. 
+
+## Dependency changes
+
+- [co](https://github.com/tj/co) is no longer bundled with Koa.  Require or import it directly.
+- [composition](https://github.com/thenables/composition) is no longer used and deprecated.
+
+## v1.x support
+
+The v1.x branch is still supported but should not receive feature updates.  Except for this migration
+guide, documentation will target the latest version.
+
+## Help out
+
+If you encounter migration related issues not covered by this migration guide, please consider 
+submitting a documentation pull request.

lib/application.js

@@ -107,8 +107,7 @@ module.exports = class Application extends Emitter {
     if (isGeneratorFunction(fn)) {
       deprecate('Support for generators will be removed in v3. ' +
                 'See the documentation for examples of how to convert old middleware ' +
-                'https://github.com/koajs/koa/blob/master/Readme.md' +
-                '#old-signature-middleware-v1x---deprecated');
+                'https://github.com/koajs/koa/blob/master/docs/migration.md');
       fn = convert(fn);
     }
     debug('use %s', fn._name || fn.name || '-');