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+
+# koa-convert
+
+[![npm version](https://img.shields.io/npm/v/koa-convert.svg)](https://npmjs.org/package/koa-convert)
+[![build status](https://travis-ci.org/gyson/koa-convert.svg)](https://travis-ci.org/gyson/koa-convert)
+
+Convert koa legacy ( 0.x & 1.x ) generator middleware to modern promise middleware ( 2.x ).
+
+It could also convert modern promise middleware back to legacy generator middleware ( useful to help modern middleware support koa 0.x or 1.x ).
+
+## Note
+
+It should be able to convert any legacy generator middleware to modern promise middleware ( or convert it back ).
+
+Please let me know ( send a issue ) if you fail to do so.
+
+## Installation
+
+```
+$ npm install koa-convert
+```
+
+## Usage
+
+```js
+const Koa = require('koa') // koa v2.x
+const convert = require('koa-convert')
+const app = new Koa()
+
+app.use(modernMiddleware)
+
+app.use(convert(legacyMiddleware))
+
+app.use(convert.compose(legacyMiddleware, modernMiddleware))
+
+function * legacyMiddleware (next) {
+  // before
+  yield next
+  // after
+}
+
+function modernMiddleware (ctx, next) {
+  // before
+  return next().then(() => {
+    // after
+  })
+}
+```
+
+## Distinguish legacy and modern middleware
+
+In koa 0.x and 1.x ( without experimental flag ), `app.use` has an assertion that all ( legacy ) middleware must be generator function and it's tested with `fn.constructor.name == 'GeneratorFunction'` at [here](https://github.com/koajs/koa/blob/7fe29d92f1e826d9ce36029e1b9263b94cba8a7c/lib/application.js#L105).
+
+Therefore, we can distinguish legacy and modern middleware with `fn.constructor.name == 'GeneratorFunction'`.
+
+## Migration
+
+`app.use(legacyMiddleware)` is everywhere in 0.x and 1.x and it would be painful to manually change all of them to `app.use(convert(legacyMiddleware))`.
+
+You can use following snippet to make migration easier.
+
+```js
+const _use = app.use
+app.use = x => _use.call(app, convert(x))
+```
+
+The above snippet will override `app.use` method and implicitly convert all legacy generator middleware to modern promise middleware.
+
+Therefore, you can have both `app.use(modernMiddleware)` and `app.use(legacyMiddleware)` and your 0.x or 1.x should work without modification.
+
+Complete example:
+
+```js
+const Koa = require('koa') // v2.x
+const convert = require('koa-convert')
+const app = new Koa()
+
+// ---------- override app.use method ----------
+
+const _use = app.use
+app.use = x => _use.call(app, convert(x))
+
+// ---------- end ----------
+
+app.use(modernMiddleware)
+
+// this will be converted to modern promise middleware implicitly
+app.use(legacyMiddleware)
+
+function * legacyMiddleware (next) {
+  // before
+  yield next
+  // after
+}
+
+function modernMiddleware (ctx, next) {
+  // before
+  return next().then(() => {
+    // after
+  })
+}
+```
+
+## API
+
+#### `convert()`
+
+Convert legacy generator middleware to modern promise middleware.
+
+```js
+modernMiddleware = convert(legacyMiddleware)
+```
+
+#### `convert.compose()`
+
+Convert and compose multiple middleware (could mix legacy and modern ones) and return modern promise middleware.
+
+```js
+composedModernMiddleware = convert.compose(legacyMiddleware, modernMiddleware)
+// or
+composedModernMiddleware = convert.compose([legacyMiddleware, modernMiddleware])
+```
+
+#### `convert.back()`
+
+Convert modern promise middleware back to legacy generator middleware.
+
+This is useful to help modern promise middleware support koa 0.x or 1.x.
+
+```js
+legacyMiddleware = convert.back(modernMiddleware)
+```
+
+## License
+
+MIT