add implementation, readme and test

.gitignore

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+node_modules

README.md

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+
+# koa-convert
+
+Convert koa generator-based middleware to promise-based middleware.
+
+## Installation
+
+```
+$ npm install koa-convert
+```
+
+## Related Issues
+
+* koa [#415](https://github.com/koajs/koa/issues/415)
+* koa-compose [#27](https://github.com/koajs/compose/pull/27)
+
+## Usage
+
+```js
+//
+// convert a generator-based middleware to promise-based middleware
+//
+let promiseBased = convert(function* generatorBased(next) {
+    yield next
+    // or
+    // yield* next
+})
+
+//
+// convert array of middleware
+//
+let mws = [
+    // will convert it to promise-based middleware
+    function* generatorMW (next) {
+        yield next
+    },
+    // will convert it to promise-based middleware
+    function* generatorMW(next) {
+        yield* next
+    },
+    // return itself if it's not generator-based middleware
+    function (ctx, next) {
+        return next()
+    },
+    // return itself if it's not generator-based middleware
+    async function (ctx, next) {
+        await next()
+    },
+].map(convert)
+```
+
+## License
+
+MIT

index.js

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+'use strict';
+
+const co = require('co')
+
+module.exports = convert
+
+function convert(mw) {
+    if (typeof mw !== 'function') {
+        throw new TypeError(mw + ' is not function')
+    }
+    if (mw.constructor.name === 'GeneratorFunction') {
+        return function (ctx, next) {
+            return co.call(ctx, mw.call(ctx, createGenerator(next)))
+        }
+    } else {
+        // assume it's Promise-based middleware
+        return mw
+    }
+}
+
+function* createGenerator(next) {
+    return yield next()
+}

package.json

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+{
+  "name": "koa-convert",
+  "version": "0.0.1",
+  "description": "convert koa generator-based middleware to promise-based middleware",
+  "main": "index.js",
+  "scripts": {
+    "test": "mocha test.js"
+  },
+  "repository": {
+    "type": "git",
+    "url": "git+https://github.com/gyson/koa-convert.git"
+  },
+  "author": "gyson <eilian.yunsong@gmail.com>",
+  "license": "MIT",
+  "bugs": {
+    "url": "https://github.com/gyson/koa-convert/issues"
+  },
+  "homepage": "https://github.com/gyson/koa-convert#readme",
+  "dependencies": {
+    "co": "^4.6.0"
+  },
+  "devDependencies": {
+    "mocha": "^2.3.3"
+  }
+}

test.js

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+'use strict';
+
+const co = require('co')
+const assert = require('assert')
+const convert = require('./index')
+
+describe('Koa Convert', function () {
+    it('should works', function (done) {
+        let call = []
+        let ctx = {}
+        let mw = convert(function* (next) {
+            assert.ok(ctx === this)
+            call.push(1)
+        })
+
+        mw(ctx, function () {
+            done(new Error('this should not be called'))
+        }).then(function () {
+            assert.deepEqual(call, [1])
+            done()
+        })
+    })
+
+    it('should works with `yield next`', function (done) {
+        let call = []
+        let ctx = {}
+        let mw = convert(function* (next) {
+            assert.ok(ctx === this)
+            call.push(1)
+            yield next
+            call.push(3)
+        })
+
+        mw(ctx, function () {
+            call.push(2)
+            return Promise.resolve()
+        }).then(function () {
+            assert.deepEqual(call, [1, 2, 3])
+            done()
+        })
+    })
+
+    it('should works with `yield* next`', function (done) {
+        let call = []
+        let ctx = {}
+        let mw = convert(function* (next) {
+            assert.ok(ctx === this)
+            call.push(1)
+            yield* next
+            call.push(3)
+        })
+
+        mw(ctx, function () {
+            call.push(2)
+            return Promise.resolve()
+        }).then(function () {
+            assert.deepEqual(call, [1, 2, 3])
+            done()
+        })
+    })
+})