edit generator/generator comprehesion

docs/generator.md

@@ -377,52 +377,46 @@ try {
 这种函数体内捕获错误的机制，大大方便了对错误的处理。如果使用回调函数的写法，想要捕获多个错误，就不得不为每个函数写一个错误处理语句。
 
 ```javascript
-
 foo('a', function (a) {
-    if (a.error) {
+  if (a.error) {
-        throw new Error(a.error);
+    throw new Error(a.error);
+  }
+
+  foo('b', function (b) {
+    if (b.error) {
+      throw new Error(b.error);
     }
 
-    foo('b', function (b) {
+    foo('c', function (c) {
-        if (b.error) {
+      if (c.error) {
-            throw new Error(b.error);
+        throw new Error(c.error);
-        }
+      }
 
-        foo('c', function (c) {
+      console.log(a, b, c);
-            if (c.error) {
-                throw new Error(c.error);
-            }
-
-            console.log(a, b, c);
-        });
     });
+  });
 });
-
 ```
 
 使用Generator函数可以大大简化上面的代码。
 
 ```javascript
-
 function* g(){
   try {
-        var a = yield foo('a');
+    var a = yield foo('a');
-        var b = yield foo('b');
+    var b = yield foo('b');
-        var c = yield foo('c');
+    var c = yield foo('c');
-    } catch (e) {
+  } catch (e) {
-        console.log(e);
+    console.log(e);
-    }
+  }
 
   console.log(a, b, c);
-
 }
-
 ```
 
 反过来，Generator函数内抛出的错误，也可以被函数体外的catch捕获。
 
 ```javascript
-
 function *foo() {
   var x = yield 3;
   var y = x.toUpperCase();
@@ -435,11 +429,9 @@ it.next(); // { value:3, done:false }
 
 try {
   it.next(42);
-}
+} catch (err) {
-catch (err) {
   console.log(err);
 }
-
 ```
 
 上面代码中，第二个next方法向函数体内传入一个参数42，数值是没有toUpperCase方法的，所以会抛出一个TypeError错误，被函数体外的catch捕获。
@@ -669,25 +661,70 @@ result
 如果一个对象的属性是Generator函数，可以简写成下面的形式。
 
 ```javascript
-
 let obj = {
   * myGeneratorMethod() {
     ···
   }
 };
-
 ```
 
-它的完整形式如下，两者是等价的。
+上面代码中，myGeneratorMethod属性前面有一个星号，表示这个属性是一个Generator函数。
+
+它的完整形式如下，与上面的写法是等价的。
 
 ```javascript
-
 let obj = {
   myGeneratorMethod: function* () {
-    ···
+    // ···
   }
 };
+```
 
+## Generator函数推导
+
+ES7在数组推导的基础上，提出了Generator函数推导（Generator comprehension）。
+
+```javascript
+let generator = function* () {
+  for (let i = 0; i < 6; i++) {
+    yield i;
+  }
+}
+
+let squared = ( for (n of generator()) n * n );
+// 等同于
+// let squared = Array.from(generator()).map(n => n * n);
+
+console.log(...squared);
+// 0 1 4 9 16 25
+```
+
+“推导”这种语法结构，在ES6只能用于数组，ES7将其推广到了Generator函数。for...of循环会自动调用遍历器的next方法，将返回值的value属性作为数组的一个成员。
+
+Generator函数推导是对数组结构的一种模拟，它的最大优点是惰性求值，即直到真正用到时才会求值，这样可以保证效率。请看下面的例子。
+
+```javascript
+let bigArray = new Array(100000);
+for (let i = 0; i < 100000; i++) {
+  bigArray[i] = i;
+}
+
+let first = bigArray.map(n => n * n)[0];
+console.log(first);
+```
+
+上面例子遍历一个大数组，但是在真正遍历之前，这个数组已经生成了，占用了系统资源。如果改用Generator函数推导，就能避免这一点。下面代码只在用到时，才会生成一个大数组。
+
+```javascript
+let bigGenerator = function* () {
+  for (let i = 0; i < 100000; i++) {
+    yield i;
+  }
+}
+
+let squared = ( for (n of bigGenerator()) n * n );
+
+console.log(squared.next());
 ```
 
 ## 含义

docs/reference.md

@@ -59,24 +59,28 @@
 - Daniel Zautner, [Meta-programming JavaScript Using Proxies](http://dzautner.com/meta-programming-javascript-using-proxies/): 使用Proxy实现元编程
 - MDN, [Symbol](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol): Symbol类型的详细介绍
 
-## Iterator和Generator
+## Iterator
 
 - Mozilla Developer Network, [Iterators and generators](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Iterators_and_Generators)
 - Mozilla Developer Network, [The Iterator protocol](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/The_Iterator_protocol)
 - Jason Orendorff, [ES6 In Depth: Iterators and the for-of loop](https://hacks.mozilla.org/2015/04/es6-in-depth-iterators-and-the-for-of-loop/): 遍历器与for...of循环的介绍
+- Axel Rauschmayer, [Iterators and generators in ECMAScript 6](http://www.2ality.com/2013/06/iterators-generators.html): 探讨Iterator和Generator的设计目的
+- Axel Rauschmayer, [Iterables and iterators in ECMAScript 6](http://www.2ality.com/2015/02/es6-iteration.html): Iterator的详细介绍
+- Kyle Simpson, [Iterating ES6 Numbers](http://blog.getify.com/iterating-es6-numbers/): 在数值对象上部署遍历器
+- Mahdi Dibaiee, [ES7 Array and Generator comprehensions](http://dibaiee.ir/es7-array-generator-comprehensions/)：ES7的Generator推导
+
+## Generator
+
 - Matt Baker, [Replacing callbacks with ES6 Generators](http://flippinawesome.org/2014/02/10/replacing-callbacks-with-es6-generators/)
 - Steven Sanderson, [Experiments with Koa and JavaScript Generators](http://blog.stevensanderson.com/2013/12/21/experiments-with-koa-and-javascript-generators/)
 - jmar777, [What's the Big Deal with Generators?](http://devsmash.com/blog/whats-the-big-deal-with-generators)
 - Marc Harter, [Generators in Node.js: Common Misconceptions and Three Good Use Cases](http://strongloop.com/strongblog/how-to-generators-node-js-yield-use-cases/): 讨论Generator函数的作用
-- Axel Rauschmayer, [Iterators and generators in ECMAScript 6](http://www.2ality.com/2013/06/iterators-generators.html): 探讨Iterator和Generator的设计目的
-- Axel Rauschmayer, [Iterables and iterators in ECMAScript 6](http://www.2ality.com/2015/02/es6-iteration.html): Iterator的详细介绍
 - StackOverflow, [ES6 yield : what happens to the arguments of the first call next()?](http://stackoverflow.com/questions/20977379/es6-yield-what-happens-to-the-arguments-of-the-first-call-next): 第一次使用next方法时不能带有参数
 - Kyle Simpson, [ES6 Generators: Complete Series](http://davidwalsh.name/es6-generators): 由浅入深探讨Generator的系列文章，共四篇
 - Gajus Kuizinas, [The Definitive Guide to the JavaScript Generators](http://gajus.com/blog/2/the-definetive-guide-to-the-javascript-generators): 对Generator的综合介绍
 - Jan Krems, [Generators Are Like Arrays](https://gist.github.com/jkrems/04a2b34fb9893e4c2b5c): 讨论Generator可以被当作数据结构看待
 - Harold Cooper, [Coroutine Event Loops in Javascript](http://syzygy.st/javascript-coroutines/): Generator用于实现状态机
 - Ruslan Ismagilov, [learn-generators](https://github.com/isRuslan/learn-generators): 编程练习，共6道题
-- Kyle Simpson, [Iterating ES6 Numbers](http://blog.getify.com/iterating-es6-numbers/): 在数值对象上部署遍历器
 - Steven Sanderson, [Experiments with Koa and JavaScript Generators](http://blog.stevensanderson.com/2013/12/21/experiments-with-koa-and-javascript-generators/): Generator入门介绍，以Koa框架为例
 
 ## Promise对象