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deploy: d14d1d0a9cf38240bfa93c6f2a5d961f306145a6
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<li><em>Go语言圣经读书笔记(挖坑中...): <a href="https://github.com/chai2010/gopl-notes-zh">https://github.com/chai2010/gopl-notes-zh</a></em></li>
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<hr>
<h2 id="55-函数值"><a class="header" href="#55-函数值">5.5. 函数值</a></h2>
<p>在Go中，函数被看作第一类值（first-class values）：函数像其他值一样，拥有类型，可以被赋值给其他变量，传递给函数，从函数返回。对函数值（function value）的调用类似函数调用。例子如下：</p>
<pre><code class="language-Go"> func square(n int) int { return n * n }
func negative(n int) int { return -n }
func product(m, n int) int { return m * n }
f := square
fmt.Println(f(3)) // &quot;9&quot;
f = negative
fmt.Println(f(3)) // &quot;-3&quot;
fmt.Printf(&quot;%T\n&quot;, f) // &quot;func(int) int&quot;
f = product // compile error: can't assign func(int, int) int to func(int) int
</code></pre>
<p>函数类型的零值是nil。调用值为nil的函数值会引起panic错误：</p>
<pre><code class="language-Go"> var f func(int) int
f(3) // 此处f的值为nil, 会引起panic错误
</code></pre>
<p>函数值可以与nil比较：</p>
<pre><code class="language-Go"> var f func(int) int
if f != nil {
f(3)
}
</code></pre>
<p>但是函数值之间是不可比较的，也不能用函数值作为map的key。</p>
<p>函数值使得我们不仅仅可以通过数据来参数化函数，亦可通过行为。标准库中包含许多这样的例子。下面的代码展示了如何使用这个技巧。strings.Map对字符串中的每个字符调用add1函数，并将每个add1函数的返回值组成一个新的字符串返回给调用者。</p>
<pre><code class="language-Go"> func add1(r rune) rune { return r + 1 }
fmt.Println(strings.Map(add1, &quot;HAL-9000&quot;)) // &quot;IBM.:111&quot;
fmt.Println(strings.Map(add1, &quot;VMS&quot;)) // &quot;WNT&quot;
fmt.Println(strings.Map(add1, &quot;Admix&quot;)) // &quot;Benjy&quot;
</code></pre>
<p>5.2节的findLinks函数使用了辅助函数visit，遍历和操作了HTML页面的所有结点。使用函数值，我们可以将遍历结点的逻辑和操作结点的逻辑分离，使得我们可以复用遍历的逻辑，从而对结点进行不同的操作。</p>
<p><u><i>gopl.io/ch5/outline2</i></u></p>
<pre><code class="language-Go">// forEachNode针对每个结点x，都会调用pre(x)和post(x)。
// pre和post都是可选的。
// 遍历孩子结点之前，pre被调用
// 遍历孩子结点之后，post被调用
func forEachNode(n *html.Node, pre, post func(n *html.Node)) {
if pre != nil {
pre(n)
}
for c := n.FirstChild; c != nil; c = c.NextSibling {
forEachNode(c, pre, post)
}
if post != nil {
post(n)
}
}
</code></pre>
<p>该函数接收2个函数作为参数，分别在结点的孩子被访问前和访问后调用。这样的设计给调用者更大的灵活性。举个例子，现在我们有startElement和endElement两个函数用于输出HTML元素的开始标签和结束标签<code>&lt;b&gt;...&lt;/b&gt;</code>：</p>
<pre><code class="language-Go">var depth int
func startElement(n *html.Node) {
if n.Type == html.ElementNode {
fmt.Printf(&quot;%*s&lt;%s&gt;\n&quot;, depth*2, &quot;&quot;, n.Data)
depth++
}
}
func endElement(n *html.Node) {
if n.Type == html.ElementNode {
depth--
fmt.Printf(&quot;%*s&lt;/%s&gt;\n&quot;, depth*2, &quot;&quot;, n.Data)
}
}
</code></pre>
<p>上面的代码利用fmt.Printf的一个小技巧控制输出的缩进。<code>%*s</code>中的<code>*</code>会在字符串之前填充一些空格。在例子中，每次输出会先填充<code>depth*2</code>数量的空格，再输出&quot;&quot;，最后再输出HTML标签。</p>
<p>如果我们像下面这样调用forEachNode：</p>
<pre><code class="language-Go">forEachNode(doc, startElement, endElement)
</code></pre>
<p>与之前的outline程序相比，我们得到了更加详细的页面结构：</p>
<pre><code>$ go build gopl.io/ch5/outline2
$ ./outline2 http://gopl.io
&lt;html&gt;
&lt;head&gt;
&lt;meta&gt;
&lt;/meta&gt;
&lt;title&gt;
&lt;/title&gt;
&lt;style&gt;
&lt;/style&gt;
&lt;/head&gt;
&lt;body&gt;
&lt;table&gt;
&lt;tbody&gt;
&lt;tr&gt;
&lt;td&gt;
&lt;a&gt;
&lt;img&gt;
&lt;/img&gt;
...
</code></pre>
<p><strong>练习 5.7：</strong> 完善startElement和endElement函数，使其成为通用的HTML输出器。要求：输出注释结点，文本结点以及每个元素的属性（&lt; a href='...'&gt;）。使用简略格式输出没有孩子结点的元素（即用<code>&lt;img/&gt;</code>代替<code>&lt;img&gt;&lt;/img&gt;</code>）。编写测试，验证程序输出的格式正确。（详见11章）</p>
<p><strong>练习 5.8：</strong> 修改pre和post函数，使其返回布尔类型的返回值。返回false时，中止forEachNoded的遍历。使用修改后的代码编写ElementByID函数，根据用户输入的id查找第一个拥有该id元素的HTML元素，查找成功后，停止遍历。</p>
<pre><code class="language-Go">func ElementByID(doc *html.Node, id string) *html.Node
</code></pre>
<p><strong>练习 5.9：</strong> 编写函数expand，将s中的&quot;foo&quot;替换为f(&quot;foo&quot;)的返回值。</p>
<pre><code class="language-Go">func expand(s string, f func(string) string) string
</code></pre>
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