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<ol class="chapter"><li class="chapter-item expanded affix "><a href="../index.html">Go语言圣经</a></li><li class="chapter-item expanded affix "><a href="../preface.html">前言</a></li><li class="chapter-item expanded "><a href="../ch1/ch1.html"><strong aria-hidden="true">1.</strong> 入门</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch1/ch1-01.html"><strong aria-hidden="true">1.1.</strong> Hello, World</a></li><li class="chapter-item expanded "><a href="../ch1/ch1-02.html"><strong aria-hidden="true">1.2.</strong> 命令行参数</a></li><li class="chapter-item expanded "><a href="../ch1/ch1-03.html"><strong aria-hidden="true">1.3.</strong> 查找重复的行</a></li><li class="chapter-item expanded "><a href="../ch1/ch1-04.html"><strong aria-hidden="true">1.4.</strong> GIF动画</a></li><li class="chapter-item expanded "><a href="../ch1/ch1-05.html"><strong aria-hidden="true">1.5.</strong> 获取URL</a></li><li class="chapter-item expanded "><a href="../ch1/ch1-06.html"><strong aria-hidden="true">1.6.</strong> 并发获取多个URL</a></li><li class="chapter-item expanded "><a href="../ch1/ch1-07.html"><strong aria-hidden="true">1.7.</strong> Web服务</a></li><li class="chapter-item expanded "><a href="../ch1/ch1-08.html"><strong aria-hidden="true">1.8.</strong> 本章要点</a></li></ol></li><li class="chapter-item expanded "><a href="../ch2/ch2.html"><strong aria-hidden="true">2.</strong> 程序结构</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch2/ch2-01.html"><strong aria-hidden="true">2.1.</strong> 命名</a></li><li class="chapter-item expanded "><a href="../ch2/ch2-02.html"><strong aria-hidden="true">2.2.</strong> 声明</a></li><li class="chapter-item expanded "><a href="../ch2/ch2-03.html"><strong aria-hidden="true">2.3.</strong> 变量</a></li><li class="chapter-item expanded "><a href="../ch2/ch2-04.html"><strong aria-hidden="true">2.4.</strong> 赋值</a></li><li class="chapter-item expanded "><a href="../ch2/ch2-05.html"><strong aria-hidden="true">2.5.</strong> 类型</a></li><li class="chapter-item expanded "><a href="../ch2/ch2-06.html"><strong aria-hidden="true">2.6.</strong> 包和文件</a></li><li class="chapter-item expanded "><a href="../ch2/ch2-07.html"><strong aria-hidden="true">2.7.</strong> 作用域</a></li></ol></li><li class="chapter-item expanded "><a href="../ch3/ch3.html"><strong aria-hidden="true">3.</strong> 基础数据类型</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch3/ch3-01.html"><strong aria-hidden="true">3.1.</strong> 整型</a></li><li class="chapter-item expanded "><a href="../ch3/ch3-02.html"><strong aria-hidden="true">3.2.</strong> 浮点数</a></li><li class="chapter-item expanded "><a href="../ch3/ch3-03.html"><strong aria-hidden="true">3.3.</strong> 复数</a></li><li class="chapter-item expanded "><a href="../ch3/ch3-04.html"><strong aria-hidden="true">3.4.</strong> 布尔型</a></li><li class="chapter-item expanded "><a href="../ch3/ch3-05.html"><strong aria-hidden="true">3.5.</strong> 字符串</a></li><li class="chapter-item expanded "><a href="../ch3/ch3-06.html"><strong aria-hidden="true">3.6.</strong> 常量</a></li></ol></li><li class="chapter-item expanded "><a href="../ch4/ch4.html"><strong aria-hidden="true">4.</strong> 复合数据类型</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch4/ch4-01.html"><strong aria-hidden="true">4.1.</strong> 数组</a></li><li class="chapter-item expanded "><a href="../ch4/ch4-02.html"><strong aria-hidden="true">4.2.</strong> Slice</a></li><li class="chapter-item expanded "><a href="../ch4/ch4-03.html"><strong aria-hidden="true">4.3.</strong> Map</a></li><li class="chapter-item expanded "><a href="../ch4/ch4-04.html"><strong aria-hidden="true">4.4.</strong> 结构体</a></li><li class="chapter-item expanded "><a href="../ch4/ch4-05.html"><strong aria-hidden="true">4.5.</strong> JSON</a></li><li class="chapter-item expanded "><a href="../ch4/ch4-06.html"><strong aria-hidd
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<main>
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<!-- 头部 -->
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<ul dir="auto">
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<li><em>KusonStack一站式可编程配置技术栈: <a href="https://github.com/KusionStack/kusion">https://github.com/KusionStack/kusion</a></em></li>
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<li><em>凹语言™: <a href="https://github.com/wa-lang/wa">https://github.com/wa-lang/wa</a></em></li>
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<hr>
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<h2 id="55-函数值"><a class="header" href="#55-函数值">5.5. 函数值</a></h2>
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<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>
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<pre><code class="language-Go"> func square(n int) int { return n * n }
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func negative(n int) int { return -n }
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func product(m, n int) int { return m * n }
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f := square
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fmt.Println(f(3)) // "9"
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f = negative
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fmt.Println(f(3)) // "-3"
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fmt.Printf("%T\n", f) // "func(int) int"
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f = product // compile error: can't assign func(int, int) int to func(int) int
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</code></pre>
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<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>
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<pre><code class="language-Go"> var f func(int) int
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f(3) // 此处f的值为nil, 会引起panic错误
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</code></pre>
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<p>函数值可以与nil比较:</p>
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<pre><code class="language-Go"> var f func(int) int
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if f != nil {
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f(3)
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}
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</code></pre>
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<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>
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<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>
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<pre><code class="language-Go"> func add1(r rune) rune { return r + 1 }
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fmt.Println(strings.Map(add1, "HAL-9000")) // "IBM.:111"
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fmt.Println(strings.Map(add1, "VMS")) // "WNT"
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fmt.Println(strings.Map(add1, "Admix")) // "Benjy"
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</code></pre>
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<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>
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<p><u><i>gopl.io/ch5/outline2</i></u></p>
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<pre><code class="language-Go">// forEachNode针对每个结点x,都会调用pre(x)和post(x)。
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// pre和post都是可选的。
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// 遍历孩子结点之前,pre被调用
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// 遍历孩子结点之后,post被调用
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func forEachNode(n *html.Node, pre, post func(n *html.Node)) {
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if pre != nil {
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pre(n)
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}
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for c := n.FirstChild; c != nil; c = c.NextSibling {
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forEachNode(c, pre, post)
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}
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if post != nil {
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post(n)
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}
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}
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</code></pre>
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<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><b>...</b></code>:</p>
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<pre><code class="language-Go">var depth int
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func startElement(n *html.Node) {
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if n.Type == html.ElementNode {
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fmt.Printf("%*s<%s>\n", depth*2, "", n.Data)
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depth++
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}
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}
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func endElement(n *html.Node) {
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if n.Type == html.ElementNode {
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depth--
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fmt.Printf("%*s</%s>\n", depth*2, "", n.Data)
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}
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}
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</code></pre>
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<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"",最后再输出HTML标签。</p>
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<p>如果我们像下面这样调用forEachNode:</p>
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<pre><code class="language-Go">forEachNode(doc, startElement, endElement)
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</code></pre>
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<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>
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<pre><code>$ go build gopl.io/ch5/outline2
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$ ./outline2 http://gopl.io
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<html>
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<head>
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<meta>
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</meta>
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<title>
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</title>
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<style>
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</style>
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</head>
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<body>
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<table>
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<tbody>
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<tr>
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<td>
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<a>
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<img>
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</img>
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...
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</code></pre>
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<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(< a href='...'>)。使用简略格式输出没有孩子结点的元素(即用<code><img/></code>代替<code><img></img></code>)。编写测试,验证程序输出的格式正确。(详见11章)</p>
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<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>
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<pre><code class="language-Go">func ElementByID(doc *html.Node, id string) *html.Node
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|||
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</code></pre>
|
|||
|
<p><strong>练习 5.9:</strong> 编写函数expand,将s中的"foo"替换为f("foo")的返回值。</p>
|
|||
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<pre><code class="language-Go">func expand(s string, f func(string) string) string
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|||
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