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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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<li><em>KusonStack一站式可编程配置技术栈(Go): <a href="https://github.com/KusionStack/kusion">https://github.com/KusionStack/kusion</a></em></li>
<li><em>KCL 配置编程语言(Rust): <a href="https://github.com/KusionStack/KCLVM">https://github.com/KusionStack/KCLVM</a></em></li>
<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>
<h2 id="56-匿名函数"><a class="header" href="#56-匿名函数">5.6. 匿名函数</a></h2>
<p>拥有函数名的函数只能在包级语法块中被声明通过函数字面量function literal我们可绕过这一限制在任何表达式中表示一个函数值。函数字面量的语法和函数声明相似区别在于func关键字后没有函数名。函数值字面量是一种表达式它的值被称为匿名函数anonymous function</p>
<p>函数字面量允许我们在使用函数时再定义它。通过这种技巧我们可以改写之前对strings.Map的调用</p>
<pre><code class="language-Go">strings.Map(func(r rune) rune { return r + 1 }, &quot;HAL-9000&quot;)
</code></pre>
<p>更为重要的是通过这种方式定义的函数可以访问完整的词法环境lexical environment这意味着在函数中定义的内部函数可以引用该函数的变量如下例所示</p>
<p><u><i>gopl.io/ch5/squares</i></u></p>
<pre><code class="language-Go">// squares返回一个匿名函数。
// 该匿名函数每次被调用时都会返回下一个数的平方。
func squares() func() int {
var x int
return func() int {
x++
return x * x
}
}
func main() {
f := squares()
fmt.Println(f()) // &quot;1&quot;
fmt.Println(f()) // &quot;4&quot;
fmt.Println(f()) // &quot;9&quot;
fmt.Println(f()) // &quot;16&quot;
}
</code></pre>
<p>函数squares返回另一个类型为 func() int 的函数。对squares的一次调用会生成一个局部变量x并返回一个匿名函数。每次调用匿名函数时该函数都会先使x的值加1再返回x的平方。第二次调用squares时会生成第二个x变量并返回一个新的匿名函数。新匿名函数操作的是第二个x变量。</p>
<p>squares的例子证明函数值不仅仅是一串代码还记录了状态。在squares中定义的匿名内部函数可以访问和更新squares中的局部变量这意味着匿名函数和squares中存在变量引用。这就是函数值属于引用类型和函数值不可比较的原因。Go使用闭包closures技术实现函数值Go程序员也把函数值叫做闭包。</p>
<p>通过这个例子我们看到变量的生命周期不由它的作用域决定squares返回后变量x仍然隐式的存在于f中。</p>
<p>接下来,我们讨论一个有点学术性的例子,考虑这样一个问题:给定一些计算机课程,每个课程都有前置课程,只有完成了前置课程才可以开始当前课程的学习;我们的目标是选择出一组课程,这组课程必须确保按顺序学习时,能全部被完成。每个课程的前置课程如下:</p>
<p><u><i>gopl.io/ch5/toposort</i></u></p>
<pre><code class="language-Go">// prereqs记录了每个课程的前置课程
var prereqs = map[string][]string{
&quot;algorithms&quot;: {&quot;data structures&quot;},
&quot;calculus&quot;: {&quot;linear algebra&quot;},
&quot;compilers&quot;: {
&quot;data structures&quot;,
&quot;formal languages&quot;,
&quot;computer organization&quot;,
},
&quot;data structures&quot;: {&quot;discrete math&quot;},
&quot;databases&quot;: {&quot;data structures&quot;},
&quot;discrete math&quot;: {&quot;intro to programming&quot;},
&quot;formal languages&quot;: {&quot;discrete math&quot;},
&quot;networks&quot;: {&quot;operating systems&quot;},
&quot;operating systems&quot;: {&quot;data structures&quot;, &quot;computer organization&quot;},
&quot;programming languages&quot;: {&quot;data structures&quot;, &quot;computer organization&quot;},
}
</code></pre>
<p>这类问题被称作拓扑排序。从概念上说,前置条件可以构成有向图。图中的顶点表示课程,边表示课程间的依赖关系。显然,图中应该无环,这也就是说从某点出发的边,最终不会回到该点。下面的代码用深度优先搜索了整张图,获得了符合要求的课程序列。</p>
<pre><code class="language-Go">func main() {
for i, course := range topoSort(prereqs) {
fmt.Printf(&quot;%d:\t%s\n&quot;, i+1, course)
}
}
func topoSort(m map[string][]string) []string {
var order []string
seen := make(map[string]bool)
var visitAll func(items []string)
visitAll = func(items []string) {
for _, item := range items {
if !seen[item] {
seen[item] = true
visitAll(m[item])
order = append(order, item)
}
}
}
var keys []string
for key := range m {
keys = append(keys, key)
}
sort.Strings(keys)
visitAll(keys)
return order
}
</code></pre>
<p>当匿名函数需要被递归调用时,我们必须首先声明一个变量(在上面的例子中,我们首先声明了 visitAll再将匿名函数赋值给这个变量。如果不分成两步函数字面量无法与visitAll绑定我们也无法递归调用该匿名函数。</p>
<pre><code class="language-Go">visitAll := func(items []string) {
// ...
visitAll(m[item]) // compile error: undefined: visitAll
// ...
}
</code></pre>
<p>在toposort程序的输出如下所示它的输出顺序是大多人想看到的固定顺序输出但是这需要我们多花点心思才能做到。哈希表prepreqs的value是遍历顺序固定的切片而不再试遍历顺序随机的map所以我们对prereqs的key值进行排序保证每次运行toposort程序都以相同的遍历顺序遍历prereqs。</p>
<pre><code>1: intro to programming
2: discrete math
3: data structures
4: algorithms
5: linear algebra
6: calculus
7: formal languages
8: computer organization
9: compilers
10: databases
11: operating systems
12: networks
13: programming languages
</code></pre>
<p>让我们回到findLinks这个例子。我们将代码移动到了links包下将函数重命名为Extract在第八章我们会再次用到这个函数。新的匿名函数被引入用于替换原来的visit函数。该匿名函数负责将新连接添加到切片中。在Extract中使用forEachNode遍历HTML页面由于Extract只需要在遍历结点前操作结点所以forEachNode的post参数被传入nil。</p>
<p><u><i>gopl.io/ch5/links</i></u></p>
<pre><code class="language-Go">// Package links provides a link-extraction function.
package links
import (
&quot;fmt&quot;
&quot;net/http&quot;
&quot;golang.org/x/net/html&quot;
)
// Extract makes an HTTP GET request to the specified URL, parses
// the response as HTML, and returns the links in the HTML document.
func Extract(url string) ([]string, error) {
resp, err := http.Get(url)
if err != nil {
return nil, err
}
if resp.StatusCode != http.StatusOK {
resp.Body.Close()
return nil, fmt.Errorf(&quot;getting %s: %s&quot;, url, resp.Status)
}
doc, err := html.Parse(resp.Body)
resp.Body.Close()
if err != nil {
return nil, fmt.Errorf(&quot;parsing %s as HTML: %v&quot;, url, err)
}
var links []string
visitNode := func(n *html.Node) {
if n.Type == html.ElementNode &amp;&amp; n.Data == &quot;a&quot; {
for _, a := range n.Attr {
if a.Key != &quot;href&quot; {
continue
}
link, err := resp.Request.URL.Parse(a.Val)
if err != nil {
continue // ignore bad URLs
}
links = append(links, link.String())
}
}
}
forEachNode(doc, visitNode, nil)
return links, nil
}
</code></pre>
<p>上面的代码对之前的版本做了改进现在links中存储的不是href属性的原始值而是通过resp.Request.URL解析后的值。解析后这些连接以绝对路径的形式存在可以直接被http.Get访问。</p>
<p>网页抓取的核心问题就是如何遍历图。在topoSort的例子中已经展示了深度优先遍历在网页抓取中我们会展示如何用广度优先遍历图。在第8章我们会介绍如何将深度优先和广度优先结合使用。</p>
<p>下面的函数实现了广度优先算法。调用者需要输入一个初始的待访问列表和一个函数f。待访问列表中的每个元素被定义为string类型。广度优先算法会为每个元素调用一次f。每次f执行完毕后会返回一组待访问元素。这些元素会被加入到待访问列表中。当待访问列表中的所有元素都被访问后breadthFirst函数运行结束。为了避免同一个元素被访问两次代码中维护了一个map。</p>
<p><u><i>gopl.io/ch5/findlinks3</i></u></p>
<pre><code class="language-Go">// breadthFirst calls f for each item in the worklist.
// Any items returned by f are added to the worklist.
// f is called at most once for each item.
func breadthFirst(f func(item string) []string, worklist []string) {
seen := make(map[string]bool)
for len(worklist) &gt; 0 {
items := worklist
worklist = nil
for _, item := range items {
if !seen[item] {
seen[item] = true
worklist = append(worklist, f(item)...)
}
}
}
}
</code></pre>
<p>就像我们在章节3解释的那样append的参数“f(item)...”会将f返回的一组元素一个个添加到worklist中。</p>
<p>在我们网页抓取器中元素的类型是url。crawl函数会将URL输出提取其中的新链接并将这些新链接返回。我们会将crawl作为参数传递给breadthFirst。</p>
<pre><code class="language-go">func crawl(url string) []string {
fmt.Println(url)
list, err := links.Extract(url)
if err != nil {
log.Print(err)
}
return list
}
</code></pre>
<p>为了使抓取器开始运行我们用命令行输入的参数作为初始的待访问url。</p>
<pre><code class="language-Go">func main() {
// Crawl the web breadth-first,
// starting from the command-line arguments.
breadthFirst(crawl, os.Args[1:])
}
</code></pre>
<p>让我们从 https://golang.org 开始,下面是程序的输出结果:</p>
<pre><code>$ go build gopl.io/ch5/findlinks3
$ ./findlinks3 https://golang.org
https://golang.org/
https://golang.org/doc/
https://golang.org/pkg/
https://golang.org/project/
https://code.google.com/p/go-tour/
https://golang.org/doc/code.html
https://www.youtube.com/watch?v=XCsL89YtqCs
http://research.swtch.com/gotour
</code></pre>
<p>当所有发现的链接都已经被访问或电脑的内存耗尽时,程序运行结束。</p>
<p><strong>练习5.10</strong> 重写topoSort函数用map代替切片并移除对key的排序代码。验证结果的正确性结果不唯一</p>
<p><strong>练习5.11</strong> 现在线性代数的老师把微积分设为了前置课程。完善topSort使其能检测有向图中的环。</p>
<p><strong>练习5.12</strong> gopl.io/ch5/outline25.5节的startElement和endElement共用了全局变量depth将它们修改为匿名函数使其共享outline中的局部变量。</p>
<p><strong>练习5.13</strong> 修改crawl使其能保存发现的页面必要时可以创建目录来保存这些页面。只保存来自原始域名下的页面。假设初始页面在golang.org下就不要保存vimeo.com下的页面。</p>
<p><strong>练习5.14</strong> 使用breadthFirst遍历其他数据结构。比如topoSort例子中的课程依赖关系有向图、个人计算机的文件层次结构你所在城市的公交或地铁线路无向图</p>
<h3 id="561-警告捕获迭代变量"><a class="header" href="#561-警告捕获迭代变量">5.6.1. 警告:捕获迭代变量</a></h3>
<p>本节将介绍Go词法作用域的一个陷阱。请务必仔细的阅读弄清楚发生问题的原因。即使是经验丰富的程序员也会在这个问题上犯错误。</p>
<p>考虑这样一个问题你被要求首先创建一些目录再将目录删除。在下面的例子中我们用函数值来完成删除操作。下面的示例代码需要引入os包。为了使代码简单我们忽略了所有的异常处理。</p>
<pre><code class="language-Go">var rmdirs []func()
for _, d := range tempDirs() {
dir := d // NOTE: necessary!
os.MkdirAll(dir, 0755) // creates parent directories too
rmdirs = append(rmdirs, func() {
os.RemoveAll(dir)
})
}
// ...do some work…
for _, rmdir := range rmdirs {
rmdir() // clean up
}
</code></pre>
<p>你可能会感到困惑为什么要在循环体中用循环变量d赋值一个新的局部变量而不是像下面的代码一样直接使用循环变量dir。需要注意下面的代码是错误的。</p>
<pre><code class="language-go">var rmdirs []func()
for _, dir := range tempDirs() {
os.MkdirAll(dir, 0755)
rmdirs = append(rmdirs, func() {
os.RemoveAll(dir) // NOTE: incorrect!
})
}
</code></pre>
<p>问题的原因在于循环变量的作用域。在上面的程序中for循环语句引入了新的词法块循环变量dir在这个词法块中被声明。在该循环中生成的所有函数值都共享相同的循环变量。需要注意函数值中记录的是循环变量的内存地址而不是循环变量某一时刻的值。以dir为例后续的迭代会不断更新dir的值当删除操作执行时for循环已完成dir中存储的值等于最后一次迭代的值。这意味着每次对os.RemoveAll的调用删除的都是相同的目录。</p>
<p>通常为了解决这个问题我们会引入一个与循环变量同名的局部变量作为循环变量的副本。比如下面的变量dir虽然这看起来很奇怪但却很有用。</p>
<pre><code class="language-Go">for _, dir := range tempDirs() {
dir := dir // declares inner dir, initialized to outer dir
// ...
}
</code></pre>
<p>这个问题不仅存在基于range的循环在下面的例子中对循环变量i的使用也存在同样的问题</p>
<pre><code class="language-Go">var rmdirs []func()
dirs := tempDirs()
for i := 0; i &lt; len(dirs); i++ {
os.MkdirAll(dirs[i], 0755) // OK
rmdirs = append(rmdirs, func() {
os.RemoveAll(dirs[i]) // NOTE: incorrect!
})
}
</code></pre>
<p>如果你使用go语句第八章或者defer语句5.8节会经常遇到此类问题。这不是go或defer本身导致的而是因为它们都会等待循环结束后再执行函数值。</p>
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