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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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<h2 id="53-多返回值"><a class="header" href="#53-多返回值">5.3. 多返回值</a></h2>
<p>在Go中一个函数可以返回多个值。我们已经在之前例子中看到许多标准库中的函数返回2个值一个是期望得到的返回值另一个是函数出错时的错误信息。下面的例子会展示如何编写多返回值的函数。</p>
<p>下面的程序是findlinks的改进版本。修改后的findlinks可以自己发起HTTP请求这样我们就不必再运行fetch。因为HTTP请求和解析操作可能会失败因此findlinks声明了2个返回值链接列表和错误信息。一般而言HTML的解析器可以处理HTML页面的错误结点构造出HTML页面结构所以解析HTML很少失败。这意味着如果findlinks函数失败了很可能是由于I/O的错误导致的。</p>
<p><u><i>gopl.io/ch5/findlinks2</i></u></p>
<pre><code class="language-Go">func main() {
for _, url := range os.Args[1:] {
links, err := findLinks(url)
if err != nil {
fmt.Fprintf(os.Stderr, &quot;findlinks2: %v\n&quot;, err)
continue
}
for _, link := range links {
fmt.Println(link)
}
}
}
// findLinks performs an HTTP GET request for url, parses the
// response as HTML, and extracts and returns the links.
func findLinks(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)
}
return visit(nil, doc), nil
}
</code></pre>
<p>在findlinks中有4处return语句每一处return都返回了一组值。前三处return将http和html包中的错误信息传递给findlinks的调用者。第一处return直接返回错误信息其他两处通过fmt.Errorf§7.8输出详细的错误信息。如果findlinks成功结束最后的return语句将一组解析获得的连接返回给用户。</p>
<p>在findlinks中我们必须确保resp.Body被关闭释放网络资源。虽然Go的垃圾回收机制会回收不被使用的内存但是这不包括操作系统层面的资源比如打开的文件、网络连接。因此我们必须显式的释放这些资源。</p>
<p>调用多返回值函数时,返回给调用者的是一组值,调用者必须显式的将这些值分配给变量:</p>
<pre><code class="language-Go">links, err := findLinks(url)
</code></pre>
<p>如果某个值不被使用可以将其分配给blank identifier:</p>
<pre><code class="language-Go">links, _ := findLinks(url) // errors ignored
</code></pre>
<p>一个函数内部可以将另一个有多返回值的函数调用作为返回值下面的例子展示了与findLinks有相同功能的函数两者的区别在于下面的例子先输出参数</p>
<pre><code class="language-Go">func findLinksLog(url string) ([]string, error) {
log.Printf(&quot;findLinks %s&quot;, url)
return findLinks(url)
}
</code></pre>
<p>当你调用接受多参数的函数时可以将一个返回多参数的函数调用作为该函数的参数。虽然这很少出现在实际生产代码中但这个特性在debug时很方便我们只需要一条语句就可以输出所有的返回值。下面的代码是等价的</p>
<pre><code class="language-Go">log.Println(findLinks(url))
links, err := findLinks(url)
log.Println(links, err)
</code></pre>
<p>准确的变量名可以传达函数返回值的含义。尤其在返回值的类型都相同时,就像下面这样:</p>
<pre><code class="language-Go">func Size(rect image.Rectangle) (width, height int)
func Split(path string) (dir, file string)
func HourMinSec(t time.Time) (hour, minute, second int)
</code></pre>
<p>虽然良好的命名很重要但你也不必为每一个返回值都取一个适当的名字。比如按照惯例函数的最后一个bool类型的返回值表示函数是否运行成功error类型的返回值代表函数的错误信息对于这些类似的惯例我们不必思考合适的命名它们都无需解释。</p>
<p>如果一个函数所有的返回值都有显式的变量名那么该函数的return语句可以省略操作数。这称之为bare return。</p>
<pre><code class="language-Go">// CountWordsAndImages does an HTTP GET request for the HTML
// document url and returns the number of words and images in it.
func CountWordsAndImages(url string) (words, images int, err error) {
resp, err := http.Get(url)
if err != nil {
return
}
doc, err := html.Parse(resp.Body)
resp.Body.Close()
if err != nil {
err = fmt.Errorf(&quot;parsing HTML: %s&quot;, err)
return
}
words, images = countWordsAndImages(doc)
return
}
func countWordsAndImages(n *html.Node) (words, images int) { /* ... */ }
</code></pre>
<p>按照返回值列表的次序返回所有的返回值在上面的例子中每一个return语句等价于</p>
<pre><code class="language-Go">return words, images, err
</code></pre>
<p>当一个函数有多处return语句以及许多返回值时bare return 可以减少代码的重复但是使得代码难以被理解。举个例子如果你没有仔细的审查代码很难发现前2处return等价于 return 0,0,errGo会将返回值 words和images在函数体的开始处根据它们的类型将其初始化为0最后一处return等价于 return words, image, nil。基于以上原因不宜过度使用bare return。</p>
<p><strong>练习 5.5</strong> 实现countWordsAndImages。参考练习4.9如何分词)</p>
<p><strong>练习 5.6</strong> 修改gopl.io/ch3/surface§3.2中的corner函数将返回值命名并使用bare return。</p>
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