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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="124-示例-编码为s表达式"><a class="header" href="#124-示例-编码为s表达式">12.4. 示例: 编码为S表达式</a></h2>
<p>Display是一个用于显示结构化数据的调试工具但是它并不能将任意的Go语言对象编码为通用消息然后用于进程间通信。</p>
<p>正如我们在4.5节中中看到的Go语言的标准库支持了包括JSON、XML和ASN.1等多种编码格式。还有另一种依然被广泛使用的格式是S表达式格式采用Lisp语言的语法。但是和其他编码格式不同的是Go语言自带的标准库并不支持S表达式主要是因为它没有一个公认的标准规范。</p>
<p>在本节中我们将定义一个包用于将任意的Go语言对象编码为S表达式格式它支持以下结构</p>
<pre><code>42 integer
&quot;hello&quot; string带有Go风格的引号
foo symbol未用引号括起来的名字
(1 2 3) list 括号包起来的0个或多个元素
</code></pre>
<p>布尔型习惯上使用t符号表示true空列表或nil符号表示false但是为了简单起见我们暂时忽略布尔类型。同时忽略的还有chan管道和函数因为通过反射并无法知道它们的确切状态。我们忽略的还有浮点数、复数和interface。支持它们是练习12.3的任务。</p>
<p>我们将Go语言的类型编码为S表达式的方法如下。整数和字符串以显而易见的方式编码。空值编码为nil符号。数组和slice被编码为列表。</p>
<p>结构体被编码为成员对象的列表每个成员对象对应一个有两个元素的子列表子列表的第一个元素是成员的名字第二个元素是成员的值。Map被编码为键值对的列表。传统上S表达式使用点状符号列表(key . value)结构来表示key/value对而不是用一个含双元素的列表不过为了简单我们忽略了点状符号列表。</p>
<p>编码是由一个encode递归函数完成如下所示。它的结构本质上和前面的Display函数类似</p>
<p><u><i>gopl.io/ch12/sexpr</i></u></p>
<pre><code class="language-Go">func encode(buf *bytes.Buffer, v reflect.Value) error {
switch v.Kind() {
case reflect.Invalid:
buf.WriteString(&quot;nil&quot;)
case reflect.Int, reflect.Int8, reflect.Int16,
reflect.Int32, reflect.Int64:
fmt.Fprintf(buf, &quot;%d&quot;, v.Int())
case reflect.Uint, reflect.Uint8, reflect.Uint16,
reflect.Uint32, reflect.Uint64, reflect.Uintptr:
fmt.Fprintf(buf, &quot;%d&quot;, v.Uint())
case reflect.String:
fmt.Fprintf(buf, &quot;%q&quot;, v.String())
case reflect.Ptr:
return encode(buf, v.Elem())
case reflect.Array, reflect.Slice: // (value ...)
buf.WriteByte('(')
for i := 0; i &lt; v.Len(); i++ {
if i &gt; 0 {
buf.WriteByte(' ')
}
if err := encode(buf, v.Index(i)); err != nil {
return err
}
}
buf.WriteByte(')')
case reflect.Struct: // ((name value) ...)
buf.WriteByte('(')
for i := 0; i &lt; v.NumField(); i++ {
if i &gt; 0 {
buf.WriteByte(' ')
}
fmt.Fprintf(buf, &quot;(%s &quot;, v.Type().Field(i).Name)
if err := encode(buf, v.Field(i)); err != nil {
return err
}
buf.WriteByte(')')
}
buf.WriteByte(')')
case reflect.Map: // ((key value) ...)
buf.WriteByte('(')
for i, key := range v.MapKeys() {
if i &gt; 0 {
buf.WriteByte(' ')
}
buf.WriteByte('(')
if err := encode(buf, key); err != nil {
return err
}
buf.WriteByte(' ')
if err := encode(buf, v.MapIndex(key)); err != nil {
return err
}
buf.WriteByte(')')
}
buf.WriteByte(')')
default: // float, complex, bool, chan, func, interface
return fmt.Errorf(&quot;unsupported type: %s&quot;, v.Type())
}
return nil
}
</code></pre>
<p>Marshal函数是对encode的包装以保持和encoding/...下其它包有着相似的API</p>
<pre><code class="language-Go">// Marshal encodes a Go value in S-expression form.
func Marshal(v interface{}) ([]byte, error) {
var buf bytes.Buffer
if err := encode(&amp;buf, reflect.ValueOf(v)); err != nil {
return nil, err
}
return buf.Bytes(), nil
}
</code></pre>
<p>下面是Marshal对12.3节的strangelove变量编码后的结果</p>
<pre><code>((Title &quot;Dr. Strangelove&quot;) (Subtitle &quot;How I Learned to Stop Worrying and Lo
ve the Bomb&quot;) (Year 1964) (Actor ((&quot;Grp. Capt. Lionel Mandrake&quot; &quot;Peter Sell
ers&quot;) (&quot;Pres. Merkin Muffley&quot; &quot;Peter Sellers&quot;) (&quot;Gen. Buck Turgidson&quot; &quot;Geor
ge C. Scott&quot;) (&quot;Brig. Gen. Jack D. Ripper&quot; &quot;Sterling Hayden&quot;) (&quot;Maj. T.J. \
&quot;King\&quot; Kong&quot; &quot;Slim Pickens&quot;) (&quot;Dr. Strangelove&quot; &quot;Peter Sellers&quot;))) (Oscars
(&quot;Best Actor (Nomin.)&quot; &quot;Best Adapted Screenplay (Nomin.)&quot; &quot;Best Director (N
omin.)&quot; &quot;Best Picture (Nomin.)&quot;)) (Sequel nil))
</code></pre>
<p>整个输出编码为一行中以减少输出的大小但是也很难阅读。下面是对S表达式手动格式化的结果。编写一个S表达式的美化格式化函数将作为一个具有挑战性的练习任务不过 http://gopl.io 也提供了一个简单的版本。</p>
<pre><code>((Title &quot;Dr. Strangelove&quot;)
(Subtitle &quot;How I Learned to Stop Worrying and Love the Bomb&quot;)
(Year 1964)
(Actor ((&quot;Grp. Capt. Lionel Mandrake&quot; &quot;Peter Sellers&quot;)
(&quot;Pres. Merkin Muffley&quot; &quot;Peter Sellers&quot;)
(&quot;Gen. Buck Turgidson&quot; &quot;George C. Scott&quot;)
(&quot;Brig. Gen. Jack D. Ripper&quot; &quot;Sterling Hayden&quot;)
(&quot;Maj. T.J. \&quot;King\&quot; Kong&quot; &quot;Slim Pickens&quot;)
(&quot;Dr. Strangelove&quot; &quot;Peter Sellers&quot;)))
(Oscars (&quot;Best Actor (Nomin.)&quot;
&quot;Best Adapted Screenplay (Nomin.)&quot;
&quot;Best Director (Nomin.)&quot;
&quot;Best Picture (Nomin.)&quot;))
(Sequel nil))
</code></pre>
<p>和fmt.Print、json.Marshal、Display函数类似sexpr.Marshal函数处理带环的数据结构也会陷入死循环。</p>
<p>在12.6节中我们将给出S表达式解码器的实现步骤但是在那之前我们还需要先了解如何通过反射技术来更新程序的变量。</p>
<p><strong>练习 12.3</strong> 实现encode函数缺少的分支。将布尔类型编码为t和nil浮点数编码为Go语言的格式复数1+2i编码为#C(1.0 2.0)格式。接口编码为类型名和值对,例如(&quot;[]int&quot; (1 2 3)但是这个形式可能会造成歧义reflect.Type.String方法对于不同的类型可能返回相同的结果。</p>
<p><strong>练习 12.4</strong> 修改encode函数以上面的格式化形式输出S表达式。</p>
<p><strong>练习 12.5</strong> 修改encode函数用JSON格式代替S表达式格式。然后使用标准库提供的json.Unmarshal解码器来验证函数是正确的。</p>
<p><strong>练习 12.6</strong> 修改encode作为一个优化忽略对是零值对象的编码。</p>
<p><strong>练习 12.7</strong> 创建一个基于流式的API用于S表达式的解码和json.Decoder(§4.5)函数功能类似。</p>
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