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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: <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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<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>
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<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>
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<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>
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<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>
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<pre><code>42 integer
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"hello" string（带有Go风格的引号）
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foo symbol（未用引号括起来的名字）
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(1 2 3) list （括号包起来的0个或多个元素）
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</code></pre>
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<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>
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<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>
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<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>
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<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>
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<p><u><i>gopl.io/ch12/sexpr</i></u></p>
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<pre><code class="language-Go">func encode(buf *bytes.Buffer, v reflect.Value) error {
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switch v.Kind() {
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case reflect.Invalid:
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buf.WriteString("nil")
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case reflect.Int, reflect.Int8, reflect.Int16,
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reflect.Int32, reflect.Int64:
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fmt.Fprintf(buf, "%d", v.Int())
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case reflect.Uint, reflect.Uint8, reflect.Uint16,
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reflect.Uint32, reflect.Uint64, reflect.Uintptr:
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fmt.Fprintf(buf, "%d", v.Uint())
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case reflect.String:
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fmt.Fprintf(buf, "%q", v.String())
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case reflect.Ptr:
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return encode(buf, v.Elem())
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case reflect.Array, reflect.Slice: // (value ...)
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buf.WriteByte('(')
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for i := 0; i < v.Len(); i++ {
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if i > 0 {
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buf.WriteByte(' ')
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}
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if err := encode(buf, v.Index(i)); err != nil {
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return err
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}
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}
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buf.WriteByte(')')
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case reflect.Struct: // ((name value) ...)
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buf.WriteByte('(')
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for i := 0; i < v.NumField(); i++ {
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if i > 0 {
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buf.WriteByte(' ')
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}
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fmt.Fprintf(buf, "(%s ", v.Type().Field(i).Name)
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if err := encode(buf, v.Field(i)); err != nil {
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return err
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}
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buf.WriteByte(')')
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}
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buf.WriteByte(')')
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case reflect.Map: // ((key value) ...)
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buf.WriteByte('(')
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for i, key := range v.MapKeys() {
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if i > 0 {
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buf.WriteByte(' ')
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}
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buf.WriteByte('(')
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if err := encode(buf, key); err != nil {
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return err
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}
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buf.WriteByte(' ')
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if err := encode(buf, v.MapIndex(key)); err != nil {
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return err
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}
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buf.WriteByte(')')
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}
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buf.WriteByte(')')
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|||
|
|
default: // float, complex, bool, chan, func, interface
|
|||
|
|
return fmt.Errorf("unsupported type: %s", 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(&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 "Dr. Strangelove") (Subtitle "How I Learned to Stop Worrying and Lo
|
|||
|
|
ve the Bomb") (Year 1964) (Actor (("Grp. Capt. Lionel Mandrake" "Peter Sell
|
|||
|
|
ers") ("Pres. Merkin Muffley" "Peter Sellers") ("Gen. Buck Turgidson" "Geor
|
|||
|
|
ge C. Scott") ("Brig. Gen. Jack D. Ripper" "Sterling Hayden") ("Maj. T.J. \
|
|||
|
|
"King\" Kong" "Slim Pickens") ("Dr. Strangelove" "Peter Sellers"))) (Oscars
|
|||
|
|
("Best Actor (Nomin.)" "Best Adapted Screenplay (Nomin.)" "Best Director (N
|
|||
|
|
omin.)" "Best Picture (Nomin.)")) (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 "Dr. Strangelove")
|
|||
|
|
(Subtitle "How I Learned to Stop Worrying and Love the Bomb")
|
|||
|
|
(Year 1964)
|
|||
|
|
(Actor (("Grp. Capt. Lionel Mandrake" "Peter Sellers")
|
|||
|
|
("Pres. Merkin Muffley" "Peter Sellers")
|
|||
|
|
("Gen. Buck Turgidson" "George C. Scott")
|
|||
|
|
("Brig. Gen. Jack D. Ripper" "Sterling Hayden")
|
|||
|
|
("Maj. T.J. \"King\" Kong" "Slim Pickens")
|
|||
|
|
("Dr. Strangelove" "Peter Sellers")))
|
|||
|
|
(Oscars ("Best Actor (Nomin.)"
|
|||
|
|
"Best Adapted Screenplay (Nomin.)"
|
|||
|
|
"Best Director (Nomin.)"
|
|||
|
|
"Best Picture (Nomin.)"))
|
|||
|
|
(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（"[]int" (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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