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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="126-示例-解码s表达式"><a class="header" href="#126-示例-解码s表达式">12.6. 示例: 解码S表达式</a></h2>
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<p>标准库中encoding/...下每个包中提供的Marshal编码函数都有一个对应的Unmarshal函数用于解码。例如，我们在4.5节中看到的，要将包含JSON编码格式的字节slice数据解码为我们自己的Movie类型（§12.3），我们可以这样做：</p>
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<pre><code class="language-Go">data := []byte{/* ... */}
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var movie Movie
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err := json.Unmarshal(data, &movie)
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</code></pre>
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<p>Unmarshal函数使用了反射机制类修改movie变量的每个成员，根据输入的内容为Movie成员创建对应的map、结构体和slice。</p>
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<p>现在让我们为S表达式编码实现一个简易的Unmarshal，类似于前面的json.Unmarshal标准库函数，对应我们之前实现的sexpr.Marshal函数的逆操作。我们必须提醒一下，一个健壮的和通用的实现通常需要比例子更多的代码，为了便于演示我们采用了精简的实现。我们只支持S表达式有限的子集，同时处理错误的方式也比较粗暴，代码的目的是为了演示反射的用法，而不是构造一个实用的S表达式的解码器。</p>
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<p>词法分析器lexer使用了标准库中的text/scanner包将输入流的字节数据解析为一个个类似注释、标识符、字符串面值和数字面值之类的标记。输入扫描器scanner的Scan方法将提前扫描和返回下一个记号，对于rune类型。大多数记号，比如“(”，对应一个单一rune可表示的Unicode字符，但是text/scanner也可以用小的负数表示记号标识符、字符串等由多个字符组成的记号。调用Scan方法将返回这些记号的类型，接着调用TokenText方法将返回记号对应的文本内容。</p>
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<p>因为每个解析器可能需要多次使用当前的记号，但是Scan会一直向前扫描，所以我们包装了一个lexer扫描器辅助类型，用于跟踪最近由Scan方法返回的记号。</p>
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<p><u><i>gopl.io/ch12/sexpr</i></u></p>
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<pre><code class="language-Go">type lexer struct {
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scan scanner.Scanner
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token rune // the current token
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}
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func (lex *lexer) next() { lex.token = lex.scan.Scan() }
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func (lex *lexer) text() string { return lex.scan.TokenText() }
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func (lex *lexer) consume(want rune) {
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if lex.token != want { // NOTE: Not an example of good error handling.
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panic(fmt.Sprintf("got %q, want %q", lex.text(), want))
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}
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lex.next()
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}
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</code></pre>
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<p>现在让我们转到语法解析器。它主要包含两个功能。第一个是read函数，用于读取S表达式的当前标记，然后根据S表达式的当前标记更新可取地址的reflect.Value对应的变量v。</p>
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<pre><code class="language-Go">func read(lex *lexer, v reflect.Value) {
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switch lex.token {
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case scanner.Ident:
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// The only valid identifiers are
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// "nil" and struct field names.
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if lex.text() == "nil" {
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v.Set(reflect.Zero(v.Type()))
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lex.next()
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return
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}
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case scanner.String:
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s, _ := strconv.Unquote(lex.text()) // NOTE: ignoring errors
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v.SetString(s)
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lex.next()
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return
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case scanner.Int:
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i, _ := strconv.Atoi(lex.text()) // NOTE: ignoring errors
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v.SetInt(int64(i))
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lex.next()
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return
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case '(':
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lex.next()
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readList(lex, v)
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lex.next() // consume ')'
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return
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}
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panic(fmt.Sprintf("unexpected token %q", lex.text()))
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}
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</code></pre>
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<p>我们的S表达式使用标识符区分两个不同类型，结构体成员名和nil值的指针。read函数值处理nil类型的标识符。当遇到scanner.Ident为“nil”时，使用reflect.Zero函数将变量v设置为零值。而其它任何类型的标识符，我们都作为错误处理。后面的readList函数将处理结构体的成员名。</p>
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<p>一个“(”标记对应一个列表的开始。第二个函数readList，将一个列表解码到一个聚合类型中（map、结构体、slice或数组），具体类型依赖于传入待填充变量的类型。每次遇到这种情况，循环继续解析每个元素直到遇到于开始标记匹配的结束标记“)”，endList函数用于检测结束标记。</p>
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<p>最有趣的部分是递归。最简单的是对数组类型的处理。直到遇到“)”结束标记，我们使用Index函数来获取数组每个元素的地址，然后递归调用read函数处理。和其它错误类似，如果输入数据导致解码器的引用超出了数组的范围，解码器将抛出panic异常。slice也采用类似方法解析，不同的是我们将为每个元素创建新的变量，然后将元素添加到slice的末尾。</p>
|
|||
|
|
<p>在循环处理结构体和map每个元素时必须解码一个(key value)格式的对应子列表。对于结构体，key部分对于成员的名字。和数组类似，我们使用FieldByName找到结构体对应成员的变量，然后递归调用read函数处理。对于map，key可能是任意类型，对元素的处理方式和slice类似，我们创建一个新的变量，然后递归填充它，最后将新解析到的key/value对添加到map。</p>
|
|||
|
|
<pre><code class="language-Go">func readList(lex *lexer, v reflect.Value) {
|
|||
|
|
switch v.Kind() {
|
|||
|
|
case reflect.Array: // (item ...)
|
|||
|
|
for i := 0; !endList(lex); i++ {
|
|||
|
|
read(lex, v.Index(i))
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
case reflect.Slice: // (item ...)
|
|||
|
|
for !endList(lex) {
|
|||
|
|
item := reflect.New(v.Type().Elem()).Elem()
|
|||
|
|
read(lex, item)
|
|||
|
|
v.Set(reflect.Append(v, item))
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
case reflect.Struct: // ((name value) ...)
|
|||
|
|
for !endList(lex) {
|
|||
|
|
lex.consume('(')
|
|||
|
|
if lex.token != scanner.Ident {
|
|||
|
|
panic(fmt.Sprintf("got token %q, want field name", lex.text()))
|
|||
|
|
}
|
|||
|
|
name := lex.text()
|
|||
|
|
lex.next()
|
|||
|
|
read(lex, v.FieldByName(name))
|
|||
|
|
lex.consume(')')
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
case reflect.Map: // ((key value) ...)
|
|||
|
|
v.Set(reflect.MakeMap(v.Type()))
|
|||
|
|
for !endList(lex) {
|
|||
|
|
lex.consume('(')
|
|||
|
|
key := reflect.New(v.Type().Key()).Elem()
|
|||
|
|
read(lex, key)
|
|||
|
|
value := reflect.New(v.Type().Elem()).Elem()
|
|||
|
|
read(lex, value)
|
|||
|
|
v.SetMapIndex(key, value)
|
|||
|
|
lex.consume(')')
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
default:
|
|||
|
|
panic(fmt.Sprintf("cannot decode list into %v", v.Type()))
|
|||
|
|
}
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
func endList(lex *lexer) bool {
|
|||
|
|
switch lex.token {
|
|||
|
|
case scanner.EOF:
|
|||
|
|
panic("end of file")
|
|||
|
|
case ')':
|
|||
|
|
return true
|
|||
|
|
}
|
|||
|
|
return false
|
|||
|
|
}
|
|||
|
|
</code></pre>
|
|||
|
|
<p>最后，我们将解析器包装为导出的Unmarshal解码函数，隐藏了一些初始化和清理等边缘处理。内部解析器以panic的方式抛出错误，但是Unmarshal函数通过在defer语句调用recover函数来捕获内部panic（§5.10），然后返回一个对panic对应的错误信息。</p>
|
|||
|
|
<pre><code class="language-Go">// Unmarshal parses S-expression data and populates the variable
|
|||
|
|
// whose address is in the non-nil pointer out.
|
|||
|
|
func Unmarshal(data []byte, out interface{}) (err error) {
|
|||
|
|
lex := &lexer{scan: scanner.Scanner{Mode: scanner.GoTokens}}
|
|||
|
|
lex.scan.Init(bytes.NewReader(data))
|
|||
|
|
lex.next() // get the first token
|
|||
|
|
defer func() {
|
|||
|
|
// NOTE: this is not an example of ideal error handling.
|
|||
|
|
if x := recover(); x != nil {
|
|||
|
|
err = fmt.Errorf("error at %s: %v", lex.scan.Position, x)
|
|||
|
|
}
|
|||
|
|
}()
|
|||
|
|
read(lex, reflect.ValueOf(out).Elem())
|
|||
|
|
return nil
|
|||
|
|
}
|
|||
|
|
</code></pre>
|
|||
|
|
<p>生产实现不应该对任何输入问题都用panic形式报告，而且应该报告一些错误相关的信息，例如出现错误输入的行号和位置等。尽管如此，我们希望通过这个例子来展示类似encoding/json等包底层代码的实现思路，以及如何使用反射机制来填充数据结构。</p>
|
|||
|
|
<p><strong>练习 12.8：</strong> sexpr.Unmarshal函数和json.Unmarshal一样，都要求在解码前输入完整的字节slice。定义一个和json.Decoder类似的sexpr.Decoder类型，支持从一个io.Reader流解码。修改sexpr.Unmarshal函数，使用这个新的类型实现。</p>
|
|||
|
|
<p><strong>练习 12.9：</strong> 编写一个基于标记的API用于解码S表达式，参考xml.Decoder（7.14）的风格。你将需要五种类型的标记：Symbol、String、Int、StartList和EndList。</p>
|
|||
|
|
<p><strong>练习 12.10：</strong> 扩展sexpr.Unmarshal函数，支持布尔型、浮点数和interface类型的解码，使用 <strong>练习 12.3：</strong> 的方案。（提示：要解码接口，你需要将name映射到每个支持类型的reflect.Type。）</p>
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