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< ol class = "chapter" >
< li class = "chapter-item expanded " >
< a href = "../index.html" > Go语言圣经< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../preface-zh.html" > 译者序< / a >
< / li >
< li class = "chapter-item expanded " >
< 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 >
< 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 class = "chapter-item expanded " >
< a href = "../ch2/ch2.html" > < strong aria-hidden = "true" > 2.< / strong > 程序结构< / a >
< / 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 class = "chapter-item expanded " >
< a href = "../ch3/ch3.html" > < strong aria-hidden = "true" > 3.< / strong > 基础数据类型< / a >
< / 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 class = "chapter-item expanded " >
< a href = "../ch4/ch4.html" > < strong aria-hidden = "true" > 4.< / strong > 复合数据类型< / a >
< / 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-hidden = "true" > 4.6.< / strong > 文本和HTML模板< / a >
< / li >
< / ol >
< li class = "chapter-item expanded " >
< a href = "../ch5/ch5.html" > < strong aria-hidden = "true" > 5.< / strong > 函数< / a >
< / li >
< ol class = "section" >
< li class = "chapter-item expanded " >
< a href = "../ch5/ch5-01.html" > < strong aria-hidden = "true" > 5.1.< / strong > 函数声明< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch5/ch5-02.html" > < strong aria-hidden = "true" > 5.2.< / strong > 递归< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch5/ch5-03.html" > < strong aria-hidden = "true" > 5.3.< / strong > 多返回值< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch5/ch5-04.html" > < strong aria-hidden = "true" > 5.4.< / strong > 错误< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch5/ch5-05.html" > < strong aria-hidden = "true" > 5.5.< / strong > 函数值< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch5/ch5-06.html" class = "active" > < strong aria-hidden = "true" > 5.6.< / strong > 匿名函数< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch5/ch5-07.html" > < strong aria-hidden = "true" > 5.7.< / strong > 可变参数< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch5/ch5-08.html" > < strong aria-hidden = "true" > 5.8.< / strong > Deferred函数< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch5/ch5-09.html" > < strong aria-hidden = "true" > 5.9.< / strong > Panic异常< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch5/ch5-10.html" > < strong aria-hidden = "true" > 5.10.< / strong > Recover捕获异常< / a >
< / li >
< / ol >
< li class = "chapter-item expanded " >
< a href = "../ch6/ch6.html" > < strong aria-hidden = "true" > 6.< / strong > 方法< / a >
< / li >
< ol class = "section" >
< li class = "chapter-item expanded " >
< a href = "../ch6/ch6-01.html" > < strong aria-hidden = "true" > 6.1.< / strong > 方法声明< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch6/ch6-02.html" > < strong aria-hidden = "true" > 6.2.< / strong > 基于指针对象的方法< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch6/ch6-03.html" > < strong aria-hidden = "true" > 6.3.< / strong > 通过嵌入结构体来扩展类型< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch6/ch6-04.html" > < strong aria-hidden = "true" > 6.4.< / strong > 方法值和方法表达式< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch6/ch6-05.html" > < strong aria-hidden = "true" > 6.5.< / strong > 示例: Bit数组< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch6/ch6-06.html" > < strong aria-hidden = "true" > 6.6.< / strong > 封装< / a >
< / li >
< / ol >
< li class = "chapter-item expanded " >
< a href = "../ch7/ch7.html" > < strong aria-hidden = "true" > 7.< / strong > 接口< / a >
< / li >
< ol class = "section" >
< li class = "chapter-item expanded " >
< a href = "../ch7/ch7-01.html" > < strong aria-hidden = "true" > 7.1.< / strong > 接口是合约< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch7/ch7-02.html" > < strong aria-hidden = "true" > 7.2.< / strong > 接口类型< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch7/ch7-03.html" > < strong aria-hidden = "true" > 7.3.< / strong > 实现接口的条件< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch7/ch7-04.html" > < strong aria-hidden = "true" > 7.4.< / strong > flag.Value接口< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch7/ch7-05.html" > < strong aria-hidden = "true" > 7.5.< / strong > 接口值< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch7/ch7-06.html" > < strong aria-hidden = "true" > 7.6.< / strong > sort.Interface接口< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch7/ch7-07.html" > < strong aria-hidden = "true" > 7.7.< / strong > http.Handler接口< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch7/ch7-08.html" > < strong aria-hidden = "true" > 7.8.< / strong > error接口< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch7/ch7-09.html" > < strong aria-hidden = "true" > 7.9.< / strong > 示例: 表达式求值< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch7/ch7-10.html" > < strong aria-hidden = "true" > 7.10.< / strong > 类型断言< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch7/ch7-11.html" > < strong aria-hidden = "true" > 7.11.< / strong > 基于类型断言识别错误类型< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch7/ch7-12.html" > < strong aria-hidden = "true" > 7.12.< / strong > 通过类型断言查询接口< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch7/ch7-13.html" > < strong aria-hidden = "true" > 7.13.< / strong > 类型分支< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch7/ch7-14.html" > < strong aria-hidden = "true" > 7.14.< / strong > 示例: 基于标记的XML解码< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch7/ch7-15.html" > < strong aria-hidden = "true" > 7.15.< / strong > 补充几点< / a >
< / li >
< / ol >
< li class = "chapter-item expanded " >
< a href = "../ch8/ch8.html" > < strong aria-hidden = "true" > 8.< / strong > Goroutines和Channels< / a >
< / li >
< ol class = "section" >
< li class = "chapter-item expanded " >
< a href = "../ch8/ch8-01.html" > < strong aria-hidden = "true" > 8.1.< / strong > Goroutines< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch8/ch8-02.html" > < strong aria-hidden = "true" > 8.2.< / strong > 示例: 并发的Clock服务< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch8/ch8-03.html" > < strong aria-hidden = "true" > 8.3.< / strong > 示例: 并发的Echo服务< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch8/ch8-04.html" > < strong aria-hidden = "true" > 8.4.< / strong > Channels< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch8/ch8-05.html" > < strong aria-hidden = "true" > 8.5.< / strong > 并发的循环< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch8/ch8-06.html" > < strong aria-hidden = "true" > 8.6.< / strong > 示例: 并发的Web爬虫< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch8/ch8-07.html" > < strong aria-hidden = "true" > 8.7.< / strong > 基于select的多路复用< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch8/ch8-08.html" > < strong aria-hidden = "true" > 8.8.< / strong > 示例: 并发的目录遍历< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch8/ch8-09.html" > < strong aria-hidden = "true" > 8.9.< / strong > 并发的退出< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch8/ch8-10.html" > < strong aria-hidden = "true" > 8.10.< / strong > 示例: 聊天服务< / a >
< / li >
< / ol >
< li class = "chapter-item expanded " >
< a href = "../ch9/ch9.html" > < strong aria-hidden = "true" > 9.< / strong > 基于共享变量的并发< / a >
< / li >
< ol class = "section" >
< li class = "chapter-item expanded " >
< a href = "../ch9/ch9-01.html" > < strong aria-hidden = "true" > 9.1.< / strong > 竞争条件< / a >
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< li class = "chapter-item expanded " >
< a href = "../ch9/ch9-02.html" > < strong aria-hidden = "true" > 9.2.< / strong > sync.Mutex互斥锁< / a >
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< li class = "chapter-item expanded " >
< a href = "../ch9/ch9-03.html" > < strong aria-hidden = "true" > 9.3.< / strong > sync.RWMutex读写锁< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch9/ch9-04.html" > < strong aria-hidden = "true" > 9.4.< / strong > 内存同步< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch9/ch9-05.html" > < strong aria-hidden = "true" > 9.5.< / strong > sync.Once惰性初始化< / a >
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< li class = "chapter-item expanded " >
< a href = "../ch9/ch9-06.html" > < strong aria-hidden = "true" > 9.6.< / strong > 竞争条件检测< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch9/ch9-07.html" > < strong aria-hidden = "true" > 9.7.< / strong > 示例: 并发的非阻塞缓存< / a >
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< li class = "chapter-item expanded " >
< a href = "../ch9/ch9-08.html" > < strong aria-hidden = "true" > 9.8.< / strong > Goroutines和线程< / a >
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< / ol >
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< a href = "../ch10/ch10.html" > < strong aria-hidden = "true" > 10.< / strong > 包和工具< / a >
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< ol class = "section" >
< li class = "chapter-item expanded " >
< a href = "../ch10/ch10-01.html" > < strong aria-hidden = "true" > 10.1.< / strong > 包简介< / a >
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< li class = "chapter-item expanded " >
< a href = "../ch10/ch10-02.html" > < strong aria-hidden = "true" > 10.2.< / strong > 导入路径< / a >
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< li class = "chapter-item expanded " >
< a href = "../ch10/ch10-03.html" > < strong aria-hidden = "true" > 10.3.< / strong > 包声明< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch10/ch10-04.html" > < strong aria-hidden = "true" > 10.4.< / strong > 导入声明< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch10/ch10-05.html" > < strong aria-hidden = "true" > 10.5.< / strong > 包的匿名导入< / a >
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< a href = "../ch10/ch10-06.html" > < strong aria-hidden = "true" > 10.6.< / strong > 包和命名< / a >
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< a href = "../ch10/ch10-07.html" > < strong aria-hidden = "true" > 10.7.< / strong > 工具< / a >
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< / ol >
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< a href = "../ch11/ch11.html" > < strong aria-hidden = "true" > 11.< / strong > 测试< / a >
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< a href = "../ch11/ch11-01.html" > < strong aria-hidden = "true" > 11.1.< / strong > go test< / a >
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< a href = "../ch11/ch11-02.html" > < strong aria-hidden = "true" > 11.2.< / strong > 测试函数< / a >
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< a href = "../ch11/ch11-03.html" > < strong aria-hidden = "true" > 11.3.< / strong > 测试覆盖率< / a >
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< a href = "../ch11/ch11-04.html" > < strong aria-hidden = "true" > 11.4.< / strong > 基准测试< / a >
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< a href = "../ch11/ch11-05.html" > < strong aria-hidden = "true" > 11.5.< / strong > 剖析< / a >
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< a href = "../ch11/ch11-06.html" > < strong aria-hidden = "true" > 11.6.< / strong > 示例函数< / a >
< / li >
< / ol >
< li class = "chapter-item expanded " >
< a href = "../ch12/ch12.html" > < strong aria-hidden = "true" > 12.< / strong > 反射< / a >
< / li >
< ol class = "section" >
< li class = "chapter-item expanded " >
< a href = "../ch12/ch12-01.html" > < strong aria-hidden = "true" > 12.1.< / strong > 为何需要反射?< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch12/ch12-02.html" > < strong aria-hidden = "true" > 12.2.< / strong > reflect.Type和reflect.Value< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch12/ch12-03.html" > < strong aria-hidden = "true" > 12.3.< / strong > Display递归打印< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch12/ch12-04.html" > < strong aria-hidden = "true" > 12.4.< / strong > 示例: 编码S表达式< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch12/ch12-05.html" > < strong aria-hidden = "true" > 12.5.< / strong > 通过reflect.Value修改值< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch12/ch12-06.html" > < strong aria-hidden = "true" > 12.6.< / strong > 示例: 解码S表达式< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch12/ch12-07.html" > < strong aria-hidden = "true" > 12.7.< / strong > 获取结构体字段标签< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch12/ch12-08.html" > < strong aria-hidden = "true" > 12.8.< / strong > 显示一个类型的方法集< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch12/ch12-09.html" > < strong aria-hidden = "true" > 12.9.< / strong > 几点忠告< / a >
< / li >
< / ol >
< li class = "chapter-item expanded " >
< a href = "../ch13/ch13.html" > < strong aria-hidden = "true" > 13.< / strong > 底层编程< / a >
< / li >
< ol class = "section" >
< li class = "chapter-item expanded " >
< a href = "../ch13/ch13-01.html" > < strong aria-hidden = "true" > 13.1.< / strong > unsafe.Sizeof, Alignof 和 Offsetof< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch13/ch13-02.html" > < strong aria-hidden = "true" > 13.2.< / strong > unsafe.Pointer< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch13/ch13-03.html" > < strong aria-hidden = "true" > 13.3.< / strong > 示例: 深度相等判断< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch13/ch13-04.html" > < strong aria-hidden = "true" > 13.4.< / strong > 通过cgo调用C代码< / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../ch13/ch13-05.html" > < strong aria-hidden = "true" > 13.5.< / strong > 几点忠告< / a >
< / li >
< / ol >
< li class = "chapter-item expanded " >
< a href = "../appendix/appendix.html" > < strong aria-hidden = "true" > 14.< / strong > 附录< / a >
< / li >
< ol class = "section" >
< li class = "chapter-item expanded " >
< a href = "../appendix/appendix-a-errata.html" > < strong aria-hidden = "true" > 14.1.< / strong > 附录A： < / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../appendix/appendix-b-author.html" > < strong aria-hidden = "true" > 14.2.< / strong > 附录B： < / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../appendix/appendix-c-cpoyright.html" > < strong aria-hidden = "true" > 14.3.< / strong > 附录C： < / a >
< / li >
< li class = "chapter-item expanded " >
< a href = "../appendix/appendix-d-translations.html" > < strong aria-hidden = "true" > 14.4.< / strong > 附录D： < / a >
< / li >
< / ol >
< / ol >
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< main >
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< ul dir = "auto" > < li > < em > 凹语言(Go实现, 面向WASM设计): < a href = "https://github.com/wa-lang/wa" > https://github.com/wa-lang/wa< / a > < / em > < / li > < li > < em > WaBook(Go语言实现的MD电子书构建工具): < a href = "https://github.com/wa-lang/wabook" > https://github.com/wa-lang/wabook< / a > < / em > < / li > < / ul > < hr >
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< h2 > 5.6. 匿名函数< / h2 >
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< p > 拥有函数名的函数只能在包级语法块中被声明， （ ） ， ， ， ， （ ） < / p >
< p > 函数字面量允许我们在使用函数时， ， ： < / p >
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< pre > < code class = "language-Go" > strings.Map(func(r rune) rune { return r + 1 }, " HAL-9000" )
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< / code > < / pre >
< p > 更为重要的是， （ ） ， ， ： < / p >
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< p > <!-- raw HTML omitted --> <!-- raw HTML omitted --> gopl.io/ch5/squares<!-- raw HTML omitted --> <!-- raw HTML omitted --> < / 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()) // " 1"
fmt.Println(f()) // " 4"
fmt.Println(f()) // " 9"
fmt.Println(f()) // " 16"
}
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< / 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， ， ， ， < / p >
< p > squares的例子证明， ， ， ， （ ） ， < / p >
< p > 通过这个例子， ： ， < / 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 >
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< p > <!-- raw HTML omitted --> <!-- raw HTML omitted --> gopl.io/ch5/toposort<!-- raw HTML omitted --> <!-- raw HTML omitted --> < / p >
< pre > < code class = "language-Go" > // prereqs记录了每个课程的前置课程
var prereqs = map[string][]string{
" algorithms" : {" data structures" },
" calculus" : {" linear algebra" },
" compilers" : {
" data structures" ,
" formal languages" ,
" computer organization" ,
},
" data structures" : {" discrete math" },
" databases" : {" data structures" },
" discrete math" : {" intro to programming" },
" formal languages" : {" discrete math" },
" networks" : {" operating systems" },
" operating systems" : {" data structures" , " computer organization" },
" programming languages" : {" data structures" , " computer organization" },
}
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< / 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 >
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< pre > < code class = "language-Go" > func main() {
for i, course := range topoSort(prereqs) {
fmt.Printf(" %d:\t%s\n" , 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
}
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< / 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） ， ， ， < / p >
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< pre > < code class = "language-Go" > visitAll := func(items []string) {
// ...
visitAll(m[item]) // compile error: undefined: visitAll
// ...
}
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< / code > < / pre >
< p > 在toposort程序的输出如下所示， ， ， ， ， ， < / p >
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< 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
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< / 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， ， ， ， ， ， < / p >
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< p > <!-- raw HTML omitted --> <!-- raw HTML omitted --> gopl.io/ch5/links<!-- raw HTML omitted --> <!-- raw HTML omitted --> < / p >
< pre > < code class = "language-Go" > // Package links provides a link-extraction function.
package links
import (
" fmt"
" net/http"
" golang.org/x/net/html"
)
// 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(" getting %s: %s" , url, resp.Status)
}
doc, err := html.Parse(resp.Body)
resp.Body.Close()
if err != nil {
return nil, fmt.Errorf(" parsing %s as HTML: %v" , url, err)
}
var links []string
visitNode := func(n *html.Node) {
if n.Type == html.ElementNode & & n.Data == " a" {
for _, a := range n.Attr {
if a.Key != " href" {
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
}
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< / code > < / pre >
< p > 上面的代码对之前的版本做了改进， ， ， ， < / p >
< p > 网页抓取的核心问题就是如何遍历图。在topoSort的例子中， ， ， ， < / 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， ， ， < / p >
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< p > <!-- raw HTML omitted --> <!-- raw HTML omitted --> gopl.io/ch5/findlinks3<!-- raw HTML omitted --> <!-- raw HTML omitted --> < / 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) > 0 {
items := worklist
worklist = nil
for _, item := range items {
if !seen[item] {
seen[item] = true
worklist = append(worklist, f(item)...)
}
}
}
}
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< / code > < / pre >
< p > 就像我们在章节3解释的那样， ， < / p >
< p > 在我们网页抓取器中， ， ， < / p >
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< 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
}
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< / code > < / pre >
< p > 为了使抓取器开始运行， < / p >
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< pre > < code class = "language-Go" > func main() {
// Crawl the web breadth-first,
// starting from the command-line arguments.
breadthFirst(crawl, os.Args[1:])
}
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< / code > < / pre >
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< p > 让我们从 < a href = "https://golang.org" > https://golang.org< / a > 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
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< / 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， （ ） < / 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， < / p >
< p > < strong > 练习5.12： < / strong > gopl.io/ch5/outline2（ ） ， ， < / p >
< p > < strong > 练习5.13： < / strong > 修改crawl， ， ， ， < / p >
< p > < strong > 练习5.14： < / strong > 使用breadthFirst遍历其他数据结构。比如， （ ） （ ） ； （ ） < / p >
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< h3 > 5.6.1. 警告：捕获迭代变量< / h3 >
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< p > 本节， ， < / p >
< p > 考虑这样一个问题： ， ， < / p >
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< 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
}
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< / code > < / pre >
< p > 你可能会感到困惑， ， ， < / p >
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< 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!
})
}
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< / 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， ， ， ， ， ， ， ， ， < / p >
< p > 通常， ， ， ， ， < / p >
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< pre > < code class = "language-Go" > for _, dir := range tempDirs() {
dir := dir // declares inner dir, initialized to outer dir
// ...
}
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< / code > < / pre >
< p > 这个问题不仅存在基于range的循环， ， ： < / p >
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< pre > < code class = "language-Go" > var rmdirs []func()
dirs := tempDirs()
for i := 0; i < len(dirs); i++ {
os.MkdirAll(dirs[i], 0755) // OK
rmdirs = append(rmdirs, func() {
os.RemoveAll(dirs[i]) // NOTE: incorrect!
})
}
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< / code > < / pre >
< p > 如果你使用go语句（ ） （ ） ， ， < / p >
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< footer class = "page-footer" >
< span > © 2015-2016 | < a href = "https://github.com/gopl-zh" > Go语言圣经中文版< / a > , 仅学习交流使用< / span >
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< nav class = "nav-wrapper" aria-label = "Page navigation" >
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