Rust闭包的虫洞穿梭

{"type":"doc","content":[{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"1. 闭包是什么"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"闭包（Closure）的概念由来已久。无论哪种语言，闭包的概念都被以下几个特征共同约束："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"匿名函数"},{"type":"text","text":"（非独有，函数指针也可以）；"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"可以调用闭包，并显式传递参数"},{"type":"text","text":"（非独有，函数指针也可以）；"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"以变量形式存在，可以传来传去"},{"type":"text","text":"（非独有，函数指针也可以）；"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"可以在闭包内直接捕获并使用定义所处作用域的值（独有）"},{"type":"text","text":"；"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"神奇的是最后一点，理解起来也比较别扭的，习惯就好了。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"为了说明上述特征，可以看一个Rust例子。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"rust"},"content":[{"type":"text","text":"fn display(age: u32, print_info: T)\n where T: Fn(u32)\n{\n print_info(age);\n}\n\nfn main() {\n let name = String::from(\"Ethan\");\n\n let print_info_closure = |age|{\n println!(\"name is {}\", name);\n println!(\"age is {}\", age);\n };\n\n let age = 18;\n display(age, print_info_closure);\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"运行代码："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"name is Ethan"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"age is 18"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"首先，闭包作为匿名函数存在了"},{"type":"codeinline","content":[{"type":"text","text":"print_info_closure"}]},{"type":"text","text":"栈变量中，然后传递给了函数"},{"type":"codeinline","content":[{"type":"text","text":"display"}]},{"type":"text","text":"作为参数，在"},{"type":"codeinline","content":[{"type":"text","text":"display"}]},{"type":"text","text":"内部调用了闭包，并传递了参数"},{"type":"codeinline","content":[{"type":"text","text":"age"}]},{"type":"text","text":"。最后神奇的事情出现了：在函数"},{"type":"codeinline","content":[{"type":"text","text":"display"}]},{"type":"text","text":"中调用的闭包居然打印出了函数"},{"type":"codeinline","content":[{"type":"text","text":"main"}]},{"type":"text","text":"作用域中的变量"},{"type":"codeinline","content":[{"type":"text","text":"name"}]},{"type":"text","text":"。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/b5/b5d5b71e8b41eec3249e9515b26df9e2.jpeg","alt":null,"title":"","style":[{"key":"width","value":"100%"},{"key":"bordertype","value":"none"}],"href":"","fromPaste":false,"pastePass":false}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"闭包的精髓，就在于它同时涉及两个作用域"},{"type":"text","text":"，就仿佛打开了一个\"虫洞\"，让不同作用域的变量穿梭其中。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"rust"},"content":[{"type":"text","text":"let 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||{};"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"单独一行代码，就藏着这个奥妙："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"赋值"},{"type":"codeinline","content":[{"type":"text","text":"="}]},{"type":"text","text":"的左侧，是存储闭包的变量，它处在一个作用域中，也就是我们说的闭包"},{"type":"text","marks":[{"type":"strong"}],"text":"定义处"},{"type":"text","text":"的环境上下文；"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"赋值"},{"type":"codeinline","content":[{"type":"text","text":"="}]},{"type":"text","text":"的右侧，那对花括号"},{"type":"codeinline","content":[{"type":"text","text":"{}"}]},{"type":"text","text":"里，也是一个作用域，它在闭包"},{"type":"text","marks":[{"type":"strong"}],"text":"被调用处"},{"type":"text","text":"动态产生；"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"无论左侧右侧，都定义了闭包的属性，天然的联通了两个作用域。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"对于闭包，Rust如此，其他语言也大抵如此。不过，Rust不是还有所有权、生命周期这一档子事儿么，所以还可以深入分析下。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"2. Rust闭包捕获上下文的方式"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如本篇题目，Rust闭包如何捕获上下文？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"换个问法，"},{"type":"codeinline","content":[{"type":"text","text":"main"}]},{"type":"text","text":"作用域中的变量"},{"type":"codeinline","content":[{"type":"text","text":"name"}]},{"type":"text","text":"是以何种方式进入闭包的作用域的（第1节例子）？转移or借用？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"It Depends，视情况而定。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Rust在std中定义了3种trait："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"FnOnce：闭包内对外部变量存在转移操作，导致外部变量不可用（所以只能call一次）"},{"type":"text","text":"；"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"FnMut：闭包内对外部变量直接使用，并进行修改"},{"type":"text","text":"；"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"Fn：闭包内对外部变量直接使用，不进行修改"},{"type":"text","text":"；"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"后者能办到的，前者一定能办到。反之则不然。所以，编译器对闭包签名进行推理时："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"实现FnMut的，同时也实现了FnOnce；"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"实现Fn的，同时也实现了FnMut和FnOnce。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"第1节的例子，将"},{"type":"codeinline","content":[{"type":"text","text":"display"}]},{"type":"text","text":"的泛型参数从Fn改成FnMut，也可以无警告通过。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"rust"},"content":[{"type":"text","text":"fn display(age: u32, mut print_info: T)\n where T: FnMut(u32)\n{\n print_info(age);\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"对环境变量进行捕获的闭包，需要额外的空间支持才能将环境变量进行存储。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"3. 作为参数的闭包签名"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"上面代码"},{"type":"codeinline","content":[{"type":"text","text":"display"}]},{"type":"text","text":"函数定义，要接受一个闭包作为参数，揭示了如何显式的描述闭包的签名：在泛型参数上添加trait约束，比如"},{"type":"codeinline","content":[{"type":"text","text":"T: 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main(){\n\t\tlet lifttime_closure = |a, b|{\n println!(\"{}\", a);\n println!(\"{}\", b);\n b\n };\n let a = String::from(\"abc\");\n let c;\n {\n let b = String::from(\"xyz\");\n c = lifttime_closure(&a, &b);\n }\n println!(\"{}\", c);\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"以上代码无法通过编译，成功检测出了悬垂引用："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"error[E0597]: "},{"type":"codeinline","content":[{"type":"text","text":"b"}]},{"type":"text","text":" does not live long enough"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"显然，对于闭包，编译期可以对引用的生命周期进行检查，以保证引用始终有效。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"这个例子，与其解释闭包与函数的区别，不如解释匿名函数与具名函数的区别："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"具名函数是签名在先的，对于编译器来说，调用方和函数内部实现，只要分别遵守签名的约定即可。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"匿名函数的签名则是被推理出来的，编译器要"},{"type":"text","marks":[{"type":"strong"}],"text":"看全看透"},{"type":"text","text":"调用方的实际输入，以及函数内部的实际返回，检查自然也就顺带做掉了。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"4. 函数返回闭包"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"第1节的例子，我们将一个闭包作为函数参数传入，那么根据闭包的特性，它应该能够作为函数的返回值。答案是肯定的。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"基于前面介绍的Fn trait，我们定义一个返回闭包的函数，代码如下："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"rust"},"content":[{"type":"text","text":"fn closure_return() -> Fn() -> (){\n\t||{} \n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"可是，编译失败了："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"error[E0746]: return type cannot have an unboxed trait object"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"doesn't have a size known at compile-time"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"失败信息显示，编译器无法确定函数返回值的大小。一个闭包有多大呢？并不重要。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"开门见山，通用的解决方法是：为了能够返回闭包，可以使用一次装箱，从而将栈内存变量装箱存入堆内存，这样无论闭包有多大，函数返回值都是一个确定大小的指针。下面的代码里，使用"},{"type":"codeinline","content":[{"type":"text","text":"Box::new"}]},{"type":"text","text":"即可完成装箱。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"rust"},"content":[{"type":"text","text":"fn closure_inside() -> Box ()>\n{\n let mut age = 1;\n let mut name = String::from(\"Ethan\");\n\n let age_closure = move || {\n name.push_str(\" Yuan\");\n age += 1;\n println!(\"name is {}\", name);\n println!(\"age is {}\", age);\n };\n\n Box::new(age_closure)\n}\n\nfn main(){\n let mut age_closure = closure_inside();\n age_closure();\n age_closure();\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"运行结果如下："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"name is Ethan Yuan"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"age is 2"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"name is Ethan Yuan Yuan"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"age is 3"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"上面的代码，除了让函数成功返回闭包之外，还有一个目的，我们想让闭包捕获函数内部环境中的值，但这次有些不同："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"第1节代码示例，我们把外层的环境上下文，通过将闭包传入内层函数，这个不难理解，因为外层变量的生命周期更长，内层函数访问时，外层变量还活着；"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"而本节代码所做的，是通过闭包将内层函数的环境变量传出来给外层环境；"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"内层函数调用完成后就会销毁内层环境变量，那如何做到呢？幸好，Rust有"},{"type":"text","marks":[{"type":"strong"}],"text":"所有权转移"},{"type":"text","text":"。只要能促成内层函数的环境变量向闭包进行所有权的转移，这个操作顺理成章。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"正因为Rust具有所有权转移的概念，返回闭包（同时捕获环境变量）的机理，Rust的要比任何具有垃圾回收语言（JavaScript、Java、C#）的解释都更简单明了。后者总会给人一丝不安：内部函数调用都结束了，居然局部变量还活着。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"代码中的所有权转移，这里使用了关键字"},{"type":"codeinline","content":[{"type":"text","text":"move"}]},{"type":"text","text":"，它可以在构建闭包时，强制将要捕获变量的所有权转移至闭包内部的特别存储区。需要注意的是，使用"},{"type":"codeinline","content":[{"type":"text","text":"move"}]},{"type":"text","text":"，并不影响闭包的"},{"type":"codeinline","content":[{"type":"text","text":"trait"}]},{"type":"text","text":"，本例中可以看到闭包是"},{"type":"codeinline","content":[{"type":"text","text":"FnMut"}]},{"type":"text","text":"，而不是"},{"type":"codeinline","content":[{"type":"text","text":"FnOnce"}]},{"type":"text","text":"。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}}]}