闭包

闭包是可以捕获外部环境的函数。例如，一个捕获了 x 变量的闭包：

|val| val + x

闭包的语法和功能使其非常方便即时使用。调用闭包与调用函数完全相同。但是，输入和返回类型都*可以*推断，而输入变量名称*必须*指定。

闭包的其他特性包括：

• 使用 || 而不是 () 来包围输入变量。
• 对于单行表达式，可以选择省略主体分隔符（{}）（其他情况下必须使用）。
• 能够捕获外部环境变量。

fn main() {
    let outer_var = 42;
    
    // A regular function can't refer to variables in the enclosing environment
    //fn function(i: i32) -> i32 { i + outer_var }
    // TODO: uncomment the line above and see the compiler error. The compiler
    // suggests that we define a closure instead.

    // Closures are anonymous, here we are binding them to references.
    // Annotation is identical to function annotation but is optional
    // as are the `{}` wrapping the body. These nameless functions
    // are assigned to appropriately named variables.
    let closure_annotated = |i: i32| -> i32 { i + outer_var };
    let closure_inferred  = |i     |          i + outer_var  ;

    // Call the closures.
    println!("closure_annotated: {}", closure_annotated(1));
    println!("closure_inferred: {}", closure_inferred(1));
    // Once closure's type has been inferred, it cannot be inferred again with another type.
    //println!("cannot reuse closure_inferred with another type: {}", closure_inferred(42i64));
    // TODO: uncomment the line above and see the compiler error.

    // A closure taking no arguments which returns an `i32`.
    // The return type is inferred.
    let one = || 1;
    println!("closure returning one: {}", one());

}