我应该如何从父函数返回匿名函数来生成闭包？

Question

我用这段代码 sn-p 解释我的问题。由于 main() 它按我的预期工作。现在我想把它移到 main() 之外的一个函数中。我还没有找到应该如何返回的语法。该文档似乎显示了许多方法，但我找不到确切的方法。

fn main() {
    println!("Hello, Rust!");

    let mut _i = 0;
    let mut closure = || -> i32 {
        _i = _i + 1;
        _i
    };

    println!("closure returns: {}", closure());
    println!("closure returns: {}", closure());
    println!("closure returns: {}", closure());

    // OK, returns 1, 2, 3, ...

    // now i want to encapsulate this in a function, something like:
    let fn_closure = get_counter();

    println!("closure returns: {}", fn_closure()); 
    println!("closure returns: {}", fn_closure()); 
    println!("closure returns: {}", fn_closure()); 

    // expect: returns 1, 2, 3, ...
}

// try failed - how returns the closure?
fn get_counter() -> || -> i32 { //? 
    let mut _i = 1;
    let mut f = || -> i32 {
        _i = _i + 1;
        _i
    };
    f
}

换句话说，我想重现这个用 C# 但用 Rust 编写的相同示例

Console.WriteLine("Hello, C#!");

var closure = GetCounter();

Console.WriteLine("closure returns: {0}", closure());
Console.WriteLine("closure returns: {0}", closure());
Console.WriteLine("closure returns: {0}", closure());

// OK, output is 1 , 2 , 3, ...

static Func<int> GetCounter()
{
    int _i = 0;
    return () => ++_i;
}

附：也许这是一个非常简单的概念性问题，对不起。

Answer 1

您可以使用返回类型impl FnMut() -> i32。这可以看作是“a concrete type that implements this trait(FnMut)” 的简写。在其他团队中，我们可以使用实现特征的某种类型的impl Trait syntax in the return position to return a value。从文档中引用。

闭包和迭代器创建只有编译器知道或 指定时间很长的类型。 impl Trait 语法让你 简明扼要地指定一个函数返回某种实现的类型 Iterator trait 不需要写出很长的类型。

fn main() {
    println!("Hello, Rust!");

    let mut _i = 0;
    let mut closure = || -> i32 {
        _i = _i + 1;
        _i
    };

    println!("closure returns: {}", closure());
    println!("closure returns: {}", closure());
    println!("closure returns: {}", closure());


    let mut fn_closure = get_counter();

    println!("closure returns: {}", fn_closure()); 
    println!("closure returns: {}", fn_closure()); 
    println!("closure returns: {}", fn_closure()); 

}

fn get_counter() -> impl FnMut() -> i32 {  
    let mut _i = 1;
    let f = move || -> i32 {
        _i = _i + 1;
        _i
    };
    f
}

另见

• When does a closure implement Fn, FnMut and FnOnce?