如何指定闭包参数的生命周期？ [英] How can I specify a lifetime for closure arguments?

问题描述

Playpen链接： http://is.gd/EpX6lM

Playpen link: http://is.gd/EpX6lM

我有一个闭包，它需要一个切片，并返回它的子切片。在rust-1.0.0-beta-2上编译以下代码失败：

I have a closure that takes a slice and returns a subslice of it. Compiling the following code on rust-1.0.0-beta-2 fails:

trait OptionalFirst {
    fn optional_first<'a>(&self, x: &'a [usize]) -> &'a [usize];
}

impl<F> OptionalFirst for F where F: Fn(&[usize]) -> &[usize] {
    fn optional_first<'a>(&self, x: &'a [usize]) -> &'a [usize] {
        (*self)(x)
    }
}

fn main() {
    let bc: Box<OptionalFirst> = Box::new(
        |x: &[usize]| -> &[usize] {
            if x.len() != 0 {
                &x[..1]
            }
            else {
                &x[..0]
            }
        }) as Box<OptionalFirst>;

    let a: [usize; 3] = [1, 2, 3];
    let b: &[usize] = bc.optional_first(&a);
    println!("{:?}", b);
}

我知道如何在闭包的类型中定义一个生命周期$ c> for<'a> ），但我不知道如何在闭包的实现中指定它。

I know how to define a lifetime in a closure's type (using for <'a>), but I don't know how to specify it in the closure's implementation.

推荐答案

您的实现 impl< F> OptionalFirst for F其中F：Fn（& [usize]） - > & [usize] 期望有一个绑定生命周期参数，对于约束 F：Fn（& [usize]） - & [usize] 已扩展为完整格式： F：for&'a' Fn（&'a [usize]） - > &'a [usize] 。

Your implementation impl<F> OptionalFirst for F where F: Fn(&[usize]) -> &[usize] is expecting a bound lifetime parameter, for the constraint F: Fn(&[usize]) -> &[usize] is, expanded to full form: F: for<'a> Fn(&'a [usize]) -> &'a [usize].

也就是说，在调用函数时，它将确定为生命周期（它们是泛型）。

That is, at the time you call the function, it will determine what values to select for the lifetime (they are generics).

但是，闭包不能有任何绑定的生命周期参数;它们是使用具体生命周期参数卡住的。它们缺乏将输出生命周期一般性地连接到输入生命周期的设施：它们是非常设计具体的而不是通用的。我没有深入思考这一点，但它可能可能抵消这一般的生命周期参数;

A closure, however, cannot have any bound lifetime parameters; they are by stuck using concrete lifetime parameters. They lack the facility to wire output lifetimes to input lifetimes generically as you want: they are by very design concrete and not generic. I haven’t thought about this in great depth, but it might be possible to counteract this for generic lifetime parameters; it is not, however, something that is implemented as far as I am aware.

如果你想要这样的东西，尝试使用一个函数，而不是一个闭包。

If you want something like this, try using a function rather than a closure. When you’re not using any of the environment there’s no benefit to using closures beyond the typically lower verbosity.

以下是你最终得到的结果：

Here’s what you end up with:

fn bc(x: &[usize]) -> &[usize] {
    if x.len() != 0 {
        &x[..1]
    } else {
        &x[..0]
    }
}

幼儿围栏

这篇关于如何指定闭包参数的生命周期？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！