[英]How can I specify a lifetime for closure arguments?
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I have a closure that takes a slice and returns a subslice of it. 我有一个需要切片并返回其子切片的闭包。 Compiling the following code on rust-1.0.0-beta-2 fails:
在rust-1.0.0-beta-2上编译以下代码失败:
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);
}
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. 我知道如何在闭包的类型中定义生存期(
for <'a>
),但是我不知道如何在闭包的实现中指定生存期。
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]
. impl<F> OptionalFirst for F where F: Fn(&[usize]) -> &[usize]
实现impl<F> OptionalFirst for F where F: Fn(&[usize]) -> &[usize]
对于约束F: Fn(&[usize]) -> &[usize]
impl<F> OptionalFirst for F where F: Fn(&[usize]) -> &[usize]
期望绑定的生存期参数。是,展开为完整形式: F: for<'a> Fn(&'a [usize]) -> &'a [usize]
。
That is, at the time you call the function, it will determine what values to select for the lifetime (they are generics). 也就是说,在调用该函数时,它将确定为生命周期选择哪些值(它们是泛型的)。
A closure, however, cannot have any bound lifetime parameters; 但是,闭包不能具有任何绑定的生存期参数; they are by stuck using concrete lifetime parameters.
他们通过使用具体的寿命参数而陷入困境。 They lack the facility to wire output lifetimes to input lifetimes generically as you want: they are by very design concrete and not generic.
它们缺乏将输出寿命与输入寿命连接起来的功能,就像您想要的那样:它们是非常具体的设计,而不是通用的。 I haven't thought about this in great depth, but it might be possible to counteract this for generic lifetime parameters;
我还没有对此进行深入的思考,但是对于通用的生命周期参数, 可能有可能抵消它。 it is not, however, something that is implemented as far as I am aware.
据我所知,它并没有实现。
If you want something like this, try using a function rather than a closure. 如果您想要这样,请尝试使用函数而不是闭包。 When you're not using any of the environment there's no benefit to using closures beyond the typically lower verbosity.
当您不使用任何环境时,使用闭包通常没有什么好处,这没有任何好处。
Here's what you end up with: 这是您最终得到的结果:
fn bc(x: &[usize]) -> &[usize] {
if x.len() != 0 {
&x[..1]
} else {
&x[..0]
}
}
You can create a closure with bound lifetime parameters, you just have to get the compiler to infer that type correctly. 您可以使用绑定的生存期参数创建一个闭包,只需要让编译器正确推断该类型即可。 It can be done like this:
可以这样完成:
fn main() {
let bc: Box<Fn(&[usize]) -> &[usize]> = Box::new(
|x| {
if x.len() != 0 {
&x[..1]
}
else {
&x[..0]
}
});
let a: [usize; 3] = [1, 2, 3];
let b: &[usize] = bc(&a);
println!("{:?}", b);
}
However, what I don't know is how to cast it further into Box<OptionalFirst>
. 但是,我不知道如何将其进一步转换为
Box<OptionalFirst>
。
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