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按相反顺序克隆集合

iterator rust

-1

Harry · 技术社区 · 8 月前

如何以相反的方式克隆集合。我试着用 DoubleEndedIterator 向后迭代以克隆集合的每个元素。

fn clone_backward<List, ItemType>(list: &List) -> List
where
    for<'a> &'a List: IntoIterator<Item = &'a ItemType>,
    for<'a> <&'a List as IntoIterator>::IntoIter: DoubleEndedIterator,
    for<'a> &'a List::IntoIter: DoubleEndedIterator<Item = &'a ItemType>,
    List: FromIterator<ItemType> + IntoIterator,
    ItemType: Clone,
{
    list.into_iter()
    .rev()
    .cloned()
    .collect()
}

fn main() {
    let first_list = vec![1, 2, 3, 4, 5];
    let second_list: Vec<i32> = clone_backward(&first_list);
        
    assert_eq!(vec![5, 4, 3, 2, 1], second_list);
}

错误

error[E0277]: the trait bound `for<'a> <&'a _ as IntoIterator>::IntoIter: DoubleEndedIterator` is not satisfied
  --> src/main.rs:17:48
   |
17 |     let second_list: Vec<i32> = clone_backward(&first_list);
   |                                 -------------- ^^^^^^^^^^^ the trait `for<'a> DoubleEndedIterator` is not implemented for `<&'a _ as IntoIterator>::IntoIter`
   |                                 |
   |                                 required by a bound introduced by this call
   |
note: this is a known limitation of the trait solver that will be lifted in the future
  --> src/main.rs:17:48
   |
17 |     let second_list: Vec<i32> = clone_backward(&first_list);
   |                                 ---------------^^^^^^^^^^^-
   |                                 |              |
   |                                 |              the trait solver is unable to infer the generic types that should be inferred from this argument
   |                                 add turbofish arguments to this call to specify the types manually, even if it's redundant
note: required by a bound in `clone_backward`
  --> src/main.rs:4:51
   |
1  | fn clone_backward<List, ItemType>(list: &List) -> List
   |    -------------- required by a bound in this function
...
4  |     for<'a> <&'a List as IntoIterator>::IntoIter: DoubleEndedIterator,
   |                                                   ^^^^^^^^^^^^^^^^^^^ required by this bound in `clone_backward`

error[E0277]: the trait bound `for<'a> &'a _: DoubleEndedIterator` is not satisfied
  --> src/main.rs:17:48
   |
17 |     let second_list: Vec<i32> = clone_backward(&first_list);
   |                                 -------------- ^^^^^^^^^^^ the trait `for<'a> DoubleEndedIterator` is not implemented for `&'a _`
   |                                 |
   |                                 required by a bound introduced by this call
   |
note: required by a bound in `clone_backward`
  --> src/main.rs:5:33
   |
1  | fn clone_backward<List, ItemType>(list: &List) -> List
   |    -------------- required by a bound in this function
...
5  |     for<'a> &'a List::IntoIter: DoubleEndedIterator<Item = &'a ItemType>,
   |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `clone_backward`
help: consider removing the leading `&`-reference
   |
17 -     let second_list: Vec<i32> = clone_backward(&first_list);
17 +     let second_list: Vec<i32> = clone_backward(first_list);
   |

For more information about this error, try `rustc --explain E0277`.
error: could not compile `playground` (bin "playground") due to 2 previous errors

2 回复 | 直到 8 月前

1

Finomnis 8 月前

在撰写本文时,特质解决者似乎无法解决这种间接问题。

似乎需要手动注释类型,如下所示:

fn clone_backward<List, ItemType>(list: &List) -> List
where
    for<'a> &'a List: IntoIterator<Item = &'a ItemType>,
    for<'a> <&'a List as IntoIterator>::IntoIter: DoubleEndedIterator,
    List: FromIterator<ItemType>,
    ItemType: Clone,
{
    list.into_iter().rev().cloned().collect()
}

fn main() {
    let first_list = vec![1, 2, 3, 4, 5];
    let second_list: Vec<i32> = clone_backward::<Vec<i32>, _>(&first_list);

    assert_eq!(vec![5, 4, 3, 2, 1], second_list);
}

(注意,我删除了一些不必要的注释)

你可以进一步优化这一步,这样你只需要注释返回类型,因为(通过一点注释的痛苦)向量的元素可以由编译器确定:

fn clone_backward<List>(list: &List) -> List
where
    for<'a> &'a List: IntoIterator,
    for<'a> <&'a List as IntoIterator>::IntoIter: DoubleEndedIterator,
    for<'a> <&'a List as IntoIterator>::Item: std::ops::Deref,
    for<'a> <<&'a List as IntoIterator>::Item as std::ops::Deref>::Target: Clone,
    for<'a> List: FromIterator<<<&'a List as IntoIterator>::Item as std::ops::Deref>::Target>,
{
    list.into_iter().rev().map(|val| (*val).clone()).collect()
}

fn main() {
    let first_list = vec![1, 2, 3, 4, 5];
    let second_list: Vec<i32> = clone_backward::<Vec<i32>>(&first_list);

    assert_eq!(vec![5, 4, 3, 2, 1], second_list);
}

但我认为,在撰写本文时,这已经是最好的了。

2

0

Finomnis 8 月前

就我个人而言,我并不完全了解你的确切用例(这个问题在任何特定用例中都很模糊),但只要在需要的地方使用这4行代码,这个问题就可以很容易地得到解决,就我个人来说,如果把它拉到一个专用的fn中,我会比任何事情都更困惑。这是一种常见且易于识别的模式的变体, .iter().{cloned/map/filter/flatten}.collect() .

正如错误消息所示,该问题源于编译器中的临时限制,希望很快能得到修补。编译器无法处理泛型,特别是当涉及引用时,IntoIterator::IntoIter上的trait边界。

其他选择是取消fn的通用性;让它采用特定的数据类型,从而绕过限制(特别是考虑是否需要将此fn作为回调传递),或者你可以让fn用户直接创建并提供DEI本身,基本上传递 clone_backward(first_list.iter().rev()) 尽管这将进一步使函数变得可读性较差、不可链接,否则现在是2行的,非常简单。