Rust：如何從函數返回迭代器並使用它？

Question

我正在嘗試通過一個小日歷項目來教自己Rust。

在這里，我試圖在給定日期生成一個跨越整整三個月的日期列表。 我想返回一個可以迭代這些日期的迭代器。 這是我的第一次嘗試：

fn three_months_range(tm: time::Tm) -> std::iter::Iterator<Item=time::Tm> {
    let fpm: time::Tm = first_of_previous_month(&tm);
    (0..)
        .map(|i| fpm + time::Duration::days(i))
        .take_while(|&t| t.tm_mon != (tm.tm_mon + 2) % 12)
}

不幸的是，這不編譯，我收到一個錯誤。

src/main.rs:49:40: 49:75 error: the trait `core::marker::Sized` is not implemented for the type `core::iter::Iterator<Item=time::Tm> + 'static` [E0277]
src/main.rs:49 fn three_months_range(tm: time::Tm) -> std::iter::Iterator <Item=time::Tm> {
                                                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/main.rs:49:40: 49:75 help: run `rustc --explain E0277` to see a detailed explanation
src/main.rs:49:40: 49:75 note: `core::iter::Iterator<Item=time::Tm> + 'static` does not have a constant size known at compile-time
src/main.rs:49:40: 49:75 note: the return type of a function must have a statically known size

“函數的返回類型必須具有靜態已知的大小”。 好吧，經過一些研究，似乎解決方案是通過Box返回迭代器。 （仍然，我想知道標准庫map ， filter ， take_while ...方法如何設法返回迭代器而不是盒子）。

好吧，這是第二次成功編譯的嘗試：

fn three_months_range(tm: time::Tm) -> Box<iter::Iterator<Item=time::Tm>> {
    let fpm: time::Tm = first_of_previous_month(&tm);
    Box::new(
        (0..)
        .map(move |i| fpm + time::Duration::days(i))
        .take_while(move |&t| t.tm_mon != (tm.tm_mon + 2) % 12)
    )
}

不幸的是，我沒有設法使用這個迭代器。 例如，假設我想構建一個包含每個日期月份日期的向量（1,2,3，...，31,1,2，...，30,1,2，... 31）：

let days_vec: Vec<u64> = 
    ( *three_months_range(time::now_utc()) )
    .map( |&t: &time::Tm| t.tm_mday )
    .collect();

→

src/main.rs:14:10: 14:42 error: the trait `core::marker::Sized` is not implemented for the type `core::iter::Iterator<Item=time::Tm> + 'static` [E0277]
src/main.rs:14         .map( |&t: &time::Tm| t.tm_mday )
                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/main.rs:14:10: 14:42 help: run `rustc --explain E0277` to see a detailed explanation
src/main.rs:14:10: 14:42 note: `core::iter::Iterator<Item=time::Tm> + 'static` does not have a constant size known at compile-time
src/main.rs:15:10: 15:19 error: no method named `collect` found for type `core::iter::Map<core::iter::Iterator<Item=time::Tm> + 'static, [closure@src/main.rs:14:15: 14:40]>` in the current scope
src/main.rs:15         .collect();
                        ^~~~~~~~~
src/main.rs:15:10: 15:19 note: the method `collect` exists but the following trait bounds were not satisfied: `core::iter::Iterator<Item=time::Tm> : core::marker::Sized`, `[closure@src/main.rs:14:15: 14:40] : core::ops::FnMut<(time::Tm,)>`, `core::iter::Map<core::iter::Iterator<Item=time::Tm> + 'static, [closure@src/main.rs:14:15: 14:40]> : core::iter::Iterator`
src/main.rs:14:10: 14:42 error: type mismatch: the type `[closure@src/main.rs:14:15: 14:40]` implements the trait `for<'r> core::ops::FnMut<(&'r time::Tm,)>`, but the trait `core::ops::FnMut<(time::Tm,)>` is required (expected struct `time::Tm`, found &-ptr) [E0281]
src/main.rs:14         .map( |&t: &time::Tm| t.tm_mday )
                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/main.rs:14:10: 14:42 help: run `rustc --explain E0281` to see a detailed explanation
src/main.rs:14:10: 14:42 error: type mismatch: the type `[closure@src/main.rs:14:15: 14:40]` implements the trait `for<'r> core::ops::FnOnce<(&'r time::Tm,)>`, but the trait `core::ops::FnOnce<(time::Tm,)>` is required (expected struct `time::Tm`, found &-ptr) [E0281]
src/main.rs:14         .map( |&t: &time::Tm| t.tm_mday )
                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/main.rs:14:10: 14:42 help: run `rustc --explain E0281` to see a detailed explanation
error: aborting due to 4 previous errors

這是很多錯誤。

那么我在這里做錯了什么？

是否有一種相對簡單的方法來轉換Rust中的迭代器和/或從函數返回它們？

Answer 1

問題是你試圖將Iterator從Box取出（你不能因為它是一個特征對象，因此不是Sized ）。 但是Box是透明的，你可以直接在它上面使用map ：

let days_vec: Vec<u64> = 
    three_months_range(time::now_utc())
    .map( |&t: &time::Tm| t.tm_mday )
    .collect();

請注意， map函數希望您按值而不是通過引用獲取參數。 所以調用可能如下所示：

.map(|t| t.tm_mday)