[英]How can I specify a trait bound on an associated type that is a Result?
我正在嘗試編寫一個可以采用返回Result
的Stream
的函數。 問題在於為Result
的成功值指定邊界。
這是一個說明問題的示例(是的,它不會在沒有異步運行時的情況下運行,但這並不是真正的問題):
use std::{pin::Pin, task::{Context, Poll}};
use futures::{Stream, StreamExt};
struct Data;
impl Stream for Data {
type Item = Result<String, std::io::Error>;
#[inline]
fn poll_next(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Option<Self::Item>> {
Poll::Ready(Some(Ok("Hello".to_owned())))
}
}
async fn handle_stream<S>(mut stream: S) where S: Stream + Unpin, <S as Stream>::Item: AsRef<str> {
while let Some(item) = stream.next().await {
// TODO
}
}
fn main() {
let mut data = Data{};
async move {
handle_stream(data).await;
};
}
下面的編譯器錯誤是有道理的,因為Result
沒有實現AsRef
:
error[E0277]: the trait bound `Result<String, std::io::Error>: AsRef<str>` is not satisfied
--> src/main.rs:24:5
|
24 | handle_stream(data).await;
| ^^^^^^^^^^^^^ the trait `AsRef<str>` is not implemented for `Result<String, std::io::Error>`
|
note: required by a bound in `handle_stream`
--> src/main.rs:15:88
|
15 | async fn handle_stream<S>(mut stream: S) where S: Stream + Unpin, <S as Stream>::Item: AsRef<str> {
| ^^^^^^^^^^ required by this bound in `handle_stream`
For more information about this error, try `rustc --explain E0277`.
但是我試圖指定S
的關聯Item
實際上應該是Result
失敗,因為Result
不是特征:
error[E0404]: expected trait, found enum `Result`
--> src/main.rs:15:88
|
15 | async fn handle_stream<S>(mut stream: S) where S: Stream + Unpin, <S as Stream>::Item: Result<AsRef<str>> {
| ^^^^^^^^^^^^^^^^^^ not a trait
For more information about this error, try `rustc --explain E0404`.
如何指定我想要返回具有AsRef<str>
值的Result
的Stream
? 我意識到我可能還需要為Err
類型指定一些東西; 假設這是一個類似的語法。
等同關聯類型的語法是Trait<Assoc = Type>
。
所以:
async fn handle_stream<S, I, E>(mut stream: S)
where
S: Stream<Item = Result<I, E>> + Unpin,
I: AsRef<str>,
{
// ...
}
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