为编写异步路由器时,我无法处理异步功能hyper
。
这段代码:
use std::collections::HashMap;
use std::future::Future;
type BoxedResult<T> = Result<T, Box<dyn std::error::Error + Send + Sync>>;
type CalcFn = Box<dyn Fn(i32, i32) -> dyn Future<Output = BoxedResult<i32>>>;
async fn add(a: i32, b: i32) -> BoxedResult<i32> {
Ok(a + b)
}
async fn sub(a: i32, b: i32) -> BoxedResult<i32> {
Ok(a - b)
}
fn main() {
let mut map: HashMap<&str, CalcFn> = Default::default();
map.insert("add", Box::new(add));
map.insert("sub", Box::new(sub));
println!("map size: {}", map.len());
}
生成以下编译器错误:
error[E0271]: type mismatch resolving `<fn(i32, i32) -> impl std::future::Future {add} as std::ops::FnOnce<(i32, i32)>>::Output == dyn std::future::Future<Output = std::result::Result<i32, std::boxed::Box<dyn std::error::Error + std::marker::Send + std::marker::Sync>>>`
--> src/main.rs:17:23
|
17 | map.insert("add", Box::new(add));
| ^^^^^^^^^^^^^ expected opaque type, found trait std::future::Future
|
= note: expected type `impl std::future::Future`
found type `dyn std::future::Future<Output = std::result::Result<i32, std::boxed::Box<dyn std::error::Error + std::marker::Send + std::marker::Sync>>>`
= note: required for the cast to the object type `dyn std::ops::Fn(i32, i32) -> dyn std::future::Future<Output = std::result::Result<i32, std::boxed::Box<dyn std::error::Error + std::marker::Send + std::marker::Sync>>>`
error[E0271]: type mismatch resolving `<fn(i32, i32) -> impl std::future::Future {sub} as std::ops::FnOnce<(i32, i32)>>::Output == dyn std::future::Future<Output = std::result::Result<i32, std::boxed::Box<dyn std::error::Error + std::marker::Send + std::marker::Sync>>>`
--> src/main.rs:18:23
|
18 | map.insert("sub", Box::new(sub));
| ^^^^^^^^^^^^^ expected opaque type, found trait std::future::Future
|
= note: expected type `impl std::future::Future`
found type `dyn std::future::Future<Output = std::result::Result<i32, std::boxed::Box<dyn std::error::Error + std::marker::Send + std::marker::Sync>>>`
= note: required for the cast to the object type `dyn std::ops::Fn(i32, i32) -> dyn std::future::Future<Output = std::result::Result<i32, std::boxed::Box<dyn std::error::Error + std::marker::Send + std::marker::Sync>>>`
impl Future
和之间似乎存在冲突dyn Future
,但我不知道如何处理。
发生这种情况是因为impl Future
它是具体的唯一类型,dyn Future
而是抽象类型。HashMap
期望抽象类型,因为它只能容纳单个类型的实例。
如果我们可以对异步函数的返回类型进行装箱,则可以将这些期货添加到中HashMap
。
首先,我们需要更改以下类型CalcFn
:
type CalcFn = Box<dyn Fn(i32, i32) -> Pin<Box<dyn Future<Output = i32>>>>;
然后可以做到这一点:
let mut map: HashMap<&str, CalcFn> = Default::default();
map.insert("add", Box::new(|a, b| Box::pin(add(a, b))));
map.insert("sub", Box::new(|a, b| Box::pin(sub(a, b))));
println!("map size: {}", map.len());
//map.get("add").unwrap()(2, 3).await
这个完整的示例简化Future
的Item
类型,使用i32
的而不是一个Result
。请同时检查完整的代码。
您也可以从期货箱子像使用类型LocalBoxFuture
和BoxFuture
创建由FutureExt::boxed
和FutureExt::boxed_local
分别的方法:
use futures::future::{FutureExt, LocalBoxFuture}; // 0.3.5
use std::collections::HashMap;
type BoxedResult<T> = Result<T, Box<dyn std::error::Error + Send + Sync>>;
type CalcFn = Box<dyn Fn(i32, i32) -> LocalBoxFuture<'static, BoxedResult<i32>>>;
async fn add(a: i32, b: i32) -> BoxedResult<i32> {
Ok(a + b)
}
async fn sub(a: i32, b: i32) -> BoxedResult<i32> {
Ok(a - b)
}
async fn example() {
let mut map: HashMap<&str, CalcFn> = Default::default();
map.insert("add", Box::new(|a, b| add(a, b).boxed()));
map.insert("sub", Box::new(|a, b| sub(a, b).boxed()));
println!("map size: {}", map.len());
//map.get("add").unwrap()(2, 3).await
}
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