将未来的结果转换为另一个未来
Converting a future result in another future
我有一个 return 具有 User
特征的未来函数。我有两个具体的实现:AnonymousUser
和 BaseUser
。要获得 BaseUser
,在身份验证后,我必须去数据库获取它,这可能会成功也可能不会成功,并且 return 具有正确类型的新未来。我尝试了以下 (playground):
extern crate futures; // 0.1.23
extern crate rand; // 0.5.4
use futures::future::{ok, Future};
use std::io::Error;
trait User {}
#[derive(Debug)]
struct AnonymousUser;
impl User for AnonymousUser {}
#[derive(Debug)]
struct BaseUser;
impl User for BaseUser {}
fn fetch_base_user() -> impl Future<Item = BaseUser, Error = Error> {
ok(BaseUser)
}
fn run_future() -> impl Future<Item = impl User, Error = Error> {
match rand::random::<bool>() {
true => fetch_base_user().from_err().then(move |res| match res {
Ok(user) => ok(user),
Err(_) => ok(AnonymousUser),
}),
false => ok(AnonymousUser),
}
}
fn main() {
run_future().and_then(move |user| println!("User {:?}", user));
}
这失败了,因为 then
函数的 return 需要一个 BaseUser
:
error[E0308]: match arms have incompatible types
--> src/main.rs:23:62
|
23 | true => fetch_base_user().from_err().then(move |res| match res {
| ______________________________________________________________^
24 | | Ok(user) => ok(user),
25 | | Err(_) => ok(AnonymousUser),
| | ----------------- match arm with an incompatible type
26 | | }),
| |_________^ expected struct `BaseUser`, found struct `AnonymousUser`
|
= note: expected type `futures::FutureResult<BaseUser, _>`
found type `futures::FutureResult<AnonymousUser, _>`
我尝试强制 return 类型:
use futures::future::FutureResult;
fn run_future() -> impl Future<Item=impl User, Error=Error> {
match rand::random::<bool>() {
true => fetch_base_user().from_err().then(move |res| ->
FutureResult<impl User, Error> { // Forcing the result type here
match res {
Ok(user) => ok(user),
Err(_) => ok(AnonymousUser),
}
}),
false => ok(AnonymousUser),
}
}
失败:
error[E0562]: `impl Trait` not allowed outside of function and inherent method return types
--> src/main.rs:27:22
|
27 | FutureResult<impl User, Error> { // Forcing the result type here
| ^^^^^^^^^
我尝试使用 Box
es 对 return 进行返工,这几乎成功了 (playground)
fn run_future() -> impl Future<Item = Box<impl User>, Error = Error> {
match rand::random::<bool>() {
true => fetch_base_user()
.from_err()
.then(move |res| -> FutureResult<Box<User>, Error> {
match res {
Ok(user) => ok(Box::new(user) as Box<User>),
Err(_) => ok(Box::new(AnonymousUser) as Box<User>),
}
}),
false => ok(Box::new(AnonymousUser) as Box<User>),
}
}
失败
error[E0308]: match arms have incompatible types
--> src/main.rs:22:5
|
22 | / match rand::random::<bool>() {
23 | | true => fetch_base_user().from_err().then(move |res| match res {
24 | | Ok(user) => ok(Box::new(user) as Box<User>),
25 | | Err(_) => ok(Box::new(AnonymousUser) as Box<User>),
26 | | }),
27 | | false => ok(Box::new(AnonymousUser) as Box<User>),
| | ---------------------------------------- match arm with an incompatible type
28 | | }
| |_____^ expected struct `futures::Then`, found struct `futures::FutureResult`
|
= note: expected type `futures::Then<futures::future::FromErr<impl futures::Future, _>, futures::FutureResult<std::boxed::Box<User>, _>, [closure@src/main.rs:23:51: 26:10]>`
found type `futures::FutureResult<std::boxed::Box<User>, _>`
所以我想这只是强制两者成为相同结果类型的问题
最后 shepmaster 的评论让我通过另一个问题得到了回应:
基本上使用 Either::A 和 Either::B 就可以解决问题。如果不对参数进行装箱,我仍然无法使其工作,但这可能是一个不同的问题。
我有一个 return 具有 User
特征的未来函数。我有两个具体的实现:AnonymousUser
和 BaseUser
。要获得 BaseUser
,在身份验证后,我必须去数据库获取它,这可能会成功也可能不会成功,并且 return 具有正确类型的新未来。我尝试了以下 (playground):
extern crate futures; // 0.1.23
extern crate rand; // 0.5.4
use futures::future::{ok, Future};
use std::io::Error;
trait User {}
#[derive(Debug)]
struct AnonymousUser;
impl User for AnonymousUser {}
#[derive(Debug)]
struct BaseUser;
impl User for BaseUser {}
fn fetch_base_user() -> impl Future<Item = BaseUser, Error = Error> {
ok(BaseUser)
}
fn run_future() -> impl Future<Item = impl User, Error = Error> {
match rand::random::<bool>() {
true => fetch_base_user().from_err().then(move |res| match res {
Ok(user) => ok(user),
Err(_) => ok(AnonymousUser),
}),
false => ok(AnonymousUser),
}
}
fn main() {
run_future().and_then(move |user| println!("User {:?}", user));
}
这失败了,因为 then
函数的 return 需要一个 BaseUser
:
error[E0308]: match arms have incompatible types
--> src/main.rs:23:62
|
23 | true => fetch_base_user().from_err().then(move |res| match res {
| ______________________________________________________________^
24 | | Ok(user) => ok(user),
25 | | Err(_) => ok(AnonymousUser),
| | ----------------- match arm with an incompatible type
26 | | }),
| |_________^ expected struct `BaseUser`, found struct `AnonymousUser`
|
= note: expected type `futures::FutureResult<BaseUser, _>`
found type `futures::FutureResult<AnonymousUser, _>`
我尝试强制 return 类型:
use futures::future::FutureResult;
fn run_future() -> impl Future<Item=impl User, Error=Error> {
match rand::random::<bool>() {
true => fetch_base_user().from_err().then(move |res| ->
FutureResult<impl User, Error> { // Forcing the result type here
match res {
Ok(user) => ok(user),
Err(_) => ok(AnonymousUser),
}
}),
false => ok(AnonymousUser),
}
}
失败:
error[E0562]: `impl Trait` not allowed outside of function and inherent method return types
--> src/main.rs:27:22
|
27 | FutureResult<impl User, Error> { // Forcing the result type here
| ^^^^^^^^^
我尝试使用 Box
es 对 return 进行返工,这几乎成功了 (playground)
fn run_future() -> impl Future<Item = Box<impl User>, Error = Error> {
match rand::random::<bool>() {
true => fetch_base_user()
.from_err()
.then(move |res| -> FutureResult<Box<User>, Error> {
match res {
Ok(user) => ok(Box::new(user) as Box<User>),
Err(_) => ok(Box::new(AnonymousUser) as Box<User>),
}
}),
false => ok(Box::new(AnonymousUser) as Box<User>),
}
}
失败
error[E0308]: match arms have incompatible types
--> src/main.rs:22:5
|
22 | / match rand::random::<bool>() {
23 | | true => fetch_base_user().from_err().then(move |res| match res {
24 | | Ok(user) => ok(Box::new(user) as Box<User>),
25 | | Err(_) => ok(Box::new(AnonymousUser) as Box<User>),
26 | | }),
27 | | false => ok(Box::new(AnonymousUser) as Box<User>),
| | ---------------------------------------- match arm with an incompatible type
28 | | }
| |_____^ expected struct `futures::Then`, found struct `futures::FutureResult`
|
= note: expected type `futures::Then<futures::future::FromErr<impl futures::Future, _>, futures::FutureResult<std::boxed::Box<User>, _>, [closure@src/main.rs:23:51: 26:10]>`
found type `futures::FutureResult<std::boxed::Box<User>, _>`
所以我想这只是强制两者成为相同结果类型的问题
最后 shepmaster 的评论让我通过另一个问题得到了回应:
基本上使用 Either::A 和 Either::B 就可以解决问题。如果不对参数进行装箱,我仍然无法使其工作,但这可能是一个不同的问题。