Trait futures_util::future::TryFutureExt

pub trait TryFutureExt: TryFuture {
    fn flatten_sink<Item>(self) -> FlattenSink<Self, Self::Ok>
    where
        Self::Ok: Sink<Item, Error = Self::Error>,
        Self: Sized,
    { ... }
    fn map_ok<T, F>(self, f: F) -> MapOk<Self, F>
    where
        F: FnOnce(Self::Ok) -> T,
        Self: Sized,
    { ... }
    fn map_err<E, F>(self, f: F) -> MapErr<Self, F>
    where
        F: FnOnce(Self::Error) -> E,
        Self: Sized,
    { ... }
    fn err_into<E>(self) -> ErrInto<Self, E>
    where
        Self: Sized,
        Self::Error: Into<E>,
    { ... }
    fn and_then<Fut, F>(self, f: F) -> AndThen<Self, Fut, F>
    where
        F: FnOnce(Self::Ok) -> Fut,
        Fut: TryFuture<Error = Self::Error>,
        Self: Sized,
    { ... }
    fn or_else<Fut, F>(self, f: F) -> OrElse<Self, Fut, F>
    where
        F: FnOnce(Self::Error) -> Fut,
        Fut: TryFuture<Ok = Self::Ok>,
        Self: Sized,
    { ... }
    fn inspect_ok<F>(self, f: F) -> InspectOk<Self, F>
    where
        F: FnOnce(&Self::Ok),
        Self: Sized,
    { ... }
    fn inspect_err<F>(self, f: F) -> InspectErr<Self, F>
    where
        F: FnOnce(&Self::Error),
        Self: Sized,
    { ... }
    fn try_flatten_stream(self) -> TryFlattenStream<Self>
    where
        Self::Ok: TryStream<Error = Self::Error>,
        Self: Sized,
    { ... }
    fn unwrap_or_else<F>(self, f: F) -> UnwrapOrElse<Self, F>
    where
        Self: Sized,
        F: FnOnce(Self::Error) -> Self::Ok,
    { ... }
    fn into_future(self) -> IntoFuture<Self>
    where
        Self: Sized,
    { ... }
    fn try_poll_unpin(
        &mut self, 
        cx: &mut Context
    ) -> Poll<Result<Self::Ok, Self::Error>>
    where
        Self: Unpin,
    { ... }
}

Adapters specific to Result-returning futures

Provided methods

fn flatten_sink<Item>(self) -> FlattenSink<Self, Self::Ok> where
    Self::Ok: Sink<Item, Error = Self::Error>,
    Self: Sized, 

Flattens the execution of this future when the successful result of this future is a Sink.

This can be useful when sink initialization is deferred, and it is convenient to work with that sink as if the sink was available at the call site.

Note that this function consumes this future and returns a wrapped version of it.

Examples

use futures::future::{Future, TryFutureExt};
use futures::sink::Sink;

fn make_sink_async() -> impl Future<Output = Result<
    impl Sink<T, Error = E>,
    E,
>> { // ... }
fn take_sink(sink: impl Sink<T, Error = E>) { /* ... */ }

let fut = make_sink_async();
take_sink(fut.flatten_sink())

Important traits for MapOk<Fut, F>

impl<Fut, F, T> Future for MapOk<Fut, F> where
    Fut: TryFuture,
    F: FnOnce(Fut::Ok) -> T,  type Output = Result<T, Fut::Error>;

fn map_ok<T, F>(self, f: F) -> MapOk<Self, F> where
    F: FnOnce(Self::Ok) -> T,
    Self: Sized, 

Maps this future's success value to a different value.

This method can be used to change the Ok type of the future into a different type. It is similar to the Result::map method. You can use this method to chain along a computation once the future has been resolved.

The provided closure f will only be called if this future is resolved to an Ok. If it resolves to an Err, panics, or is dropped, then the provided closure will never be invoked.

Note that this method consumes the future it is called on and returns a wrapped version of it.

Examples

use futures::future::TryFutureExt;

let future = async { Ok::<i32, i32>(1) };
let future = future.map_ok(|x| x + 3);
assert_eq!(future.await, Ok(4));

Calling map_ok on an errored future has no effect:

use futures::future::TryFutureExt;

let future = async { Err::<i32, i32>(1) };
let future = future.map_ok(|x| x + 3);
assert_eq!(future.await, Err(1));

Important traits for MapErr<Fut, F>

impl<Fut, F, E> Future for MapErr<Fut, F> where
    Fut: TryFuture,
    F: FnOnce(Fut::Error) -> E,  type Output = Result<Fut::Ok, E>;

fn map_err<E, F>(self, f: F) -> MapErr<Self, F> where
    F: FnOnce(Self::Error) -> E,
    Self: Sized, 

Maps this future's error value to a different value.

This method can be used to change the Error type of the future into a different type. It is similar to the Result::map_err method. You can use this method for example to ensure that futures have the same Error type when using select! or join!.

The provided closure f will only be called if this future is resolved to an Err. If it resolves to an Ok, panics, or is dropped, then the provided closure will never be invoked.

Note that this method consumes the future it is called on and returns a wrapped version of it.

Examples

use futures::future::TryFutureExt;

let future = async { Err::<i32, i32>(1) };
let future = future.map_err(|x| x + 3);
assert_eq!(future.await, Err(4));

Calling map_err on a successful future has no effect:

use futures::future::TryFutureExt;

let future = async { Ok::<i32, i32>(1) };
let future = future.map_err(|x| x + 3);
assert_eq!(future.await, Ok(1));

Important traits for ErrInto<Fut, E>

impl<Fut, E> Future for ErrInto<Fut, E> where
    Fut: TryFuture,
    Fut::Error: Into<E>,  type Output = Result<Fut::Ok, E>;

fn err_into<E>(self) -> ErrInto<Self, E> where
    Self: Sized,
    Self::Error: Into<E>, 

Maps this future's Error to a new error type using the Into trait.

This method does for futures what the ?-operator does for Result: It lets the compiler infer the type of the resulting error. Just as map_err, this is useful for example to ensure that futures have the same Error type when using select! or join!.

Note that this method consumes the future it is called on and returns a wrapped version of it.

Examples

use futures::future::TryFutureExt;

let future_err_u8 = async { Err::<(), u8>(1) };
let future_err_i32 = future_err_u8.err_into::<i32>();

Important traits for AndThen<Fut1, Fut2, F>

impl<Fut1, Fut2, F> Future for AndThen<Fut1, Fut2, F> where
    Fut1: TryFuture,
    Fut2: TryFuture<Error = Fut1::Error>,
    F: FnOnce(Fut1::Ok) -> Fut2,  type Output = Result<Fut2::Ok, Fut2::Error>;

fn and_then<Fut, F>(self, f: F) -> AndThen<Self, Fut, F> where
    F: FnOnce(Self::Ok) -> Fut,
    Fut: TryFuture<Error = Self::Error>,
    Self: Sized, 

Executes another future after this one resolves successfully. The success value is passed to a closure to create this subsequent future.

The provided closure f will only be called if this future is resolved to an Ok. If this future resolves to an Err, panics, or is dropped, then the provided closure will never be invoked. The Error type of this future and the future returned by f have to match.

Note that this method consumes the future it is called on and returns a wrapped version of it.

Examples

use futures::future::TryFutureExt;

let future = async { Ok::<i32, i32>(1) };
let future = future.and_then(|x| async move { Ok::<i32, i32>(x + 3) });
assert_eq!(future.await, Ok(4));

Calling and_then on an errored future has no effect:

use futures::future::TryFutureExt;

let future = async { Err::<i32, i32>(1) };
let future = future.and_then(|x| async move { Err::<i32, i32>(x + 3) });
assert_eq!(future.await, Err(1));

Important traits for OrElse<Fut1, Fut2, F>

impl<Fut1, Fut2, F> Future for OrElse<Fut1, Fut2, F> where
    Fut1: TryFuture,
    Fut2: TryFuture<Ok = Fut1::Ok>,
    F: FnOnce(Fut1::Error) -> Fut2,  type Output = Result<Fut2::Ok, Fut2::Error>;

fn or_else<Fut, F>(self, f: F) -> OrElse<Self, Fut, F> where
    F: FnOnce(Self::Error) -> Fut,
    Fut: TryFuture<Ok = Self::Ok>,
    Self: Sized, 

Executes another future if this one resolves to an error. The error value is passed to a closure to create this subsequent future.

The provided closure f will only be called if this future is resolved to an Err. If this future resolves to an Ok, panics, or is dropped, then the provided closure will never be invoked. The Ok type of this future and the future returned by f have to match.

Note that this method consumes the future it is called on and returns a wrapped version of it.

Examples

use futures::future::TryFutureExt;

let future = async { Err::<i32, i32>(1) };
let future = future.or_else(|x| async move { Err::<i32, i32>(x + 3) });
assert_eq!(future.await, Err(4));

Calling or_else on a successful future has no effect:

use futures::future::TryFutureExt;

let future = async { Ok::<i32, i32>(1) };
let future = future.or_else(|x| async move { Ok::<i32, i32>(x + 3) });
assert_eq!(future.await, Ok(1));

Important traits for InspectOk<Fut, F>

impl<Fut, F> Future for InspectOk<Fut, F> where
    Fut: TryFuture,
    F: FnOnce(&Fut::Ok),  type Output = Result<Fut::Ok, Fut::Error>;

fn inspect_ok<F>(self, f: F) -> InspectOk<Self, F> where
    F: FnOnce(&Self::Ok),
    Self: Sized, 

Do something with the success value of a future before passing it on.

When using futures, you'll often chain several of them together. While working on such code, you might want to check out what's happening at various parts in the pipeline, without consuming the intermediate value. To do that, insert a call to inspect_ok.

Examples

use futures::future::TryFutureExt;

let future = async { Ok::<_, ()>(1) };
let new_future = future.inspect_ok(|&x| println!("about to resolve: {}", x));
assert_eq!(new_future.await, Ok(1));

Important traits for InspectErr<Fut, F>

impl<Fut, F> Future for InspectErr<Fut, F> where
    Fut: TryFuture,
    F: FnOnce(&Fut::Error),  type Output = Result<Fut::Ok, Fut::Error>;

fn inspect_err<F>(self, f: F) -> InspectErr<Self, F> where
    F: FnOnce(&Self::Error),
    Self: Sized, 

Do something with the error value of a future before passing it on.

When using futures, you'll often chain several of them together. While working on such code, you might want to check out what's happening at various parts in the pipeline, without consuming the intermediate value. To do that, insert a call to inspect_err.

Examples

use futures::future::TryFutureExt;

let future = async { Err::<(), _>(1) };
let new_future = future.inspect_err(|&x| println!("about to error: {}", x));
assert_eq!(new_future.await, Err(1));

fn try_flatten_stream(self) -> TryFlattenStream<Self> where
    Self::Ok: TryStream<Error = Self::Error>,
    Self: Sized, 

Flatten the execution of this future when the successful result of this future is a stream.

This can be useful when stream initialization is deferred, and it is convenient to work with that stream as if stream was available at the call site.

Note that this function consumes this future and returns a wrapped version of it.

Examples

use futures::future::TryFutureExt;
use futures::stream::{self, TryStreamExt};

let stream_items = vec![17, 18, 19].into_iter().map(Ok);
let future_of_a_stream = async { Ok::<_, ()>(stream::iter(stream_items)) };

let stream = future_of_a_stream.try_flatten_stream();
let list = stream.try_collect::<Vec<_>>().await;
assert_eq!(list, Ok(vec![17, 18, 19]));

Important traits for UnwrapOrElse<Fut, F>

impl<Fut, F> Future for UnwrapOrElse<Fut, F> where
    Fut: TryFuture,
    F: FnOnce(Fut::Error) -> Fut::Ok,  type Output = Fut::Ok;

fn unwrap_or_else<F>(self, f: F) -> UnwrapOrElse<Self, F> where
    Self: Sized,
    F: FnOnce(Self::Error) -> Self::Ok, 

Unwraps this future's ouput, producing a future with this future's Ok type as its Output type.

If this future is resolved successfully, the returned future will contain the original future's success value as output. Otherwise, the closure f is called with the error value to produce an alternate success value.

This method is similar to the Result::unwrap_or_else method.

Examples

use futures::future::TryFutureExt;

let future = async { Err::<(), &str>("Boom!") };
let future = future.unwrap_or_else(|_| ());
assert_eq!(future.await, ());

Important traits for IntoFuture<Fut>

impl<Fut: TryFuture> Future for IntoFuture<Fut> type Output = Result<Fut::Ok, Fut::Error>;

fn into_future(self) -> IntoFuture<Self> where
    Self: Sized, 

Wraps a TryFuture into a type that implements Future.

TryFutures currently do not implement the Future trait due to limitations of the compiler.

Examples

use futures::future::{Future, TryFuture, TryFutureExt};

fn make_try_future() -> impl TryFuture<Ok = T, Error = E> { // ... }
fn take_future(future: impl Future<Output = Result<T, E>>) { /* ... */ }

take_future(make_try_future().into_future());

fn try_poll_unpin(
    &mut self,
    cx: &mut Context
) -> Poll<Result<Self::Ok, Self::Error>> where
    Self: Unpin, 

A convenience method for calling TryFuture::try_poll on Unpin future types.

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Implementors

impl<Fut: ?Sized + TryFuture> TryFutureExt for Fut

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