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//! Futures
//!
//! This module contains a number of functions for working with `Future`s,
//! including the `FutureExt` trait which adds methods to `Future` types.
#[cfg(feature = "compat")]
use crate::compat::Compat;
use core::pin::Pin;
use futures_core::{
future::TryFuture,
stream::TryStream,
task::{Context, Poll},
};
#[cfg(feature = "sink")]
use futures_sink::Sink;
use crate::fns::{
inspect_err_fn, inspect_ok_fn, into_fn, map_err_fn, map_ok_fn, map_ok_or_else_fn,
unwrap_or_else_fn, InspectErrFn, InspectOkFn, IntoFn, MapErrFn, MapOkFn, MapOkOrElseFn,
UnwrapOrElseFn,
};
use crate::future::{assert_future, Inspect, Map};
use crate::stream::assert_stream;
// Combinators
mod into_future;
mod try_flatten;
mod try_flatten_err;
delegate_all!(
/// Future for the [`try_flatten`](TryFutureExt::try_flatten) method.
TryFlatten<Fut1, Fut2>(
try_flatten::TryFlatten<Fut1, Fut2>
): Debug + Future + FusedFuture + New[|x: Fut1| try_flatten::TryFlatten::new(x)]
);
delegate_all!(
/// Future for the [`try_flatten_err`](TryFutureExt::try_flatten_err) method.
TryFlattenErr<Fut1, Fut2>(
try_flatten_err::TryFlattenErr<Fut1, Fut2>
): Debug + Future + FusedFuture + New[|x: Fut1| try_flatten_err::TryFlattenErr::new(x)]
);
delegate_all!(
/// Future for the [`try_flatten_stream`](TryFutureExt::try_flatten_stream) method.
TryFlattenStream<Fut>(
try_flatten::TryFlatten<Fut, Fut::Ok>
): Debug + Sink + Stream + FusedStream + New[|x: Fut| try_flatten::TryFlatten::new(x)]
where Fut: TryFuture
);
#[cfg(feature = "sink")]
delegate_all!(
/// Sink for the [`flatten_sink`](TryFutureExt::flatten_sink) method.
#[cfg_attr(docsrs, doc(cfg(feature = "sink")))]
FlattenSink<Fut, Si>(
try_flatten::TryFlatten<Fut, Si>
): Debug + Sink + Stream + FusedStream + New[|x: Fut| try_flatten::TryFlatten::new(x)]
);
delegate_all!(
/// Future for the [`and_then`](TryFutureExt::and_then) method.
AndThen<Fut1, Fut2, F>(
TryFlatten<MapOk<Fut1, F>, Fut2>
): Debug + Future + FusedFuture + New[|x: Fut1, f: F| TryFlatten::new(MapOk::new(x, f))]
);
delegate_all!(
/// Future for the [`or_else`](TryFutureExt::or_else) method.
OrElse<Fut1, Fut2, F>(
TryFlattenErr<MapErr<Fut1, F>, Fut2>
): Debug + Future + FusedFuture + New[|x: Fut1, f: F| TryFlattenErr::new(MapErr::new(x, f))]
);
delegate_all!(
/// Future for the [`err_into`](TryFutureExt::err_into) method.
ErrInto<Fut, E>(
MapErr<Fut, IntoFn<E>>
): Debug + Future + FusedFuture + New[|x: Fut| MapErr::new(x, into_fn())]
);
delegate_all!(
/// Future for the [`ok_into`](TryFutureExt::ok_into) method.
OkInto<Fut, E>(
MapOk<Fut, IntoFn<E>>
): Debug + Future + FusedFuture + New[|x: Fut| MapOk::new(x, into_fn())]
);
delegate_all!(
/// Future for the [`inspect_ok`](super::TryFutureExt::inspect_ok) method.
InspectOk<Fut, F>(
Inspect<IntoFuture<Fut>, InspectOkFn<F>>
): Debug + Future + FusedFuture + New[|x: Fut, f: F| Inspect::new(IntoFuture::new(x), inspect_ok_fn(f))]
);
delegate_all!(
/// Future for the [`inspect_err`](super::TryFutureExt::inspect_err) method.
InspectErr<Fut, F>(
Inspect<IntoFuture<Fut>, InspectErrFn<F>>
): Debug + Future + FusedFuture + New[|x: Fut, f: F| Inspect::new(IntoFuture::new(x), inspect_err_fn(f))]
);
pub use self::into_future::IntoFuture;
delegate_all!(
/// Future for the [`map_ok`](TryFutureExt::map_ok) method.
MapOk<Fut, F>(
Map<IntoFuture<Fut>, MapOkFn<F>>
): Debug + Future + FusedFuture + New[|x: Fut, f: F| Map::new(IntoFuture::new(x), map_ok_fn(f))]
);
delegate_all!(
/// Future for the [`map_err`](TryFutureExt::map_err) method.
MapErr<Fut, F>(
Map<IntoFuture<Fut>, MapErrFn<F>>
): Debug + Future + FusedFuture + New[|x: Fut, f: F| Map::new(IntoFuture::new(x), map_err_fn(f))]
);
delegate_all!(
/// Future for the [`map_ok_or_else`](TryFutureExt::map_ok_or_else) method.
MapOkOrElse<Fut, F, G>(
Map<IntoFuture<Fut>, MapOkOrElseFn<F, G>>
): Debug + Future + FusedFuture + New[|x: Fut, f: F, g: G| Map::new(IntoFuture::new(x), map_ok_or_else_fn(f, g))]
);
delegate_all!(
/// Future for the [`unwrap_or_else`](TryFutureExt::unwrap_or_else) method.
UnwrapOrElse<Fut, F>(
Map<IntoFuture<Fut>, UnwrapOrElseFn<F>>
): Debug + Future + FusedFuture + New[|x: Fut, f: F| Map::new(IntoFuture::new(x), unwrap_or_else_fn(f))]
);
impl<Fut: ?Sized + TryFuture> TryFutureExt for Fut {}
/// Adapters specific to [`Result`]-returning futures
pub trait TryFutureExt: TryFuture {
/// 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;
/// # use futures::channel::mpsc::{self, SendError};
/// # type T = i32;
/// # type E = SendError;
///
/// fn make_sink_async() -> impl Future<Output = Result<
/// impl Sink<T, Error = E>,
/// E,
/// >> { // ... }
/// # let (tx, _rx) = mpsc::unbounded::<i32>();
/// # futures::future::ready(Ok(tx))
/// # }
/// fn take_sink(sink: impl Sink<T, Error = E>) { /* ... */ }
///
/// let fut = make_sink_async();
/// take_sink(fut.flatten_sink())
/// ```
#[cfg(feature = "sink")]
#[cfg_attr(docsrs, doc(cfg(feature = "sink")))]
fn flatten_sink<Item>(self) -> FlattenSink<Self, Self::Ok>
where
Self::Ok: Sink<Item, Error = Self::Error>,
Self: Sized,
{
crate::sink::assert_sink::<Item, Self::Error, _>(FlattenSink::new(self))
}
/// Maps this future's success value to a different value.
///
/// This method can be used to change the [`Ok`](TryFuture::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;
///
/// # futures::executor::block_on(async {
/// let future = async { Ok::<i32, i32>(1) };
/// let future = future.map_ok(|x| x + 3);
/// assert_eq!(future.await, Ok(4));
/// # });
/// ```
///
/// Calling [`map_ok`](TryFutureExt::map_ok) on an errored future has no
/// effect:
///
/// ```
/// use futures::future::TryFutureExt;
///
/// # futures::executor::block_on(async {
/// let future = async { Err::<i32, i32>(1) };
/// let future = future.map_ok(|x| x + 3);
/// assert_eq!(future.await, Err(1));
/// # });
/// ```
fn map_ok<T, F>(self, f: F) -> MapOk<Self, F>
where
F: FnOnce(Self::Ok) -> T,
Self: Sized,
{
assert_future::<Result<T, Self::Error>, _>(MapOk::new(self, f))
}
/// Maps this future's success value to a different value, and permits for error handling resulting in the same type.
///
/// This method can be used to coalesce your [`Ok`](TryFuture::Ok) type and [`Error`](TryFuture::Error) into another type,
/// where that type is the same for both outcomes.
///
/// 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.
///
/// The provided closure `e` 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;
///
/// # futures::executor::block_on(async {
/// let future = async { Ok::<i32, i32>(5) };
/// let future = future.map_ok_or_else(|x| x * 2, |x| x + 3);
/// assert_eq!(future.await, 8);
///
/// let future = async { Err::<i32, i32>(5) };
/// let future = future.map_ok_or_else(|x| x * 2, |x| x + 3);
/// assert_eq!(future.await, 10);
/// # });
/// ```
///
fn map_ok_or_else<T, E, F>(self, e: E, f: F) -> MapOkOrElse<Self, F, E>
where
F: FnOnce(Self::Ok) -> T,
E: FnOnce(Self::Error) -> T,
Self: Sized,
{
assert_future::<T, _>(MapOkOrElse::new(self, f, e))
}
/// Maps this future's error value to a different value.
///
/// This method can be used to change the [`Error`](TryFuture::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`](TryFuture::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;
///
/// # futures::executor::block_on(async {
/// let future = async { Err::<i32, i32>(1) };
/// let future = future.map_err(|x| x + 3);
/// assert_eq!(future.await, Err(4));
/// # });
/// ```
///
/// Calling [`map_err`](TryFutureExt::map_err) on a successful future has
/// no effect:
///
/// ```
/// use futures::future::TryFutureExt;
///
/// # futures::executor::block_on(async {
/// let future = async { Ok::<i32, i32>(1) };
/// let future = future.map_err(|x| x + 3);
/// assert_eq!(future.await, Ok(1));
/// # });
/// ```
///
/// [`join!`]: crate::join
/// [`select!`]: crate::select
fn map_err<E, F>(self, f: F) -> MapErr<Self, F>
where
F: FnOnce(Self::Error) -> E,
Self: Sized,
{
assert_future::<Result<Self::Ok, E>, _>(MapErr::new(self, f))
}
/// Maps this future's [`Error`](TryFuture::Error) to a new error type
/// using the [`Into`](std::convert::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`](TryFutureExt::map_err), this is useful for
/// example to ensure that futures have the same [`Error`](TryFuture::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;
///
/// # futures::executor::block_on(async {
/// let future_err_u8 = async { Err::<(), u8>(1) };
/// let future_err_i32 = future_err_u8.err_into::<i32>();
/// # });
/// ```
///
/// [`join!`]: crate::join
/// [`select!`]: crate::select
fn err_into<E>(self) -> ErrInto<Self, E>
where
Self: Sized,
Self::Error: Into<E>,
{
assert_future::<Result<Self::Ok, E>, _>(ErrInto::new(self))
}
/// Maps this future's [`Ok`](TryFuture::Ok) to a new type
/// using the [`Into`](std::convert::Into) trait.
fn ok_into<U>(self) -> OkInto<Self, U>
where
Self: Sized,
Self::Ok: Into<U>,
{
assert_future::<Result<U, Self::Error>, _>(OkInto::new(self))
}
/// 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`](TryFuture::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;
///
/// # futures::executor::block_on(async {
/// 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`](TryFutureExt::and_then) on an errored future has no
/// effect:
///
/// ```
/// use futures::future::TryFutureExt;
///
/// # futures::executor::block_on(async {
/// 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));
/// # });
/// ```
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,
{
assert_future::<Result<Fut::Ok, Fut::Error>, _>(AndThen::new(self, f))
}
/// 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`](TryFuture::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;
///
/// # futures::executor::block_on(async {
/// 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`](TryFutureExt::or_else) on a successful future has
/// no effect:
///
/// ```
/// use futures::future::TryFutureExt;
///
/// # futures::executor::block_on(async {
/// 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));
/// # });
/// ```
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,
{
assert_future::<Result<Fut::Ok, Fut::Error>, _>(OrElse::new(self, f))
}
/// 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
///
/// ```
/// # futures::executor::block_on(async {
/// 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));
/// # });
/// ```
fn inspect_ok<F>(self, f: F) -> InspectOk<Self, F>
where
F: FnOnce(&Self::Ok),
Self: Sized,
{
assert_future::<Result<Self::Ok, Self::Error>, _>(InspectOk::new(self, f))
}
/// 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
///
/// ```
/// # futures::executor::block_on(async {
/// 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 inspect_err<F>(self, f: F) -> InspectErr<Self, F>
where
F: FnOnce(&Self::Error),
Self: Sized,
{
assert_future::<Result<Self::Ok, Self::Error>, _>(InspectErr::new(self, f))
}
/// Flatten the execution of this future when the successful result of this
/// future is another future.
///
/// This is equivalent to `future.and_then(|x| x)`.
fn try_flatten(self) -> TryFlatten<Self, Self::Ok>
where
Self::Ok: TryFuture<Error = Self::Error>,
Self: Sized,
{
assert_future::<Result<<Self::Ok as TryFuture>::Ok, Self::Error>, _>(TryFlatten::new(self))
}
/// 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
///
/// ```
/// # futures::executor::block_on(async {
/// 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]));
/// # });
/// ```
fn try_flatten_stream(self) -> TryFlattenStream<Self>
where
Self::Ok: TryStream<Error = Self::Error>,
Self: Sized,
{
assert_stream::<Result<<Self::Ok as TryStream>::Ok, Self::Error>, _>(TryFlattenStream::new(
self,
))
}
/// Unwraps this future's output, producing a future with this future's
/// [`Ok`](TryFuture::Ok) type as its
/// [`Output`](std::future::Future::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;
///
/// # futures::executor::block_on(async {
/// let future = async { Err::<(), &str>("Boom!") };
/// let future = future.unwrap_or_else(|_| ());
/// assert_eq!(future.await, ());
/// # });
/// ```
fn unwrap_or_else<F>(self, f: F) -> UnwrapOrElse<Self, F>
where
Self: Sized,
F: FnOnce(Self::Error) -> Self::Ok,
{
assert_future::<Self::Ok, _>(UnwrapOrElse::new(self, f))
}
/// Wraps a [`TryFuture`] into a future compatible with libraries using
/// futures 0.1 future definitions. Requires the `compat` feature to enable.
#[cfg(feature = "compat")]
#[cfg_attr(docsrs, doc(cfg(feature = "compat")))]
fn compat(self) -> Compat<Self>
where
Self: Sized + Unpin,
{
Compat::new(self)
}
/// Wraps a [`TryFuture`] into a type that implements
/// [`Future`](std::future::Future).
///
/// [`TryFuture`]s currently do not implement the
/// [`Future`](std::future::Future) trait due to limitations of the
/// compiler.
///
/// # Examples
///
/// ```
/// use futures::future::{Future, TryFuture, TryFutureExt};
///
/// # type T = i32;
/// # type E = ();
/// fn make_try_future() -> impl TryFuture<Ok = T, Error = E> { // ... }
/// # async { Ok::<i32, ()>(1) }
/// # }
/// fn take_future(future: impl Future<Output = Result<T, E>>) { /* ... */ }
///
/// take_future(make_try_future().into_future());
/// ```
fn into_future(self) -> IntoFuture<Self>
where
Self: Sized,
{
assert_future::<Result<Self::Ok, Self::Error>, _>(IntoFuture::new(self))
}
/// A convenience method for calling [`TryFuture::try_poll`] on [`Unpin`]
/// future types.
fn try_poll_unpin(&mut self, cx: &mut Context<'_>) -> Poll<Result<Self::Ok, Self::Error>>
where
Self: Unpin,
{
Pin::new(self).try_poll(cx)
}
}