1 unstable release
Uses old Rust 2015
0.1.0 | Jan 21, 2018 |
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#1172 in Asynchronous
24KB
future-by-example
This document is intended to let readers start working with Rust's Future
quickly. Some other
useful reading includes:
- The official Tokio documentation
- Zero-cost futures in Rust
- Tokio Internals: Understanding Rust's asynchronous I/O framework from the bottom up
Future
The Future
trait from futures
represents an asynchronous operation that
can fail or succeed, producing a value either way. It is like an async version of
Result
. This document assumes that the reader is familiar with Result
, which is
covered in the second edition of The Rust Programming Language.
One of the most common questions about Future
seems to be, "how do I get the value out of it?"
The easiest way to do this is to call the wait
method. This runs the Future
in the current
thread, blocking all other work until it is finished.
This is not frequently the best way to run a Future
, because no other work can happen
until the Future
completes, which completely defeats the point of using asynchronous
programming in the first place. However, it can be useful in unit tests, when debugging, or at
the top level of a simple application.
See the section on reactors for better ways to run a Future
.
extern crate futures;
extern crate future_by_example;
fn main() {
use futures::Future;
use future_by_example::new_example_future;
let future = new_example_future();
let expected = Ok(2);
assert_eq!(future.wait(), expected);
}
A Future
can be modified using many functions analogous to those of Result
, such as map
,
map_err
, and then
. Here's map
:
extern crate futures;
extern crate future_by_example;
fn main() {
use futures::Future;
use future_by_example::new_example_future;
let future = new_example_future();
let mapped = future.map(|i| i * 3);
let expected = Ok(6);
assert_eq!(mapped.wait(), expected);
}
Like a Result
, two Future
s can be combined using and_then
and or_else
:
extern crate futures;
extern crate future_by_example;
fn main() {
use futures::Future;
use future_by_example::{new_example_future, new_example_future_err, ExampleFutureError};
let good = new_example_future();
let bad = new_example_future_err();
let both = good.and_then(|good| bad);
let expected = Err(ExampleFutureError::Oops);
assert_eq!(both.wait(), expected);
}
Future
also has a lot of functions that have no analog in Result
. Because we're talking
about aynchronous programming, now we have to choose whether we want to run two independent
operations one after the other (in sequence), or at the same time (in parallel).
For example, to get the results of two independent Future
s, we could use and_then
to run
them in sequence. However, that strategy is silly, because we are only making progress on one
Future
at a time. Why not run both at the same time?
Future::join
creates a new Future
that contains the results of two other Future
s.
Importantly, both of the input Future
s can make progress at the same time. The new Future
completes only when both input Future
s complete. There's also join3
, join4
and join5
for
joining larger numbers of Future
s.
extern crate futures;
extern crate future_by_example;
fn main() {
use futures::Future;
use futures::future::ok;
use future_by_example::new_example_future;
let future1 = new_example_future();
let future2 = new_example_future();
let joined = future1.join(future2);
let (value1, value2) = joined.wait().unwrap();
assert_eq!(value1, value2);
}
Whereas join
completes when both Future
s are complete, select
returns whichever
of two Future
s completes first. This is useful for implementing timeouts, among other things.
select2
is like select
except that the two Future
s can have different value types.
Creating a Future
Many libraries return Future
s for asynchronous operations such as network calls. Sometimes you
may want to create your own Future
. Implementing a Future
from scratch is difficult, but
there are other ways to create futures.
You can easily create a Future
from a value that is already available using the ok
function.
There are similiar err
and result
methods.
extern crate futures;
fn main() {
use futures::Future;
use futures::future::ok;
// Here I specify the type of the error as (); otherwise the compiler can't infer it
let future = ok::<_, ()>(String::from("hello"));
assert_eq!(Ok(String::from("hello")), future.wait());
}
Futures and types
Working with Future
s tends to produce complex types. For example, the full type of the
expression below is actually:
futures::Map<
futures::Map<
futures::Join<
futures::FutureResult<u64, ()>,
futures::FutureResult<u64, ()>
>,
[closure@src/lib.rs:...]>,
[closure@src/lib.rs:...]
>
That is, for every transformation, we add another layer to the type of our Future
! This can
sometimes be confusing. In particular, it can be challenging to identify ways to write out the
types that aren't brittle or verbose.
In order to help the Rust compiler do type inference, below we have specify the type of
expected
. It's much terser than writing the full type out, and adding another operation won't
break compilation.
extern crate futures;
fn main() {
use futures::future::ok;
use futures::Future;
let expected: Result<u64, ()> = Ok(6);
assert_eq!(
ok(5).join(ok(7)).map(|(x, y)| x + y).map(|z| z / 2).wait(),
expected
)
}
Alternatively, we can make use of _
to let the Rust compiler infer types for us.
extern crate futures;
fn main() {
use futures::future::ok;
use futures::Future;
use futures::Map;
let expected: Result<_, ()> = Ok(6);
let twelve: Map<_, _> = ok(5).join(ok(7)).map(|(x, y)| x + y);
assert_eq!(twelve.map(|z| z / 2).wait(), expected)
}
Rust requires that all types in function signatures are specified.
One way to achieve this for functions that return Future
s is to specify the full return
type in the function signature. However, specifying the exact type can be verbose, brittle, and
difficult.
It would be nice to be able to define a function like this:
fn make_twelve() -> Future<Item=u64, Error=()> {
unimplemented!()
}
However, the compiler doesn't like that:
error[E0277]: the trait bound `futures::Future<Item=u64, Error=()>: std::marker::Sized` is not satisfied
--> src/lib.rs:119:13
|
119 | let twelve = make_twelve();
| ^^^^^^ `futures::Future<Item=u64, Error=()>` does not have a constant size known at compile-time
|
= help: the trait `std::marker::Sized` is not implemented for `futures::Future<Item=u64, Error=()>`
= note: all local variables must have a statically known size
This can be solved by wrapping the return type in a Box
. One day, this will be solved in a
more elegant way with the currently unstable impl Trait functionality.
extern crate futures;
fn main() {
use futures::Future;
use futures::future::ok;
fn make_twelve() -> Box<Future<Item=u64, Error=()>> {
ok(5).join(ok(7)).map(|(x, y)| x + y).boxed()
}
let twelve = make_twelve();
assert_eq!(twelve.map(|z| z / 2).wait(), Ok(6))
}
Unlike functions, closures do not require all types in their signatures to be explicitly
defined, so they don't need to be wrapped in a Box
.
extern crate futures;
fn main() {
use futures::Future;
let make_twelve = || {
use futures::future::ok;
// We don't need to put our `Future` inside of a `Box` here.
ok(5).join(ok(7)).map(|(x, y)| x + y)
};
let expected: Result<u64, ()> = Ok(6);
let twelve = make_twelve();
assert_eq!(twelve.map(|z| z / 2).wait(), expected)
}
A more powerful way to run Futures
Composing a bunch of Futures
into a single Future
and calling wait
on it is a simple and
easy method as long as you only need to run a single Future
at a time. However, if you only
need to run a single Future
at a time, perhaps you don't need the futures
crate in the first
place! The futures
crate promises to efficiently juggle many concurrent tasks, so let's
see how that might work.
The tokio-core
crate has a struct called Core
which can run multiple
Future
s concurrently. Core::run
runs a Future
, returning its value. Unlike Future::wait
,
though, it allows the Core
to make progress on executing other Future
objects while run
running. The Future
in Core::run
is the main event loop, and it may request that new
Future
s be run by calling Handle::spawn
. Note that the Future
s run by spawn
don't get to
return a value; they exist only to perform side effects.
extern crate futures;
extern crate tokio_core;
fn main() {
use tokio_core::reactor::Core;
use futures::future::lazy;
let mut core = Core::new().unwrap();
let handle = core.handle();
let future = lazy(|| {
handle.spawn(lazy(|| {
Ok(()) // Ok(()) implements FromResult
}));
Ok(2)
});
let expected: Result<_, ()> = Ok(2usize);
assert_eq!(core.run(future), expected);
}
License: MIT/Apache-2.0
Dependencies
~6MB
~94K SLoC