Crate futures [−] [src]
Zero-cost Futures in Rust
This library is an implementation of futures in Rust which aims to provide a robust implementation of handling asynchronous computations, ergonomic composition and usage, and zero-cost abstractions over what would otherwise be written by hand.
Futures are a concept for an object which is a proxy for another value that may not be ready yet. For example issuing an HTTP request may return a future for the HTTP response, as it probably hasn't arrived yet. With an object representing a value that will eventually be available, futures allow for powerful composition of tasks through basic combinators that can perform operations like chaining computations, changing the types of futures, or waiting for two futures to complete at the same time.
Installation
Add this to your Cargo.toml
:
[dependencies]
futures = "0.1"
Examples
Let's take a look at a few examples of how futures might be used:
extern crate futures; use std::io; use std::time::Duration; use futures::{Future, Map}; // A future is actually a trait implementation, so we can generically take a // future of any integer and return back a future that will resolve to that // value plus 10 more. // // Note here that like iterators, we're returning the `Map` combinator in // the futures crate, not a boxed abstraction. This is a zero-cost // construction of a future. fn add_ten<F>(future: F) -> Map<F, fn(i32) -> i32> where F: Future<Item=i32>, { fn add(a: i32) -> i32 { a + 10 } future.map(add) } // Not only can we modify one future, but we can even compose them together! // Here we have a function which takes two futures as input, and returns a // future that will calculate the sum of their two values. // // Above we saw a direct return value of the `Map` combinator, but // performance isn't always critical and sometimes it's more ergonomic to // return a trait object like we do here. Note though that there's only one // allocation here, not any for the intermediate futures. fn add<'a, A, B>(a: A, b: B) -> Box<Future<Item=i32, Error=A::Error> + 'a> where A: Future<Item=i32> + 'a, B: Future<Item=i32, Error=A::Error> + 'a, { Box::new(a.join(b).map(|(a, b)| a + b)) } // Futures also allow chaining computations together, starting another after // the previous finishes. Here we wait for the first computation to finish, // and then decide what to do depending on the result. fn download_timeout(url: &str, timeout_dur: Duration) -> Box<Future<Item=Vec<u8>, Error=io::Error>> { use std::io; use std::net::{SocketAddr, TcpStream}; type IoFuture<T> = Box<Future<Item=T, Error=io::Error>>; // First thing to do is we need to resolve our URL to an address. This // will likely perform a DNS lookup which may take some time. let addr = resolve(url); // After we acquire the address, we next want to open up a TCP // connection. let tcp = addr.and_then(|addr| connect(&addr)); // After the TCP connection is established and ready to go, we're off to // the races! let data = tcp.and_then(|conn| download(conn)); // That all might take awhile, though, so let's not wait too long for it // to all come back. The `select` combinator here returns a future which // resolves to the first value that's ready plus the next future. // // Note we can also use the `then` combinator which which is similar to // `and_then` above except that it receives the result of the // computation, not just the successful value. // // Again note that all the above calls to `and_then` and the below calls // to `map` and such require no allocations. We only ever allocate once // we hit the `.boxed()` call at the end here, which means we've built // up a relatively involved computation with only one box, and even that // was optional! let data = data.map(Ok); let timeout = timeout(timeout_dur).map(Err); let ret = data.select(timeout).then(|result| { match result { // One future succeeded, and it was the one which was // downloading data from the connection. Ok((Ok(data), _other_future)) => Ok(data), // The timeout fired, and otherwise no error was found, so // we translate this to an error. Ok((Err(_timeout), _other_future)) => { Err(io::Error::new(io::ErrorKind::Other, "timeout")) } // A normal I/O error happened, so we pass that on through. Err((e, _other_future)) => Err(e), } }); return Box::new(ret); fn resolve(url: &str) -> IoFuture<SocketAddr> { // ... } fn connect(hostname: &SocketAddr) -> IoFuture<TcpStream> { // ... } fn download(stream: TcpStream) -> IoFuture<Vec<u8>> { // ... } fn timeout(stream: Duration) -> IoFuture<()> { // ... } }Run
Some more information can also be found in the README for now, but otherwise feel free to jump in to the docs below!
Modules
stream |
Asynchronous streams |
task |
Tasks used to drive a future computation |
Macros
task_local |
A macro to create a |
try_ready |
A macro for extracting the successful type of a |
Structs
AndThen |
Future for the |
Canceled |
Error returned from a |
CatchUnwind |
Future for the |
Collect |
A future which takes a list of futures and resolves with a vector of the completed values. |
Complete |
Represents the completion half of a oneshot through which the result of a computation is signaled. |
Done |
A future representing a value that is immediately ready. |
Empty |
A future which is never resolved. |
Failed |
A future representing a finished but erroneous computation. |
Finished |
A future representing a finished successful computation. |
Flatten |
Future for the |
FlattenStream |
Future for the |
Fuse |
A future which "fuse"s a future once it's been resolved. |
IntoStream |
Future that forwards one element from the underlying future (whether it is success of error) and emits EOF after that. |
Join |
Future for the |
Join3 |
Future for the |
Join4 |
Future for the |
Join5 |
Future for the |
Lazy |
A future which defers creation of the actual future until a callback is scheduled. |
Map |
Future for the |
MapErr |
Future for the |
Oneshot |
A future representing the completion of a computation happening elsewhere in memory. |
OrElse |
Future for the |
Select |
Future for the |
SelectAll |
Future for the |
SelectAllNext |
Future yielded as the result in a |
SelectNext |
Future yielded as the second result in a |
Then |
Future for the |
Enums
Async |
Return type of future, indicating whether a value is ready or not. |
Traits
Future |
Trait for types which are a placeholder of a value that will become available at possible some later point in time. |
IntoFuture |
Class of types which can be converted themselves into a future. |
Functions
collect |
Creates a future which represents a collection of the results of the futures given. |
done |
Creates a new "leaf future" which will resolve with the given result. |
empty |
Creates a future which never resolves, representing a computation that never finishes. |
failed |
Creates a "leaf future" from an immediate value of a failed computation. |
finished |
Creates a "leaf future" from an immediate value of a finished and successful computation. |
lazy |
Creates a new future which will eventually be the same as the one created by the closure provided. |
oneshot |
Creates a new in-memory oneshot used to represent completing a computation. |
select_all |
Creates a new future which will select over a list of futures. |
Type Definitions
BoxFuture |
A type alias for |
Poll |
Return type of the |