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//! This document is intended to let readers start working with Rust's `Future` quickly. Some other //! useful reading includes: //! //! - [The official Tokio documentation][tokio] //! - [Zero-cost futures in Rust][zero] //! - [Tokio Internals: Understanding Rust's asynchronous I/O framework from the bottom up][internals] //! //! [tokio]: https://tokio.rs/ //! [zero]: https://aturon.github.io/blog/2016/08/11/futures/ //! [internals]: https://cafbit.com/post/tokio_internals/ //! //! # `Future` //! //! The [`Future` trait][future] from [`futures`][futures] represents an asynchronous operation that //! can fail or succeed, producing a value either way. It is like an async version of //! [`Result`][result]. This document assumes that the reader is familiar with `Result`, which is //! [covered][result in trpl] 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`. //! //! [future]: https://docs.rs/futures/*/futures/future/trait.Future.html //! [futures]: https://docs.rs/futures //! [result]: https://doc.rust-lang.org/std/result/ //! [result in trpl]: https://doc.rust-lang.org/book/second-edition/ch09-02-recoverable-errors-with-result.html //! //! ```rust //! 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`: //! //! ```rust //! 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`: //! //! ```rust //! 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. //! //! ```rust //! 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. //! //! ```rust //! 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: //! //! ```text //! 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. //! //! ```rust //! 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. //! //! ```rust //! 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: //! //! ```text //! fn make_twelve() -> Future<Item=u64, Error=()> { //! unimplemented!() //! } //! ``` //! //! However, the compiler doesn't like that: //! //! ```text //! 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][impl trait] functionality. //! //![impl trait]: https://internals.rust-lang.org/t/help-test-impl-trait/6516 //! //! ```rust //! 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`. //! //! ```rust //! 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`][tokio-core] crate has a struct called [`Core`][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. //! //! [tokio-core]: https://docs.rs/tokio-core //! [core]: https://docs.rs/tokio-core/*/tokio_core/reactor/struct.Core.html //! //! ```rust //! 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); //! } //! ``` //! #![deny(warnings)] extern crate futures; extern crate tokio_core; use futures::future::FutureResult; // Ideally impl Trait will prevent us from needing to be aware of FutureResult. type ExampleFuture = FutureResult<usize, ExampleFutureError>; #[derive(Debug, PartialEq)] pub enum ExampleFutureError { Oops, } pub fn new_example_future() -> ExampleFuture { futures::future::ok(2) } pub fn new_example_future_err() -> ExampleFuture { futures::future::err(ExampleFutureError::Oops) }