mirror of https://github.com/rust-lang/book
1274 lines
48 KiB
Markdown
1274 lines
48 KiB
Markdown
<!-- DO NOT EDIT THIS FILE.
|
||
|
||
This file is periodically generated from the content in the `/src/`
|
||
directory, so all fixes need to be made in `/src/`.
|
||
-->
|
||
|
||
[TOC]
|
||
|
||
# Functional Language Features: Iterators and Closures
|
||
|
||
Rust’s design has taken inspiration from many existing languages and
|
||
techniques, and one significant influence is *functional programming*.
|
||
Programming in a functional style often includes using functions as values by
|
||
passing them in arguments, returning them from other functions, assigning them
|
||
to variables for later execution, and so forth.
|
||
|
||
In this chapter, we won’t debate the issue of what functional programming is or
|
||
isn’t but will instead discuss some features of Rust that are similar to
|
||
features in many languages often referred to as functional.
|
||
|
||
More specifically, we’ll cover:
|
||
|
||
* *Closures*, a function-like construct you can store in a variable
|
||
* *Iterators*, a way of processing a series of elements
|
||
* How to use closures and iterators to improve the I/O project in Chapter 12
|
||
* The performance of closures and iterators (spoiler alert: they’re faster than
|
||
you might think!)
|
||
|
||
We’ve already covered some other Rust features, such as pattern matching and
|
||
enums, that are also influenced by the functional style. Because mastering
|
||
closures and iterators is an important part of writing idiomatic, fast Rust
|
||
code, we’ll devote this entire chapter to them.
|
||
|
||
## Closures: Anonymous Functions That Capture Their Environment
|
||
|
||
Rust’s closures are anonymous functions you can save in a variable or pass as
|
||
arguments to other functions. You can create the closure in one place and then
|
||
call the closure elsewhere to evaluate it in a different context. Unlike
|
||
functions, closures can capture values from the scope in which they’re defined.
|
||
We’ll demonstrate how these closure features allow for code reuse and behavior
|
||
customization.
|
||
|
||
### Capturing the Environment with Closures
|
||
|
||
We’ll first examine how we can use closures to capture values from the
|
||
environment they’re defined in for later use. Here’s the scenario: every so
|
||
often, our T-shirt company gives away an exclusive, limited-edition shirt to
|
||
someone on our mailing list as a promotion. People on the mailing list can
|
||
optionally add their favorite color to their profile. If the person chosen for
|
||
a free shirt has their favorite color set, they get that color shirt. If the
|
||
person hasn’t specified a favorite color, they get whatever color the company
|
||
currently has the most of.
|
||
|
||
There are many ways to implement this. For this example, we’re going to use an
|
||
enum called `ShirtColor` that has the variants `Red` and `Blue` (limiting the
|
||
number of colors available for simplicity). We represent the company’s
|
||
inventory with an `Inventory` struct that has a field named `shirts` that
|
||
contains a `Vec<ShirtColor>` representing the shirt colors currently in stock.
|
||
The method `giveaway` defined on `Inventory` gets the optional shirt color
|
||
preference of the free-shirt winner, and returns the shirt color the person
|
||
will get. This setup is shown in Listing 13-1.
|
||
|
||
Filename: src/main.rs
|
||
|
||
```
|
||
#[derive(Debug, PartialEq, Copy, Clone)]
|
||
enum ShirtColor {
|
||
Red,
|
||
Blue,
|
||
}
|
||
|
||
struct Inventory {
|
||
shirts: Vec<ShirtColor>,
|
||
}
|
||
|
||
impl Inventory {
|
||
fn giveaway(
|
||
&self,
|
||
user_preference: Option<ShirtColor>,
|
||
) -> ShirtColor {
|
||
1 user_preference.unwrap_or_else(|| self.most_stocked())
|
||
}
|
||
|
||
fn most_stocked(&self) -> ShirtColor {
|
||
let mut num_red = 0;
|
||
let mut num_blue = 0;
|
||
|
||
for color in &self.shirts {
|
||
match color {
|
||
ShirtColor::Red => num_red += 1,
|
||
ShirtColor::Blue => num_blue += 1,
|
||
}
|
||
}
|
||
if num_red > num_blue {
|
||
ShirtColor::Red
|
||
} else {
|
||
ShirtColor::Blue
|
||
}
|
||
}
|
||
}
|
||
|
||
fn main() {
|
||
let store = Inventory {
|
||
2 shirts: vec![
|
||
ShirtColor::Blue,
|
||
ShirtColor::Red,
|
||
ShirtColor::Blue,
|
||
],
|
||
};
|
||
|
||
let user_pref1 = Some(ShirtColor::Red);
|
||
3 let giveaway1 = store.giveaway(user_pref1);
|
||
println!(
|
||
"The user with preference {:?} gets {:?}",
|
||
user_pref1, giveaway1
|
||
);
|
||
|
||
let user_pref2 = None;
|
||
4 let giveaway2 = store.giveaway(user_pref2);
|
||
println!(
|
||
"The user with preference {:?} gets {:?}",
|
||
user_pref2, giveaway2
|
||
);
|
||
}
|
||
```
|
||
|
||
Listing 13-1: Shirt company giveaway situation
|
||
|
||
The `store` defined in `main` has two blue shirts and one red shirt remaining
|
||
to distribute for this limited-edition promotion [2]. We call the `giveaway`
|
||
method for a user with a preference for a red shirt [3] and a user without any
|
||
preference [4].
|
||
|
||
Again, this code could be implemented in many ways, and here, to focus on
|
||
closures, we’ve stuck to concepts you’ve already learned, except for the body
|
||
of the `giveaway` method that uses a closure. In the `giveaway` method, we get
|
||
the user preference as a parameter of type `Option<ShirtColor>` and call the
|
||
`unwrap_or_else` method on `user_preference` [1]. The `unwrap_or_else` method
|
||
on `Option<T>` is defined by the standard library. It takes one argument: a
|
||
closure without any arguments that returns a value `T` (the same type stored in
|
||
the `Some` variant of the `Option<T>`, in this case `ShirtColor`). If the
|
||
`Option<T>` is the `Some` variant, `unwrap_or_else` returns the value from
|
||
within the `Some`. If the `Option<T>` is the `None` variant, `unwrap_or_else`
|
||
calls the closure and returns the value returned by the closure.
|
||
|
||
We specify the closure expression `|| self.most_stocked()` as the argument to
|
||
`unwrap_or_else`. This is a closure that takes no parameters itself (if the
|
||
closure had parameters, they would appear between the two vertical pipes). The
|
||
body of the closure calls `self.most_stocked()`. We’re defining the closure
|
||
here, and the implementation of `unwrap_or_else` will evaluate the closure
|
||
later if the result is needed.
|
||
|
||
Running this code prints the following:
|
||
|
||
```
|
||
The user with preference Some(Red) gets Red
|
||
The user with preference None gets Blue
|
||
```
|
||
|
||
One interesting aspect here is that we’ve passed a closure that calls
|
||
`self.most_stocked()` on the current `Inventory` instance. The standard library
|
||
didn’t need to know anything about the `Inventory` or `ShirtColor` types we
|
||
defined, or the logic we want to use in this scenario. The closure captures an
|
||
immutable reference to the `self` `Inventory` instance and passes it with the
|
||
code we specify to the `unwrap_or_else` method. Functions, on the other hand,
|
||
are not able to capture their environment in this way.
|
||
|
||
### Closure Type Inference and Annotation
|
||
|
||
There are more differences between functions and closures. Closures don’t
|
||
usually require you to annotate the types of the parameters or the return value
|
||
like `fn` functions do. Type annotations are required on functions because the
|
||
types are part of an explicit interface exposed to your users. Defining this
|
||
interface rigidly is important for ensuring that everyone agrees on what types
|
||
of values a function uses and returns. Closures, on the other hand, aren’t used
|
||
in an exposed interface like this: they’re stored in variables and used without
|
||
naming them and exposing them to users of our library.
|
||
|
||
Closures are typically short and relevant only within a narrow context rather
|
||
than in any arbitrary scenario. Within these limited contexts, the compiler can
|
||
infer the types of the parameters and the return type, similar to how it’s able
|
||
to infer the types of most variables (there are rare cases where the compiler
|
||
needs closure type annotations too).
|
||
|
||
As with variables, we can add type annotations if we want to increase
|
||
explicitness and clarity at the cost of being more verbose than is strictly
|
||
necessary. Annotating the types for a closure would look like the definition
|
||
shown in Listing 13-2. In this example, we’re defining a closure and storing it
|
||
in a variable rather than defining the closure in the spot we pass it as an
|
||
argument, as we did in Listing 13-1.
|
||
|
||
Filename: src/main.rs
|
||
|
||
```
|
||
let expensive_closure = |num: u32| -> u32 {
|
||
println!("calculating slowly...");
|
||
thread::sleep(Duration::from_secs(2));
|
||
num
|
||
};
|
||
```
|
||
|
||
Listing 13-2: Adding optional type annotations of the parameter and return
|
||
value types in the closure
|
||
|
||
With type annotations added, the syntax of closures looks more similar to the
|
||
syntax of functions. Here, we define a function that adds 1 to its parameter
|
||
and a closure that has the same behavior, for comparison. We’ve added some
|
||
spaces to line up the relevant parts. This illustrates how closure syntax is
|
||
similar to function syntax except for the use of pipes and the amount of syntax
|
||
that is optional:
|
||
|
||
```
|
||
fn add_one_v1 (x: u32) -> u32 { x + 1 }
|
||
let add_one_v2 = |x: u32| -> u32 { x + 1 };
|
||
let add_one_v3 = |x| { x + 1 };
|
||
let add_one_v4 = |x| x + 1 ;
|
||
```
|
||
|
||
The first line shows a function definition and the second line shows a fully
|
||
annotated closure definition. In the third line, we remove the type annotations
|
||
from the closure definition. In the fourth line, we remove the curly brackets,
|
||
which are optional because the closure body has only one expression. These are
|
||
all valid definitions that will produce the same behavior when they’re called.
|
||
The `add_one_v3` and `add_one_v4` lines require the closures to be evaluated to
|
||
be able to compile because the types will be inferred from their usage. This is
|
||
similar to `let v = Vec::new();` needing either type annotations or values of
|
||
some type to be inserted into the `Vec` for Rust to be able to infer the type.
|
||
|
||
For closure definitions, the compiler will infer one concrete type for each of
|
||
their parameters and for their return value. For instance, Listing 13-3 shows
|
||
the definition of a short closure that just returns the value it receives as a
|
||
parameter. This closure isn’t very useful except for the purposes of this
|
||
example. Note that we haven’t added any type annotations to the definition.
|
||
Because there are no type annotations, we can call the closure with any type,
|
||
which we’ve done here with `String` the first time. If we then try to call
|
||
`example_closure` with an integer, we’ll get an error.
|
||
|
||
Filename: src/main.rs
|
||
|
||
```
|
||
let example_closure = |x| x;
|
||
|
||
let s = example_closure(String::from("hello"));
|
||
let n = example_closure(5);
|
||
```
|
||
|
||
Listing 13-3: Attempting to call a closure whose types are inferred with two
|
||
different types
|
||
|
||
The compiler gives us this error:
|
||
|
||
```
|
||
error[E0308]: mismatched types
|
||
--> src/main.rs:5:29
|
||
|
|
||
5 | let n = example_closure(5);
|
||
| ^- help: try using a conversion method:
|
||
`.to_string()`
|
||
| |
|
||
| expected struct `String`, found integer
|
||
```
|
||
|
||
The first time we call `example_closure` with the `String` value, the compiler
|
||
infers the type of `x` and the return type of the closure to be `String`. Those
|
||
types are then locked into the closure in `example_closure`, and we get a type
|
||
error when we next try to use a different type with the same closure.
|
||
|
||
### Capturing References or Moving Ownership
|
||
|
||
Closures can capture values from their environment in three ways, which
|
||
directly map to the three ways a function can take a parameter: borrowing
|
||
immutably, borrowing mutably, and taking ownership. The closure will decide
|
||
which of these to use based on what the body of the function does with the
|
||
captured values.
|
||
|
||
In Listing 13-4, we define a closure that captures an immutable reference to
|
||
the vector named `list` because it only needs an immutable reference to print
|
||
the value.
|
||
|
||
Filename: src/main.rs
|
||
|
||
```
|
||
fn main() {
|
||
let list = vec![1, 2, 3];
|
||
println!("Before defining closure: {:?}", list);
|
||
|
||
1 let only_borrows = || println!("From closure: {:?}", list);
|
||
|
||
println!("Before calling closure: {:?}", list);
|
||
2 only_borrows();
|
||
println!("After calling closure: {:?}", list);
|
||
}
|
||
```
|
||
|
||
Listing 13-4: Defining and calling a closure that captures an immutable
|
||
reference
|
||
|
||
This example also illustrates that a variable can bind to a closure definition
|
||
[1], and we can later call the closure by using the variable name and
|
||
parentheses as if the variable name were a function name [2].
|
||
|
||
Because we can have multiple immutable references to `list` at the same time,
|
||
`list` is still accessible from the code before the closure definition, after
|
||
the closure definition but before the closure is called, and after the closure
|
||
is called. This code compiles, runs, and prints:
|
||
|
||
```
|
||
Before defining closure: [1, 2, 3]
|
||
Before calling closure: [1, 2, 3]
|
||
From closure: [1, 2, 3]
|
||
After calling closure: [1, 2, 3]
|
||
```
|
||
|
||
Next, in Listing 13-5, we change the closure body so that it adds an element to
|
||
the `list` vector. The closure now captures a mutable reference.
|
||
|
||
Filename: src/main.rs
|
||
|
||
```
|
||
fn main() {
|
||
let mut list = vec![1, 2, 3];
|
||
println!("Before defining closure: {:?}", list);
|
||
|
||
let mut borrows_mutably = || list.push(7);
|
||
|
||
borrows_mutably();
|
||
println!("After calling closure: {:?}", list);
|
||
}
|
||
```
|
||
|
||
Listing 13-5: Defining and calling a closure that captures a mutable reference
|
||
|
||
This code compiles, runs, and prints:
|
||
|
||
```
|
||
Before defining closure: [1, 2, 3]
|
||
After calling closure: [1, 2, 3, 7]
|
||
```
|
||
|
||
Note that there’s no longer a `println!` between the definition and the call of
|
||
the `borrows_mutably` closure: when `borrows_mutably` is defined, it captures a
|
||
mutable reference to `list`. We don’t use the closure again after the closure
|
||
is called, so the mutable borrow ends. Between the closure definition and the
|
||
closure call, an immutable borrow to print isn’t allowed because no other
|
||
borrows are allowed when there’s a mutable borrow. Try adding a `println!`
|
||
there to see what error message you get!
|
||
|
||
If you want to force the closure to take ownership of the values it uses in the
|
||
environment even though the body of the closure doesn’t strictly need
|
||
ownership, you can use the `move` keyword before the parameter list.
|
||
|
||
This technique is mostly useful when passing a closure to a new thread to move
|
||
the data so that it’s owned by the new thread. We’ll discuss threads and why
|
||
you would want to use them in detail in Chapter 16 when we talk about
|
||
concurrency, but for now, let’s briefly explore spawning a new thread using a
|
||
closure that needs the `move` keyword. Listing 13-6 shows Listing 13-4 modified
|
||
to print the vector in a new thread rather than in the main thread.
|
||
|
||
Filename: src/main.rs
|
||
|
||
```
|
||
use std::thread;
|
||
|
||
fn main() {
|
||
let list = vec![1, 2, 3];
|
||
println!("Before defining closure: {:?}", list);
|
||
|
||
1 thread::spawn(move || {
|
||
2 println!("From thread: {:?}", list)
|
||
}).join().unwrap();
|
||
}
|
||
```
|
||
|
||
Listing 13-6: Using `move` to force the closure for the thread to take
|
||
ownership of `list`
|
||
|
||
We spawn a new thread, giving the thread a closure to run as an argument. The
|
||
closure body prints out the list. In Listing 13-4, the closure only captured
|
||
`list` using an immutable reference because that’s the least amount of access
|
||
to `list` needed to print it. In this example, even though the closure body
|
||
still only needs an immutable reference [2], we need to specify that `list`
|
||
should be moved into the closure by putting the `move` keyword [1] at the
|
||
beginning of the closure definition. The new thread might finish before the
|
||
rest of the main thread finishes, or the main thread might finish first. If the
|
||
main thread maintains ownership of `list` but ends before the new thread and
|
||
drops `list`, the immutable reference in the thread would be invalid.
|
||
Therefore, the compiler requires that `list` be moved into the closure given to
|
||
the new thread so the reference will be valid. Try removing the `move` keyword
|
||
or using `list` in the main thread after the closure is defined to see what
|
||
compiler errors you get!
|
||
|
||
### Moving Captured Values Out of Closures and the Fn Traits
|
||
|
||
Once a closure has captured a reference or captured ownership of a value from
|
||
the environment where the closure is defined (thus affecting what, if anything,
|
||
is moved *into* the closure), the code in the body of the closure defines what
|
||
happens to the references or values when the closure is evaluated later (thus
|
||
affecting what, if anything, is moved *out of* the closure).
|
||
|
||
A closure body can do any of the following: move a captured value out of the
|
||
closure, mutate the captured value, neither move nor mutate the value, or
|
||
capture nothing from the environment to begin with.
|
||
|
||
The way a closure captures and handles values from the environment affects
|
||
which traits the closure implements, and traits are how functions and structs
|
||
can specify what kinds of closures they can use. Closures will automatically
|
||
implement one, two, or all three of these `Fn` traits, in an additive fashion,
|
||
depending on how the closure’s body handles the values:
|
||
|
||
* `FnOnce` applies to closures that can be called once. All closures implement
|
||
at least this trait because all closures can be called. A closure that moves
|
||
captured values out of its body will only implement `FnOnce` and none of the
|
||
other `Fn` traits because it can only be called once.
|
||
* `FnMut` applies to closures that don’t move captured values out of their
|
||
body, but that might mutate the captured values. These closures can be called
|
||
more than once.
|
||
* `Fn` applies to closures that don’t move captured values out of their body
|
||
and that don’t mutate captured values, as well as closures that capture nothing
|
||
from their environment. These closures can be called more than once without
|
||
mutating their environment, which is important in cases such as calling a
|
||
closure multiple times concurrently.
|
||
|
||
Let’s look at the definition of the `unwrap_or_else` method on `Option<T>` that
|
||
we used in Listing 13-1:
|
||
|
||
```
|
||
impl<T> Option<T> {
|
||
pub fn unwrap_or_else<F>(self, f: F) -> T
|
||
where
|
||
F: FnOnce() -> T
|
||
{
|
||
match self {
|
||
Some(x) => x,
|
||
None => f(),
|
||
}
|
||
}
|
||
}
|
||
```
|
||
|
||
Recall that `T` is the generic type representing the type of the value in the
|
||
`Some` variant of an `Option`. That type `T` is also the return type of the
|
||
`unwrap_or_else` function: code that calls `unwrap_or_else` on an
|
||
`Option<String>`, for example, will get a `String`.
|
||
|
||
Next, notice that the `unwrap_or_else` function has the additional generic type
|
||
parameter `F`. The `F` type is the type of the parameter named `f`, which is
|
||
the closure we provide when calling `unwrap_or_else`.
|
||
|
||
The trait bound specified on the generic type `F` is `FnOnce() -> T`, which
|
||
means `F` must be able to be called once, take no arguments, and return a `T`.
|
||
Using `FnOnce` in the trait bound expresses the constraint that
|
||
`unwrap_or_else` is only going to call `f` one time, at most. In the body of
|
||
`unwrap_or_else`, we can see that if the `Option` is `Some`, `f` won’t be
|
||
called. If the `Option` is `None`, `f` will be called once. Because all
|
||
closures implement `FnOnce`, `unwrap_or_else` accepts the largest variety of
|
||
closures and is as flexible as it can be.
|
||
|
||
> Note: Functions can implement all three of the `Fn` traits too. If what we
|
||
want to do doesn’t require capturing a value from the environment, we can use
|
||
the name of a function rather than a closure where we need something that
|
||
implements one of the `Fn` traits. For example, on an `Option<Vec<T>>` value,
|
||
we could call `unwrap_or_else(Vec::new)` to get a new, empty vector if the
|
||
value is `None`.
|
||
|
||
Now let’s look at the standard library method `sort_by_key`, defined on slices,
|
||
to see how that differs from `unwrap_or_else` and why `sort_by_key` uses
|
||
`FnMut` instead of `FnOnce` for the trait bound. The closure gets one argument
|
||
in the form of a reference to the current item in the slice being considered,
|
||
and returns a value of type `K` that can be ordered. This function is useful
|
||
when you want to sort a slice by a particular attribute of each item. In
|
||
Listing 13-7, we have a list of `Rectangle` instances and we use `sort_by_key`
|
||
to order them by their `width` attribute from low to high.
|
||
|
||
Filename: src/main.rs
|
||
|
||
```
|
||
#[derive(Debug)]
|
||
struct Rectangle {
|
||
width: u32,
|
||
height: u32,
|
||
}
|
||
|
||
fn main() {
|
||
let mut list = [
|
||
Rectangle { width: 10, height: 1 },
|
||
Rectangle { width: 3, height: 5 },
|
||
Rectangle { width: 7, height: 12 },
|
||
];
|
||
|
||
list.sort_by_key(|r| r.width);
|
||
println!("{:#?}", list);
|
||
}
|
||
```
|
||
|
||
Listing 13-7: Using `sort_by_key` to order rectangles by width
|
||
|
||
This code prints:
|
||
|
||
```
|
||
[
|
||
Rectangle {
|
||
width: 3,
|
||
height: 5,
|
||
},
|
||
Rectangle {
|
||
width: 7,
|
||
height: 12,
|
||
},
|
||
Rectangle {
|
||
width: 10,
|
||
height: 1,
|
||
},
|
||
]
|
||
```
|
||
|
||
The reason `sort_by_key` is defined to take an `FnMut` closure is that it calls
|
||
the closure multiple times: once for each item in the slice. The closure `|r|
|
||
r.width` doesn’t capture, mutate, or move anything out from its environment, so
|
||
it meets the trait bound requirements.
|
||
|
||
In contrast, Listing 13-8 shows an example of a closure that implements just
|
||
the `FnOnce` trait, because it moves a value out of the environment. The
|
||
compiler won’t let us use this closure with `sort_by_key`.
|
||
|
||
Filename: src/main.rs
|
||
|
||
```
|
||
--snip--
|
||
|
||
fn main() {
|
||
let mut list = [
|
||
Rectangle { width: 10, height: 1 },
|
||
Rectangle { width: 3, height: 5 },
|
||
Rectangle { width: 7, height: 12 },
|
||
];
|
||
|
||
let mut sort_operations = vec![];
|
||
let value = String::from("by key called");
|
||
|
||
list.sort_by_key(|r| {
|
||
sort_operations.push(value);
|
||
r.width
|
||
});
|
||
println!("{:#?}", list);
|
||
}
|
||
```
|
||
|
||
Listing 13-8: Attempting to use an `FnOnce` closure with `sort_by_key`
|
||
|
||
This is a contrived, convoluted way (that doesn’t work) to try and count the
|
||
number of times `sort_by_key` gets called when sorting `list`. This code
|
||
attempts to do this counting by pushing `value`—a `String` from the closure’s
|
||
environment—into the `sort_operations` vector. The closure captures `value` and
|
||
then moves `value` out of the closure by transferring ownership of `value` to
|
||
the `sort_operations` vector. This closure can be called once; trying to call
|
||
it a second time wouldn’t work because `value` would no longer be in the
|
||
environment to be pushed into `sort_operations` again! Therefore, this closure
|
||
only implements `FnOnce`. When we try to compile this code, we get this error
|
||
that `value` can’t be moved out of the closure because the closure must
|
||
implement `FnMut`:
|
||
|
||
```
|
||
error[E0507]: cannot move out of `value`, a captured variable in an `FnMut`
|
||
closure
|
||
--> src/main.rs:18:30
|
||
|
|
||
15 | let value = String::from("by key called");
|
||
| ----- captured outer variable
|
||
16 |
|
||
17 | list.sort_by_key(|r| {
|
||
| ______________________-
|
||
18 | | sort_operations.push(value);
|
||
| | ^^^^^ move occurs because `value` has
|
||
type `String`, which does not implement the `Copy` trait
|
||
19 | | r.width
|
||
20 | | });
|
||
| |_____- captured by this `FnMut` closure
|
||
```
|
||
|
||
The error points to the line in the closure body that moves `value` out of the
|
||
environment. To fix this, we need to change the closure body so that it doesn’t
|
||
move values out of the environment. Keeping a counter in the environment and
|
||
incrementing its value in the closure body is a more straightforward way to
|
||
count the number of times `sort_by_key` is called. The closure in Listing 13-9
|
||
works with `sort_by_key` because it is only capturing a mutable reference to
|
||
the `num_sort_operations` counter and can therefore be called more than once.
|
||
|
||
Filename: src/main.rs
|
||
|
||
```
|
||
--snip--
|
||
|
||
fn main() {
|
||
--snip--
|
||
|
||
let mut num_sort_operations = 0;
|
||
list.sort_by_key(|r| {
|
||
num_sort_operations += 1;
|
||
r.width
|
||
});
|
||
println!(
|
||
"{:#?}, sorted in {num_sort_operations} operations",
|
||
list
|
||
);
|
||
}
|
||
```
|
||
|
||
Listing 13-9: Using an `FnMut` closure with `sort_by_key` is allowed.
|
||
|
||
The `Fn` traits are important when defining or using functions or types that
|
||
make use of closures. In the next section, we’ll discuss iterators. Many
|
||
iterator methods take closure arguments, so keep these closure details in mind
|
||
as we continue!
|
||
|
||
## Processing a Series of Items with Iterators
|
||
|
||
The iterator pattern allows you to perform some task on a sequence of items in
|
||
turn. An iterator is responsible for the logic of iterating over each item and
|
||
determining when the sequence has finished. When you use iterators, you don’t
|
||
have to reimplement that logic yourself.
|
||
|
||
In Rust, iterators are *lazy*, meaning they have no effect until you call
|
||
methods that consume the iterator to use it up. For example, the code in
|
||
Listing 13-10 creates an iterator over the items in the vector `v1` by calling
|
||
the `iter` method defined on `Vec<T>`. This code by itself doesn’t do anything
|
||
useful.
|
||
|
||
```
|
||
let v1 = vec![1, 2, 3];
|
||
|
||
let v1_iter = v1.iter();
|
||
```
|
||
|
||
Listing 13-10: Creating an iterator
|
||
|
||
The iterator is stored in the `v1_iter` variable. Once we’ve created an
|
||
iterator, we can use it in a variety of ways. In Listing 3-5, we iterated over
|
||
an array using a `for` loop to execute some code on each of its items. Under
|
||
the hood, this implicitly created and then consumed an iterator, but we glossed
|
||
over how exactly that works until now.
|
||
|
||
In the example in Listing 13-11, we separate the creation of the iterator from
|
||
the use of the iterator in the `for` loop. When the `for` loop is called using
|
||
the iterator in `v1_iter`, each element in the iterator is used in one
|
||
iteration of the loop, which prints out each value.
|
||
|
||
```
|
||
let v1 = vec![1, 2, 3];
|
||
|
||
let v1_iter = v1.iter();
|
||
|
||
for val in v1_iter {
|
||
println!("Got: {val}");
|
||
}
|
||
```
|
||
|
||
Listing 13-11: Using an iterator in a `for` loop
|
||
|
||
In languages that don’t have iterators provided by their standard libraries,
|
||
you would likely write this same functionality by starting a variable at index
|
||
0, using that variable to index into the vector to get a value, and
|
||
incrementing the variable value in a loop until it reached the total number of
|
||
items in the vector.
|
||
|
||
Iterators handle all of that logic for you, cutting down on repetitive code you
|
||
could potentially mess up. Iterators give you more flexibility to use the same
|
||
logic with many different kinds of sequences, not just data structures you can
|
||
index into, like vectors. Let’s examine how iterators do that.
|
||
|
||
### The Iterator Trait and the next Method
|
||
|
||
All iterators implement a trait named `Iterator` that is defined in the
|
||
standard library. The definition of the trait looks like this:
|
||
|
||
```
|
||
pub trait Iterator {
|
||
type Item;
|
||
|
||
fn next(&mut self) -> Option<Self::Item>;
|
||
|
||
// methods with default implementations elided
|
||
}
|
||
```
|
||
|
||
Notice that this definition uses some new syntax: `type Item` and `Self::Item`,
|
||
which are defining an *associated type* with this trait. We’ll talk about
|
||
associated types in depth in Chapter 19. For now, all you need to know is that
|
||
this code says implementing the `Iterator` trait requires that you also define
|
||
an `Item` type, and this `Item` type is used in the return type of the `next`
|
||
method. In other words, the `Item` type will be the type returned from the
|
||
iterator.
|
||
|
||
The `Iterator` trait only requires implementors to define one method: the
|
||
`next` method, which returns one item of the iterator at a time, wrapped in
|
||
`Some`, and, when iteration is over, returns `None`.
|
||
|
||
We can call the `next` method on iterators directly; Listing 13-12 demonstrates
|
||
what values are returned from repeated calls to `next` on the iterator created
|
||
from the vector.
|
||
|
||
Filename: src/lib.rs
|
||
|
||
```
|
||
#[test]
|
||
fn iterator_demonstration() {
|
||
let v1 = vec![1, 2, 3];
|
||
|
||
let mut v1_iter = v1.iter();
|
||
|
||
assert_eq!(v1_iter.next(), Some(&1));
|
||
assert_eq!(v1_iter.next(), Some(&2));
|
||
assert_eq!(v1_iter.next(), Some(&3));
|
||
assert_eq!(v1_iter.next(), None);
|
||
}
|
||
```
|
||
|
||
Listing 13-12: Calling the `next` method on an iterator
|
||
|
||
Note that we needed to make `v1_iter` mutable: calling the `next` method on an
|
||
iterator changes internal state that the iterator uses to keep track of where
|
||
it is in the sequence. In other words, this code *consumes*, or uses up, the
|
||
iterator. Each call to `next` eats up an item from the iterator. We didn’t need
|
||
to make `v1_iter` mutable when we used a `for` loop because the loop took
|
||
ownership of `v1_iter` and made it mutable behind the scenes.
|
||
|
||
Also note that the values we get from the calls to `next` are immutable
|
||
references to the values in the vector. The `iter` method produces an iterator
|
||
over immutable references. If we want to create an iterator that takes
|
||
ownership of `v1` and returns owned values, we can call `into_iter` instead of
|
||
`iter`. Similarly, if we want to iterate over mutable references, we can call
|
||
`iter_mut` instead of `iter`.
|
||
|
||
### Methods That Consume the Iterator
|
||
|
||
The `Iterator` trait has a number of different methods with default
|
||
implementations provided by the standard library; you can find out about these
|
||
methods by looking in the standard library API documentation for the `Iterator`
|
||
trait. Some of these methods call the `next` method in their definition, which
|
||
is why you’re required to implement the `next` method when implementing the
|
||
`Iterator` trait.
|
||
|
||
Methods that call `next` are called *consuming adapters* because calling them
|
||
uses up the iterator. One example is the `sum` method, which takes ownership of
|
||
the iterator and iterates through the items by repeatedly calling `next`, thus
|
||
consuming the iterator. As it iterates through, it adds each item to a running
|
||
total and returns the total when iteration is complete. Listing 13-13 has a
|
||
test illustrating a use of the `sum` method.
|
||
|
||
Filename: src/lib.rs
|
||
|
||
```
|
||
#[test]
|
||
fn iterator_sum() {
|
||
let v1 = vec![1, 2, 3];
|
||
|
||
let v1_iter = v1.iter();
|
||
|
||
let total: i32 = v1_iter.sum();
|
||
|
||
assert_eq!(total, 6);
|
||
}
|
||
```
|
||
|
||
Listing 13-13: Calling the `sum` method to get the total of all items in the
|
||
iterator
|
||
|
||
We aren’t allowed to use `v1_iter` after the call to `sum` because `sum` takes
|
||
ownership of the iterator we call it on.
|
||
|
||
### Methods That Produce Other Iterators
|
||
|
||
*Iterator adapters* are methods defined on the `Iterator` trait that don’t
|
||
consume the iterator. Instead, they produce different iterators by changing
|
||
some aspect of the original iterator.
|
||
|
||
Listing 13-14 shows an example of calling the iterator adapter method `map`,
|
||
which takes a closure to call on each item as the items are iterated through.
|
||
The `map` method returns a new iterator that produces the modified items. The
|
||
closure here creates a new iterator in which each item from the vector will be
|
||
incremented by 1.
|
||
|
||
Filename: src/main.rs
|
||
|
||
```
|
||
let v1: Vec<i32> = vec![1, 2, 3];
|
||
|
||
v1.iter().map(|x| x + 1);
|
||
```
|
||
|
||
Listing 13-14: Calling the iterator adapter `map` to create a new iterator
|
||
|
||
However, this code produces a warning:
|
||
|
||
```
|
||
warning: unused `Map` that must be used
|
||
--> src/main.rs:4:5
|
||
|
|
||
4 | v1.iter().map(|x| x + 1);
|
||
| ^^^^^^^^^^^^^^^^^^^^^^^^^
|
||
|
|
||
= note: `#[warn(unused_must_use)]` on by default
|
||
= note: iterators are lazy and do nothing unless consumed
|
||
```
|
||
|
||
The code in Listing 13-14 doesn’t do anything; the closure we’ve specified
|
||
never gets called. The warning reminds us why: iterator adapters are lazy, and
|
||
we need to consume the iterator here.
|
||
|
||
To fix this warning and consume the iterator, we’ll use the `collect` method,
|
||
which we used with `env::args` in Listing 12-1. This method consumes the
|
||
iterator and collects the resultant values into a collection data type.
|
||
|
||
In Listing 13-15, we collect into a vector the results of iterating over the
|
||
iterator that’s returned from the call to `map`. This vector will end up
|
||
containing each item from the original vector, incremented by 1.
|
||
|
||
Filename: src/main.rs
|
||
|
||
```
|
||
let v1: Vec<i32> = vec![1, 2, 3];
|
||
|
||
let v2: Vec<_> = v1.iter().map(|x| x + 1).collect();
|
||
|
||
assert_eq!(v2, vec![2, 3, 4]);
|
||
```
|
||
|
||
Listing 13-15: Calling the `map` method to create a new iterator, and then
|
||
calling the `collect` method to consume the new iterator and create a vector
|
||
|
||
Because `map` takes a closure, we can specify any operation we want to perform
|
||
on each item. This is a great example of how closures let you customize some
|
||
behavior while reusing the iteration behavior that the `Iterator` trait
|
||
provides.
|
||
|
||
You can chain multiple calls to iterator adapters to perform complex actions in
|
||
a readable way. But because all iterators are lazy, you have to call one of the
|
||
consuming adapter methods to get results from calls to iterator adapters.
|
||
|
||
### Using Closures That Capture Their Environment
|
||
|
||
Many iterator adapters take closures as arguments, and commonly the closures
|
||
we’ll specify as arguments to iterator adapters will be closures that capture
|
||
their environment.
|
||
|
||
For this example, we’ll use the `filter` method that takes a closure. The
|
||
closure gets an item from the iterator and returns a `bool`. If the closure
|
||
returns `true`, the value will be included in the iteration produced by
|
||
`filter`. If the closure returns `false`, the value won’t be included.
|
||
|
||
In Listing 13-16, we use `filter` with a closure that captures the `shoe_size`
|
||
variable from its environment to iterate over a collection of `Shoe` struct
|
||
instances. It will return only shoes that are the specified size.
|
||
|
||
Filename: src/lib.rs
|
||
|
||
```
|
||
#[derive(PartialEq, Debug)]
|
||
struct Shoe {
|
||
size: u32,
|
||
style: String,
|
||
}
|
||
|
||
fn shoes_in_size(shoes: Vec<Shoe>, shoe_size: u32) -> Vec<Shoe> {
|
||
shoes.into_iter().filter(|s| s.size == shoe_size).collect()
|
||
}
|
||
|
||
#[cfg(test)]
|
||
mod tests {
|
||
use super::*;
|
||
|
||
#[test]
|
||
fn filters_by_size() {
|
||
let shoes = vec![
|
||
Shoe {
|
||
size: 10,
|
||
style: String::from("sneaker"),
|
||
},
|
||
Shoe {
|
||
size: 13,
|
||
style: String::from("sandal"),
|
||
},
|
||
Shoe {
|
||
size: 10,
|
||
style: String::from("boot"),
|
||
},
|
||
];
|
||
|
||
let in_my_size = shoes_in_size(shoes, 10);
|
||
|
||
assert_eq!(
|
||
in_my_size,
|
||
vec![
|
||
Shoe {
|
||
size: 10,
|
||
style: String::from("sneaker")
|
||
},
|
||
Shoe {
|
||
size: 10,
|
||
style: String::from("boot")
|
||
},
|
||
]
|
||
);
|
||
}
|
||
}
|
||
```
|
||
|
||
Listing 13-16: Using the `filter` method with a closure that captures
|
||
`shoe_size`
|
||
|
||
The `shoes_in_size` function takes ownership of a vector of shoes and a shoe
|
||
size as parameters. It returns a vector containing only shoes of the specified
|
||
size.
|
||
|
||
In the body of `shoes_in_size`, we call `into_iter` to create an iterator that
|
||
takes ownership of the vector. Then we call `filter` to adapt that iterator
|
||
into a new iterator that only contains elements for which the closure returns
|
||
`true`.
|
||
|
||
The closure captures the `shoe_size` parameter from the environment and
|
||
compares the value with each shoe’s size, keeping only shoes of the size
|
||
specified. Finally, calling `collect` gathers the values returned by the
|
||
adapted iterator into a vector that’s returned by the function.
|
||
|
||
The test shows that when we call `shoes_in_size`, we get back only shoes that
|
||
have the same size as the value we specified.
|
||
|
||
## Improving Our I/O Project
|
||
|
||
With this new knowledge about iterators, we can improve the I/O project in
|
||
Chapter 12 by using iterators to make places in the code clearer and more
|
||
concise. Let’s look at how iterators can improve our implementation of the
|
||
`Config::build` function and the `search` function.
|
||
|
||
### Removing a clone Using an Iterator
|
||
|
||
In Listing 12-6, we added code that took a slice of `String` values and created
|
||
an instance of the `Config` struct by indexing into the slice and cloning the
|
||
values, allowing the `Config` struct to own those values. In Listing 13-17,
|
||
we’ve reproduced the implementation of the `Config::build` function as it was
|
||
in Listing 12-23.
|
||
|
||
Filename: src/lib.rs
|
||
|
||
```
|
||
impl Config {
|
||
pub fn build(
|
||
args: &[String]
|
||
) -> Result<Config, &'static str> {
|
||
if args.len() < 3 {
|
||
return Err("not enough arguments");
|
||
}
|
||
|
||
let query = args[1].clone();
|
||
let file_path = args[2].clone();
|
||
|
||
let ignore_case = env::var("IGNORE_CASE").is_ok();
|
||
|
||
Ok(Config {
|
||
query,
|
||
file_path,
|
||
ignore_case,
|
||
})
|
||
}
|
||
}
|
||
```
|
||
|
||
Listing 13-17: Reproduction of the `Config::build` function from Listing 12-23
|
||
|
||
At the time, we said not to worry about the inefficient `clone` calls because
|
||
we would remove them in the future. Well, that time is now!
|
||
|
||
We needed `clone` here because we have a slice with `String` elements in the
|
||
parameter `args`, but the `build` function doesn’t own `args`. To return
|
||
ownership of a `Config` instance, we had to clone the values from the `query`
|
||
and `filename` fields of `Config` so the `Config` instance can own its values.
|
||
|
||
With our new knowledge about iterators, we can change the `build` function to
|
||
take ownership of an iterator as its argument instead of borrowing a slice.
|
||
We’ll use the iterator functionality instead of the code that checks the length
|
||
of the slice and indexes into specific locations. This will clarify what the
|
||
`Config::build` function is doing because the iterator will access the values.
|
||
|
||
Once `Config::build` takes ownership of the iterator and stops using indexing
|
||
operations that borrow, we can move the `String` values from the iterator into
|
||
`Config` rather than calling `clone` and making a new allocation.
|
||
|
||
#### Using the Returned Iterator Directly
|
||
|
||
Open your I/O project’s *src/main.rs* file, which should look like this:
|
||
|
||
Filename: src/main.rs
|
||
|
||
```
|
||
fn main() {
|
||
let args: Vec<String> = env::args().collect();
|
||
|
||
let config = Config::build(&args).unwrap_or_else(|err| {
|
||
eprintln!("Problem parsing arguments: {err}");
|
||
process::exit(1);
|
||
});
|
||
|
||
--snip--
|
||
}
|
||
```
|
||
|
||
We’ll first change the start of the `main` function that we had in Listing
|
||
12-24 to the code in Listing 13-18, which this time uses an iterator. This
|
||
won’t compile until we update `Config::build` as well.
|
||
|
||
Filename: src/main.rs
|
||
|
||
```
|
||
fn main() {
|
||
let config =
|
||
Config::build(env::args()).unwrap_or_else(|err| {
|
||
eprintln!("Problem parsing arguments: {err}");
|
||
process::exit(1);
|
||
});
|
||
|
||
--snip--
|
||
}
|
||
```
|
||
|
||
Listing 13-18: Passing the return value of `env::args` to `Config::build`
|
||
|
||
The `env::args` function returns an iterator! Rather than collecting the
|
||
iterator values into a vector and then passing a slice to `Config::build`, now
|
||
we’re passing ownership of the iterator returned from `env::args` to
|
||
`Config::build` directly.
|
||
|
||
Next, we need to update the definition of `Config::build`. In your I/O
|
||
project’s *src/lib.rs* file, let’s change the signature of `Config::build` to
|
||
look like Listing 13-19. This still won’t compile, because we need to update
|
||
the function body.
|
||
|
||
Filename: src/lib.rs
|
||
|
||
```
|
||
impl Config {
|
||
pub fn build(
|
||
mut args: impl Iterator<Item = String>,
|
||
) -> Result<Config, &'static str> {
|
||
--snip--
|
||
```
|
||
|
||
Listing 13-19: Updating the signature of `Config::build` to expect an iterator
|
||
|
||
The standard library documentation for the `env::args` function shows that the
|
||
type of the iterator it returns is `std::env::Args`, and that type implements
|
||
the `Iterator` trait and returns `String` values.
|
||
|
||
We’ve updated the signature of the `Config::build` function so the parameter
|
||
`args` has a generic type with the trait bounds `impl Iterator<Item = String>`
|
||
instead of `&[String]`. This usage of the `impl Trait` syntax we discussed in
|
||
“Traits as Parameters” on page XX means that `args` can be any type that
|
||
implements the `Iterator` type and returns `String` items.
|
||
|
||
Because we’re taking ownership of `args` and we’ll be mutating `args` by
|
||
iterating over it, we can add the `mut` keyword into the specification of the
|
||
`args` parameter to make it mutable.
|
||
|
||
#### Using Iterator Trait Methods Instead of Indexing
|
||
|
||
Next, we’ll fix the body of `Config::build`. Because `args` implements the
|
||
`Iterator` trait, we know we can call the `next` method on it! Listing 13-20
|
||
updates the code from Listing 12-23 to use the `next` method.
|
||
|
||
Filename: src/lib.rs
|
||
|
||
```
|
||
impl Config {
|
||
pub fn build(
|
||
mut args: impl Iterator<Item = String>,
|
||
) -> Result<Config, &'static str> {
|
||
args.next();
|
||
|
||
let query = match args.next() {
|
||
Some(arg) => arg,
|
||
None => return Err("Didn't get a query string"),
|
||
};
|
||
|
||
let file_path = match args.next() {
|
||
Some(arg) => arg,
|
||
None => return Err("Didn't get a file path"),
|
||
};
|
||
|
||
let ignore_case = env::var("IGNORE_CASE").is_ok();
|
||
|
||
Ok(Config {
|
||
query,
|
||
file_path,
|
||
ignore_case,
|
||
})
|
||
}
|
||
}
|
||
```
|
||
|
||
Listing 13-20: Changing the body of `Config::build` to use iterator methods
|
||
|
||
Remember that the first value in the return value of `env::args` is the name of
|
||
the program. We want to ignore that and get to the next value, so first we call
|
||
`next` and do nothing with the return value. Then we call `next` to get the
|
||
value we want to put in the `query` field of `Config`. If `next` returns
|
||
`Some`, we use a `match` to extract the value. If it returns `None`, it means
|
||
not enough arguments were given and we return early with an `Err` value. We do
|
||
the same thing for the `filename` value.
|
||
|
||
### Making Code Clearer with Iterator Adapters
|
||
|
||
We can also take advantage of iterators in the `search` function in our I/O
|
||
project, which is reproduced here in Listing 13-21 as it was in Listing 12-19.
|
||
|
||
Filename: src/lib.rs
|
||
|
||
```
|
||
pub fn search<'a>(
|
||
query: &str,
|
||
contents: &'a str,
|
||
) -> Vec<&'a str> {
|
||
let mut results = Vec::new();
|
||
|
||
for line in contents.lines() {
|
||
if line.contains(query) {
|
||
results.push(line);
|
||
}
|
||
}
|
||
|
||
results
|
||
}
|
||
```
|
||
|
||
Listing 13-21: The implementation of the `search` function from Listing 12-19
|
||
|
||
We can write this code in a more concise way using iterator adapter methods.
|
||
Doing so also lets us avoid having a mutable intermediate `results` vector. The
|
||
functional programming style prefers to minimize the amount of mutable state to
|
||
make code clearer. Removing the mutable state might enable a future enhancement
|
||
to make searching happen in parallel because we wouldn’t have to manage
|
||
concurrent access to the `results` vector. Listing 13-22 shows this change.
|
||
|
||
Filename: src/lib.rs
|
||
|
||
```
|
||
pub fn search<'a>(
|
||
query: &str,
|
||
contents: &'a str,
|
||
) -> Vec<&'a str> {
|
||
contents
|
||
.lines()
|
||
.filter(|line| line.contains(query))
|
||
.collect()
|
||
}
|
||
```
|
||
|
||
Listing 13-22: Using iterator adapter methods in the implementation of the
|
||
`search` function
|
||
|
||
Recall that the purpose of the `search` function is to return all lines in
|
||
`contents` that contain the `query`. Similar to the `filter` example in Listing
|
||
13-16, this code uses the `filter` adapter to keep only the lines for which
|
||
`line.contains(query)` returns `true`. We then collect the matching lines into
|
||
another vector with `collect`. Much simpler! Feel free to make the same change
|
||
to use iterator methods in the `search_case_insensitive` function as well.
|
||
|
||
### Choosing Between Loops and Iterators
|
||
|
||
The next logical question is which style you should choose in your own code and
|
||
why: the original implementation in Listing 13-21 or the version using
|
||
iterators in Listing 13-22. Most Rust programmers prefer to use the iterator
|
||
style. It’s a bit tougher to get the hang of at first, but once you get a feel
|
||
for the various iterator adapters and what they do, iterators can be easier to
|
||
understand. Instead of fiddling with the various bits of looping and building
|
||
new vectors, the code focuses on the high-level objective of the loop. This
|
||
abstracts away some of the commonplace code so it’s easier to see the concepts
|
||
that are unique to this code, such as the filtering condition each element in
|
||
the iterator must pass.
|
||
|
||
But are the two implementations truly equivalent? The intuitive assumption
|
||
might be that the lower-level loop will be faster. Let’s talk about performance.
|
||
|
||
## Comparing Performance: Loops vs. Iterators
|
||
|
||
To determine whether to use loops or iterators, you need to know which
|
||
implementation is faster: the version of the `search` function with an explicit
|
||
`for` loop or the version with iterators.
|
||
|
||
We ran a benchmark by loading the entire contents of *The Adventures of
|
||
Sherlock Holmes* by Sir Arthur Conan Doyle into a `String` and looking for the
|
||
word *the* in the contents. Here are the results of the benchmark on the
|
||
version of `search` using the `for` loop and the version using iterators:
|
||
|
||
```
|
||
test bench_search_for ... bench: 19,620,300 ns/iter (+/- 915,700)
|
||
test bench_search_iter ... bench: 19,234,900 ns/iter (+/- 657,200)
|
||
```
|
||
|
||
The iterator version was slightly faster! We won’t explain the benchmark code
|
||
here because the point is not to prove that the two versions are equivalent but
|
||
to get a general sense of how these two implementations compare
|
||
performance-wise.
|
||
|
||
For a more comprehensive benchmark, you should check using various texts of
|
||
various sizes as the `contents`, different words and words of different lengths
|
||
as the `query`, and all kinds of other variations. The point is this:
|
||
iterators, although a high-level abstraction, get compiled down to roughly the
|
||
same code as if you’d written the lower-level code yourself. Iterators are one
|
||
of Rust’s *zero-cost abstractions*, by which we mean that using the abstraction
|
||
imposes no additional runtime overhead. This is analogous to how Bjarne
|
||
Stroustrup, the original designer and implementor of C++, defines
|
||
*zero-overhead* in “Foundations of C++” (2012):
|
||
|
||
> In general, C++ implementations obey the zero-overhead principle: What you
|
||
don’t use, you don’t pay for. And further: What you do use, you couldn’t hand
|
||
code any better.As another example, the following code is taken from an audio
|
||
decoder. The decoding algorithm uses the linear prediction mathematical
|
||
operation to estimate future values based on a linear function of the previous
|
||
samples. This code uses an iterator chain to do some math on three variables in
|
||
scope: a `buffer` slice of data, an array of 12 `coefficients`, and an amount
|
||
by which to shift data in `qlp_shift`. We’ve declared the variables within this
|
||
example but not given them any values; although this code doesn’t have much
|
||
meaning outside of its context, it’s still a concise, real-world example of how
|
||
Rust translates high-level ideas to low-level code.
|
||
|
||
```
|
||
let buffer: &mut [i32];
|
||
let coefficients: [i64; 12];
|
||
let qlp_shift: i16;
|
||
|
||
for i in 12..buffer.len() {
|
||
let prediction = coefficients.iter()
|
||
.zip(&buffer[i - 12..i])
|
||
.map(|(&c, &s)| c * s as i64)
|
||
.sum::<i64>() >> qlp_shift;
|
||
let delta = buffer[i];
|
||
buffer[i] = prediction as i32 + delta;
|
||
}
|
||
```
|
||
|
||
To calculate the value of `prediction`, this code iterates through each of the
|
||
12 values in `coefficients` and uses the `zip` method to pair the coefficient
|
||
values with the previous 12 values in `buffer`. Then, for each pair, it
|
||
multiplies the values together, sums all the results, and shifts the bits in
|
||
the sum `qlp_shift` bits to the right.
|
||
|
||
Calculations in applications like audio decoders often prioritize performance
|
||
most highly. Here, we’re creating an iterator, using two adapters, and then
|
||
consuming the value. What assembly code would this Rust code compile to? Well,
|
||
as of this writing, it compiles down to the same assembly you’d write by hand.
|
||
There’s no loop at all corresponding to the iteration over the values in
|
||
`coefficients`: Rust knows that there are 12 iterations, so it “unrolls” the
|
||
loop. *Unrolling* is an optimization that removes the overhead of the loop
|
||
controlling code and instead generates repetitive code for each iteration of
|
||
the loop.
|
||
|
||
All of the coefficients get stored in registers, which means accessing the
|
||
values is very fast. There are no bounds checks on the array access at runtime.
|
||
All of these optimizations that Rust is able to apply make the resultant code
|
||
extremely efficient. Now that you know this, you can use iterators and closures
|
||
without fear! They make code seem like it’s higher level but don’t impose a
|
||
runtime performance penalty for doing so.
|
||
|
||
## Summary
|
||
|
||
Closures and iterators are Rust features inspired by functional programming
|
||
language ideas. They contribute to Rust’s capability to clearly express
|
||
high-level ideas at low-level performance. The implementations of closures and
|
||
iterators are such that runtime performance is not affected. This is part of
|
||
Rust’s goal to strive to provide zero-cost abstractions.
|
||
|
||
Now that we’ve improved the expressiveness of our I/O project, let’s look at
|
||
some more features of `cargo` that will help us share the project with the
|
||
world.
|
||
|