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diff --git a/src/doc/book/nostarch/chapter13.md b/src/doc/book/nostarch/chapter13.md index 8f7717ccb..52c1f5f97 100644 --- a/src/doc/book/nostarch/chapter13.md +++ b/src/doc/book/nostarch/chapter13.md @@ -23,15 +23,15 @@ 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!) +* 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 +## 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 @@ -43,8 +43,8 @@ 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 +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 @@ -56,9 +56,9 @@ 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: +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 @@ -74,8 +74,11 @@ struct Inventory { } impl Inventory { - fn giveaway(&self, user_preference: Option<ShirtColor>) -> ShirtColor { - user_preference.unwrap_or_else(|| self.most_stocked()) [1] + fn giveaway( + &self, + user_preference: Option<ShirtColor>, + ) -> ShirtColor { + 1 user_preference.unwrap_or_else(|| self.most_stocked()) } fn most_stocked(&self) -> ShirtColor { @@ -98,18 +101,22 @@ impl Inventory { fn main() { let store = Inventory { - shirts: vec![ShirtColor::Blue, ShirtColor::Red, ShirtColor::Blue], [2] + 2 shirts: vec![ + ShirtColor::Blue, + ShirtColor::Red, + ShirtColor::Blue, + ], }; let user_pref1 = Some(ShirtColor::Red); - let giveaway1 = store.giveaway(user_pref1); [3] + 3 let giveaway1 = store.giveaway(user_pref1); println!( "The user with preference {:?} gets {:?}", user_pref1, giveaway1 ); let user_pref2 = None; - let giveaway2 = store.giveaway(user_pref2); [4] + 4 let giveaway2 = store.giveaway(user_pref2); println!( "The user with preference {:?} gets {:?}", user_pref2, giveaway2 @@ -125,11 +132,11 @@ 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 +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 @@ -138,18 +145,14 @@ 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 bars). 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: +Running this code prints the following: ``` -$ cargo run - Compiling shirt-company v0.1.0 (file:///projects/shirt-company) - Finished dev [unoptimized + debuginfo] target(s) in 0.27s - Running `target/debug/shirt-company` The user with preference Some(Red) gets Red The user with preference None gets Blue ``` @@ -184,7 +187,7 @@ 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. +argument, as we did in Listing 13-1. Filename: src/main.rs @@ -200,11 +203,11 @@ 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: +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 } @@ -213,13 +216,13 @@ 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 +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 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 +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. @@ -251,7 +254,8 @@ error[E0308]: mismatched types --> src/main.rs:5:29 | 5 | let n = example_closure(5); - | ^- help: try using a conversion method: `.to_string()` + | ^- help: try using a conversion method: +`.to_string()` | | | expected struct `String`, found integer ``` @@ -271,7 +275,7 @@ 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: +the value. Filename: src/main.rs @@ -280,10 +284,10 @@ fn main() { let list = vec![1, 2, 3]; println!("Before defining closure: {:?}", list); - [1] let only_borrows = || println!("From closure: {:?}", list); + 1 let only_borrows = || println!("From closure: {:?}", list); println!("Before calling closure: {:?}", list); - only_borrows(); [2] + 2 only_borrows(); println!("After calling closure: {:?}", list); } ``` @@ -308,7 +312,7 @@ 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: +the `list` vector. The closure now captures a mutable reference. Filename: src/main.rs @@ -350,7 +354,7 @@ 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: +to print the vector in a new thread rather than in the main thread. Filename: src/main.rs @@ -361,8 +365,8 @@ fn main() { let list = vec![1, 2, 3]; println!("Before defining closure: {:?}", list); - [1] thread::spawn(move || { - [2] println!("From thread: {:?}", list) + 1 thread::spawn(move || { + 2 println!("From thread: {:?}", list) }).join().unwrap(); } ``` @@ -372,29 +376,30 @@ 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 +`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, we need to specify that `list` should -be moved into the closure by putting the `move` keyword 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 -maintained ownership of `list` but ended before the new thread did and dropped -`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 +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. +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 @@ -402,18 +407,18 @@ 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: -1. `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. -2. `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. -3. `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. +* `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: @@ -444,27 +449,27 @@ 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` at most one time. In the body of +`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 most different kinds -of closures and is as flexible as it can be. +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`. +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, +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: +to order them by their `width` attribute from low to high. Filename: src/main.rs @@ -510,21 +515,17 @@ This code prints: 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 out anything from its environment, so +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`: +compiler won’t let us use this closure with `sort_by_key`. Filename: src/main.rs ``` -#[derive(Debug)] -struct Rectangle { - width: u32, - height: u32, -} +--snip-- fn main() { let mut list = [ @@ -549,7 +550,7 @@ 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` +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 @@ -559,7 +560,8 @@ 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 +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"); @@ -568,7 +570,8 @@ error[E0507]: cannot move out of `value`, a captured variable in an `FnMut` clos 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 + | | ^^^^^ move occurs because `value` has +type `String`, which does not implement the `Copy` trait 19 | | r.width 20 | | }); | |_____- captured by this `FnMut` closure @@ -576,39 +579,33 @@ error[E0507]: cannot move out of `value`, a captured variable in an `FnMut` clos 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. To count the number of times `sort_by_key` -is called, keeping a counter in the environment and incrementing its value in -the closure body is a more straightforward way to calculate that. 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: +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 ``` -#[derive(Debug)] -struct Rectangle { - width: u32, - height: u32, -} +--snip-- fn main() { - let mut list = [ - Rectangle { width: 10, height: 1 }, - Rectangle { width: 3, height: 5 }, - Rectangle { width: 7, height: 12 }, - ]; + --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); + println!( + "{:#?}, sorted in {num_sort_operations} operations", + list + ); } ``` -Listing 13-9: Using an `FnMut` closure with `sort_by_key` is allowed +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 @@ -637,10 +634,10 @@ 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 in Chapter 3, 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. +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 @@ -653,7 +650,7 @@ let v1 = vec![1, 2, 3]; let v1_iter = v1.iter(); for val in v1_iter { - println!("Got: {}", val); + println!("Got: {val}"); } ``` @@ -665,12 +662,12 @@ you would likely write this same functionality by starting a variable at index incrementing the variable value in a loop until it reached the total number of items in the vector. -Iterators handle all that logic for you, cutting down on repetitive code you +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 +### 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: @@ -685,7 +682,7 @@ pub trait Iterator { } ``` -Notice this definition uses some new syntax: `type Item` and `Self::Item`, +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 @@ -694,8 +691,8 @@ 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`. +`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 @@ -733,7 +730,7 @@ 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 +### 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 @@ -742,12 +739,12 @@ 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 adaptors*, because calling them +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: +test illustrating a use of the `sum` method. Filename: src/lib.rs @@ -770,17 +767,17 @@ 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 +### Methods That Produce Other Iterators -*Iterator adaptors* are methods defined on the `Iterator` trait that don’t +*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 13-17 shows an example of calling the iterator adaptor method `map`, +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: +incremented by 1. Filename: src/main.rs @@ -790,7 +787,7 @@ let v1: Vec<i32> = vec![1, 2, 3]; v1.iter().map(|x| x + 1); ``` -Listing 13-14: Calling the iterator adaptor `map` to create a new iterator +Listing 13-14: Calling the iterator adapter `map` to create a new iterator However, this code produces a warning: @@ -806,17 +803,16 @@ warning: unused `Map` that must be used ``` The code in Listing 13-14 doesn’t do anything; the closure we’ve specified -never gets called. The warning reminds us why: iterator adaptors are lazy, and +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 in Chapter 12 with `env::args` in Listing 12-1. This method -consumes the iterator and collects the resulting values into a collection data -type. +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 the results of iterating over the iterator that’s -returned from the call to `map` into a vector. This vector will end up -containing each item from the original vector incremented by 1. +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 @@ -828,7 +824,7 @@ 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 +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 @@ -836,11 +832,11 @@ 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 adaptors to perform complex actions in +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 adaptor methods to get results from calls to iterator adaptors. +consuming adapter methods to get results from calls to iterator adapters. -### Using Closures that Capture Their Environment +### 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 @@ -915,18 +911,18 @@ 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`. +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. +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 @@ -935,19 +931,21 @@ 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 +### 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: +in Listing 12-23. Filename: src/lib.rs ``` impl Config { - pub fn build(args: &[String]) -> Result<Config, &'static str> { + pub fn build( + args: &[String] + ) -> Result<Config, &'static str> { if args.len() < 3 { return Err("not enough arguments"); } @@ -1001,7 +999,7 @@ fn main() { process::exit(1); }); - // --snip-- + --snip-- } ``` @@ -1013,12 +1011,13 @@ Filename: src/main.rs ``` fn main() { - let config = Config::build(env::args()).unwrap_or_else(|err| { - eprintln!("Problem parsing arguments: {err}"); - process::exit(1); - }); + let config = + Config::build(env::args()).unwrap_or_else(|err| { + eprintln!("Problem parsing arguments: {err}"); + process::exit(1); + }); - // --snip-- + --snip-- } ``` @@ -1031,8 +1030,8 @@ we’re passing ownership of the iterator returned from `env::args` to 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. +look like Listing 13-19. This still won’t compile, because we need to update +the function body. Filename: src/lib.rs @@ -1041,7 +1040,7 @@ impl Config { pub fn build( mut args: impl Iterator<Item = String>, ) -> Result<Config, &'static str> { - // --snip-- + --snip-- ``` Listing 13-19: Updating the signature of `Config::build` to expect an iterator @@ -1053,18 +1052,18 @@ 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 -the “Traits as Parameters” section of Chapter 10 means that `args` can be any -type that implements the `Iterator` type and returns `String` items. +“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 +#### 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: +updates the code from Listing 12-23 to use the `next` method. Filename: src/lib.rs @@ -1100,21 +1099,24 @@ 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. Second, we call `next` to get the -value we want to put in the `query` field of `Config`. If `next` returns a +`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 Adaptors +### 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: +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> { +pub fn search<'a>( + query: &str, + contents: &'a str, +) -> Vec<&'a str> { let mut results = Vec::new(); for line in contents.lines() { @@ -1129,17 +1131,20 @@ pub fn search<'a>(query: &str, contents: &'a str) -> Vec<&'a str> { 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 adaptor methods. +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: +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> { +pub fn search<'a>( + query: &str, + contents: &'a str, +) -> Vec<&'a str> { contents .lines() .filter(|line| line.contains(query)) @@ -1147,24 +1152,23 @@ pub fn search<'a>(query: &str, contents: &'a str) -> Vec<&'a str> { } ``` -Listing 13-22: Using iterator adaptor methods in the implementation of the +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` adaptor to keep only the lines that -`line.contains(query)` returns `true` for. 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. +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 or Iterators +### 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 adaptors and what they do, iterators can be easier to +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 @@ -1172,8 +1176,7 @@ 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 more low-level loop will be faster. Let’s talk about -performance. +might be that the lower-level loop will be faster. Let’s talk about performance. ## Comparing Performance: Loops vs. Iterators @@ -1192,8 +1195,8 @@ 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 +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 @@ -1201,24 +1204,22 @@ 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 using the abstraction +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. +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]; @@ -1237,12 +1238,12 @@ for i in 12..buffer.len() { 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, we -multiply the values together, sum all the results, and shift the bits in the -sum `qlp_shift` bits to the right. +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 adaptors, and then +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 @@ -1253,7 +1254,7 @@ 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 these optimizations that Rust is able to apply make the resulting code +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. @@ -1269,3 +1270,4 @@ 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. + |