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<main>
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<h2 id="advanced-functions-and-closures"><a class="header" href="#advanced-functions-and-closures">Advanced Functions and Closures</a></h2>
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<p>This section explores some advanced features related to functions and closures,
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including function pointers and returning closures.</p>
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<h3 id="function-pointers"><a class="header" href="#function-pointers">Function Pointers</a></h3>
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<p>We’ve talked about how to pass closures to functions; you can also pass regular
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functions to functions! This technique is useful when you want to pass a
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function you’ve already defined rather than defining a new closure. Functions
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coerce to the type <code>fn</code> (with a lowercase <em>f</em>), not to be confused with the
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<code>Fn</code> closure trait. The <code>fn</code> type is called a <em>function pointer</em>. Passing
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functions with function pointers will allow you to use functions as arguments
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to other functions.</p>
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<p>The syntax for specifying that a parameter is a function pointer is similar to
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that of closures, as shown in Listing 20-28, where we’ve defined a function
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<code>add_one</code> that adds 1 to its parameter. The function <code>do_twice</code> takes two
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parameters: a function pointer to any function that takes an <code>i32</code> parameter
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and returns an <code>i32</code>, and one <code>i32</code> value. The <code>do_twice</code> function calls the
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function <code>f</code> twice, passing it the <code>arg</code> value, then adds the two function call
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results together. The <code>main</code> function calls <code>do_twice</code> with the arguments
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<code>add_one</code> and <code>5</code>.</p>
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<figure class="listing" id="listing-20-28">
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<span class="file-name">Filename: src/main.rs</span>
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<pre class="playground"><code class="language-rust edition2024">fn add_one(x: i32) -> i32 {
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x + 1
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}
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fn do_twice(f: fn(i32) -> i32, arg: i32) -> i32 {
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f(arg) + f(arg)
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}
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fn main() {
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let answer = do_twice(add_one, 5);
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println!("The answer is: {answer}");
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}</code></pre>
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<figcaption><a href="#listing-20-28">Listing 20-28</a>: Using the <code>fn</code> type to accept a function pointer as an argument</figcaption>
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</figure>
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<p>This code prints <code>The answer is: 12</code>. We specify that the parameter <code>f</code> in
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<code>do_twice</code> is an <code>fn</code> that takes one parameter of type <code>i32</code> and returns an
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<code>i32</code>. We can then call <code>f</code> in the body of <code>do_twice</code>. In <code>main</code>, we can pass
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the function name <code>add_one</code> as the first argument to <code>do_twice</code>.</p>
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<p>Unlike closures, <code>fn</code> is a type rather than a trait, so we specify <code>fn</code> as the
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parameter type directly rather than declaring a generic type parameter with one
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of the <code>Fn</code> traits as a trait bound.</p>
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<p>Function pointers implement all three of the closure traits (<code>Fn</code>, <code>FnMut</code>, and
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<code>FnOnce</code>), meaning you can always pass a function pointer as an argument for a
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function that expects a closure. It’s best to write functions using a generic
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type and one of the closure traits so that your functions can accept either
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functions or closures.</p>
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<p>That said, one example of where you would want to only accept <code>fn</code> and not
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closures is when interfacing with external code that doesn’t have closures: C
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functions can accept functions as arguments, but C doesn’t have closures.</p>
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<p>As an example of where you could use either a closure defined inline or a named
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function, let’s look at a use of the <code>map</code> method provided by the <code>Iterator</code>
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trait in the standard library. To use the <code>map</code> method to turn a vector of
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numbers into a vector of strings, we could use a closure, as in Listing 20-29.</p>
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<figure class="listing" id="listing-20-29">
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<pre class="playground"><code class="language-rust edition2024"><span class="boring">fn main() {
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</span> let list_of_numbers = vec![1, 2, 3];
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let list_of_strings: Vec<String> =
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list_of_numbers.iter().map(|i| i.to_string()).collect();
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<span class="boring">}</span></code></pre>
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<figcaption><a href="#listing-20-29">Listing 20-29</a>: Using a closure with the <code>map</code> method to convert numbers to strings</figcaption>
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</figure>
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<p>Or we could name a function as the argument to <code>map</code> instead of the closure.
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Listing 20-30 shows what this would look like.</p>
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<figure class="listing" id="listing-20-30">
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<pre class="playground"><code class="language-rust edition2024"><span class="boring">fn main() {
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</span> let list_of_numbers = vec![1, 2, 3];
|
||
let list_of_strings: Vec<String> =
|
||
list_of_numbers.iter().map(ToString::to_string).collect();
|
||
<span class="boring">}</span></code></pre>
|
||
<figcaption><a href="#listing-20-30">Listing 20-30</a>: Using the <code>String::to_string</code> function with the <code>map</code> method to convert numbers to strings</figcaption>
|
||
</figure>
|
||
<p>Note that we must use the fully qualified syntax that we talked about in the
|
||
<a href="ch20-02-advanced-traits.html#advanced-traits">“Advanced Traits”</a><!-- ignore --> section because there are
|
||
multiple functions available named <code>to_string</code>.</p>
|
||
<p>Here, we’re using the <code>to_string</code> function defined in the <code>ToString</code> trait,
|
||
which the standard library has implemented for any type that implements
|
||
<code>Display</code>.</p>
|
||
<p>Recall from the <a href="ch06-01-defining-an-enum.html#enum-values">“Enum Values”</a><!-- ignore --> section in Chapter
|
||
6 that the name of each enum variant that we define also becomes an initializer
|
||
function. We can use these initializer functions as function pointers that
|
||
implement the closure traits, which means we can specify the initializer
|
||
functions as arguments for methods that take closures, as seen in Listing 20-31.</p>
|
||
<figure class="listing" id="listing-20-31">
|
||
<pre class="playground"><code class="language-rust edition2024"><span class="boring">fn main() {
|
||
</span> enum Status {
|
||
Value(u32),
|
||
Stop,
|
||
}
|
||
|
||
let list_of_statuses: Vec<Status> = (0u32..20).map(Status::Value).collect();
|
||
<span class="boring">}</span></code></pre>
|
||
<figcaption><a href="#listing-20-31">Listing 20-31</a>: Using an enum initializer with the <code>map</code> method to create a <code>Status</code> instance from numbers</figcaption>
|
||
</figure>
|
||
<p>Here, we create <code>Status::Value</code> instances using each <code>u32</code> value in the range
|
||
that <code>map</code> is called on by using the initializer function of <code>Status::Value</code>.
|
||
Some people prefer this style and some people prefer to use closures. They
|
||
compile to the same code, so use whichever style is clearer to you.</p>
|
||
<h3 id="returning-closures"><a class="header" href="#returning-closures">Returning Closures</a></h3>
|
||
<p>Closures are represented by traits, which means you can’t return closures
|
||
directly. In most cases where you might want to return a trait, you can instead
|
||
use the concrete type that implements the trait as the return value of the
|
||
function. However, you can’t usually do that with closures because they don’t
|
||
have a concrete type that is returnable; you’re not allowed to use the function
|
||
pointer <code>fn</code> as a return type if the closure captures any values from its
|
||
scope, for example.</p>
|
||
<p>Instead, you will normally use the <code>impl Trait</code> syntax we learned about in
|
||
Chapter 10. You can return any function type, using <code>Fn</code>, <code>FnOnce</code>, and <code>FnMut</code>.
|
||
For example, the code in Listing 20-32 will compile just fine.</p>
|
||
<figure class="listing" id="listing-20-32">
|
||
<pre class="playground"><code class="language-rust edition2024"><span class="boring">#![allow(unused)]
|
||
</span><span class="boring">fn main() {
|
||
</span>fn returns_closure() -> impl Fn(i32) -> i32 {
|
||
|x| x + 1
|
||
}
|
||
<span class="boring">}</span></code></pre>
|
||
<figcaption><a href="#listing-20-32">Listing 20-32</a>: Returning a closure from a function using the <code>impl Trait</code> syntax</figcaption>
|
||
</figure>
|
||
<p>However, as we noted in the <a href="ch13-01-closures.html#closure-type-inference-and-annotation">“Inferring and Annotating Closure
|
||
Types”</a><!-- ignore --> section in Chapter 13, each closure is
|
||
also its own distinct type. If you need to work with multiple functions that
|
||
have the same signature but different implementations, you will need to use a
|
||
trait object for them. Consider what happens if you write code like that shown
|
||
in Listing 20-33.</p>
|
||
<figure class="listing" id="listing-20-33">
|
||
<span class="file-name">Filename: src/main.rs</span>
|
||
<pre><code class="language-rust ignore does_not_compile">fn main() {
|
||
let handlers = vec![returns_closure(), returns_initialized_closure(123)];
|
||
for handler in handlers {
|
||
let output = handler(5);
|
||
println!("{output}");
|
||
}
|
||
}
|
||
|
||
fn returns_closure() -> impl Fn(i32) -> i32 {
|
||
|x| x + 1
|
||
}
|
||
|
||
fn returns_initialized_closure(init: i32) -> impl Fn(i32) -> i32 {
|
||
move |x| x + init
|
||
}</code></pre>
|
||
<figcaption><a href="#listing-20-33">Listing 20-33</a>: Creating a <code>Vec<T></code> of closures defined by functions that return <code>impl Fn</code> types</figcaption>
|
||
</figure>
|
||
<p>Here we have two functions, <code>returns_closure</code> and <code>returns_initialized_closure</code>,
|
||
which both return <code>impl Fn(i32) -> i32</code>. Notice that the closures that they
|
||
return are different, even though they implement the same type. If we try to
|
||
compile this, Rust lets us know that it won’t work:</p>
|
||
<pre><code class="language-text">$ cargo build
|
||
Compiling functions-example v0.1.0 (file:///projects/functions-example)
|
||
error[E0308]: mismatched types
|
||
--> src/main.rs:2:44
|
||
|
|
||
2 | let handlers = vec![returns_closure(), returns_initialized_closure(123)];
|
||
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected opaque type, found a different opaque type
|
||
...
|
||
9 | fn returns_closure() -> impl Fn(i32) -> i32 {
|
||
| ------------------- the expected opaque type
|
||
...
|
||
13 | fn returns_initialized_closure(init: i32) -> impl Fn(i32) -> i32 {
|
||
| ------------------- the found opaque type
|
||
|
|
||
= note: expected opaque type `impl Fn(i32) -> i32`
|
||
found opaque type `impl Fn(i32) -> i32`
|
||
= note: distinct uses of `impl Trait` result in different opaque types
|
||
|
||
For more information about this error, try `rustc --explain E0308`.
|
||
error: could not compile `functions-example` (bin "functions-example") due to 1 previous error
|
||
</code></pre>
|
||
<p>The error message tells us that whenever we return an <code>impl Trait</code>, Rust
|
||
creates a unique <em>opaque type</em>, a type where we cannot see into the details of
|
||
what Rust constructs for us, nor can we guess the type Rust will generate to
|
||
write ourselves. So, even though these functions return closures that implement
|
||
the same trait, <code>Fn(i32) -> i32</code>, the opaque types Rust generates for each are
|
||
distinct. (This is similar to how Rust produces different concrete types for
|
||
distinct async blocks even when they have the same output type, as we saw in
|
||
<a href="ch17-03-more-futures.html">“The <code>Pin</code> Type and the <code>Unpin</code> Trait”</a><!-- ignore --> in
|
||
Chapter 17.) We have seen a solution to this problem a few times now: We can
|
||
use a trait object, as in Listing 20-34.</p>
|
||
<figure class="listing" id="listing-20-34">
|
||
<pre class="playground"><code class="language-rust edition2024"><span class="boring">fn main() {
|
||
</span><span class="boring"> let handlers = vec![returns_closure(), returns_initialized_closure(123)];
|
||
</span><span class="boring"> for handler in handlers {
|
||
</span><span class="boring"> let output = handler(5);
|
||
</span><span class="boring"> println!("{output}");
|
||
</span><span class="boring"> }
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span>fn returns_closure() -> Box<dyn Fn(i32) -> i32> {
|
||
Box::new(|x| x + 1)
|
||
}
|
||
|
||
fn returns_initialized_closure(init: i32) -> Box<dyn Fn(i32) -> i32> {
|
||
Box::new(move |x| x + init)
|
||
}</code></pre>
|
||
<figcaption><a href="#listing-20-34">Listing 20-34</a>: Creating a <code>Vec<T></code> of closures defined by functions that return <code>Box<dyn Fn></code> so that they have the same type</figcaption>
|
||
</figure>
|
||
<p>This code will compile just fine. For more about trait objects, refer to the
|
||
section <a href="ch18-02-trait-objects.html">“Using Trait Objects To Abstract over Shared
|
||
Behavior”</a><!-- ignore --> in Chapter 18.</p>
|
||
<p>Next, let’s look at macros!</p>
|
||
|
||
</main>
|
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<template id=fa-eye-slash><span class=fa-svg><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 640 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M38.8 5.1C28.4-3.1 13.3-1.2 5.1 9.2S-1.2 34.7 9.2 42.9l592 464c10.4 8.2 25.5 6.3 33.7-4.1s6.3-25.5-4.1-33.7L525.6 386.7c39.6-40.6 66.4-86.1 79.9-118.4c3.3-7.9 3.3-16.7 0-24.6c-14.9-35.7-46.2-87.7-93-131.1C465.5 68.8 400.8 32 320 32c-68.2 0-125 26.3-169.3 60.8L38.8 5.1zM223.1 149.5C248.6 126.2 282.7 112 320 112c79.5 0 144 64.5 144 144c0 24.9-6.3 48.3-17.4 68.7L408 294.5c5.2-11.8 8-24.8 8-38.5c0-53-43-96-96-96c-2.8 0-5.6 .1-8.4 .4c5.3 9.3 8.4 20.1 8.4 31.6c0 10.2-2.4 19.8-6.6 28.3l-90.3-70.8zm223.1 298L373 389.9c-16.4 6.5-34.3 10.1-53 10.1c-79.5 0-144-64.5-144-144c0-6.9 .5-13.6 1.4-20.2L83.1 161.5C60.3 191.2 44 220.8 34.5 243.7c-3.3 7.9-3.3 16.7 0 24.6c14.9 35.7 46.2 87.7 93 131.1C174.5 443.2 239.2 480 320 480c47.8 0 89.9-12.9 126.2-32.5z"/></svg></span></template>
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<template id=fa-copy><span class=fa-svg><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M502.6 70.63l-61.25-61.25C435.4 3.371 427.2 0 418.7 0H255.1c-35.35 0-64 28.66-64 64l.0195 256C192 355.4 220.7 384 256 384h192c35.2 0 64-28.8 64-64V93.25C512 84.77 508.6 76.63 502.6 70.63zM464 320c0 8.836-7.164 16-16 16H255.1c-8.838 0-16-7.164-16-16L239.1 64.13c0-8.836 7.164-16 16-16h128L384 96c0 17.67 14.33 32 32 32h47.1V320zM272 448c0 8.836-7.164 16-16 16H63.1c-8.838 0-16-7.164-16-16L47.98 192.1c0-8.836 7.164-16 16-16H160V128H63.99c-35.35 0-64 28.65-64 64l.0098 256C.002 483.3 28.66 512 64 512h192c35.2 0 64-28.8 64-64v-32h-47.1L272 448z"/></svg></span></template>
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