262 lines
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262 lines
16 KiB
HTML
<!DOCTYPE html>
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<html lang="en">
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<head>
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<meta charset="UTF-8">
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<title>All the Places Patterns Can Be Used</title>
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</head>
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<body>
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<h2 id="all-the-places-patterns-can-be-used"><a class="header" href="#all-the-places-patterns-can-be-used">All the Places Patterns Can Be Used</a></h2>
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<p>Patterns pop up in a number of places in Rust, and you’ve been using them a lot
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without realizing it! This section discusses all the places where patterns are
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valid.</p>
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<h3 id="match-arms"><a class="header" href="#match-arms"><code>match</code> Arms</a></h3>
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<p>As discussed in Chapter 6, we use patterns in the arms of <code>match</code> expressions.
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Formally, <code>match</code> expressions are defined as the keyword <code>match</code>, a value to
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match on, and one or more match arms that consist of a pattern and an
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expression to run if the value matches that arm’s pattern, like this:</p>
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<!--
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Manually formatted rather than using Markdown intentionally: Markdown does not
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support italicizing code in the body of a block like this!
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-->
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<pre><code>match <em>VALUE</em> {
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<em>PATTERN</em> => <em>EXPRESSION</em>,
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<em>PATTERN</em> => <em>EXPRESSION</em>,
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<em>PATTERN</em> => <em>EXPRESSION</em>,
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}</code></pre>
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<p>For example, here’s the <code>match</code> expression from Listing 6-5 that matches on an
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<code>Option<i32></code> value in the variable <code>x</code>:</p>
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<pre><code class="language-rust ignore">match x {
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None => None,
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Some(i) => Some(i + 1),
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}</code></pre>
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<p>The patterns in this <code>match</code> expression are the <code>None</code> and <code>Some(i)</code> to the
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left of each arrow.</p>
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<p>One requirement for <code>match</code> expressions is that they need to be exhaustive in
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the sense that all possibilities for the value in the <code>match</code> expression must
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be accounted for. One way to ensure that you’ve covered every possibility is to
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have a catch-all pattern for the last arm: For example, a variable name
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matching any value can never fail and thus covers every remaining case.</p>
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<p>The particular pattern <code>_</code> will match anything, but it never binds to a
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variable, so it’s often used in the last match arm. The <code>_</code> pattern can be
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useful when you want to ignore any value not specified, for example. We’ll
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cover the <code>_</code> pattern in more detail in <a href="ch19-03-pattern-syntax.html#ignoring-values-in-a-pattern">“Ignoring Values in a
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Pattern”</a><!-- ignore --> later in this chapter.</p>
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<h3 id="let-statements"><a class="header" href="#let-statements"><code>let</code> Statements</a></h3>
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<p>Prior to this chapter, we had only explicitly discussed using patterns with
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<code>match</code> and <code>if let</code>, but in fact, we’ve used patterns in other places as well,
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including in <code>let</code> statements. For example, consider this straightforward
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variable assignment with <code>let</code>:</p>
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<pre class="playground"><code class="language-rust edition2024"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>let x = 5;
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<span class="boring">}</span></code></pre>
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<p>Every time you’ve used a <code>let</code> statement like this you’ve been using patterns,
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although you might not have realized it! More formally, a <code>let</code> statement looks
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like this:</p>
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<!--
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Manually formatted rather than using Markdown intentionally: Markdown does not
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support italicizing code in the body of a block like this!
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-->
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<pre>
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<code>let <em>PATTERN</em> = <em>EXPRESSION</em>;</code>
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</pre>
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<p>In statements like <code>let x = 5;</code> with a variable name in the PATTERN slot, the
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variable name is just a particularly simple form of a pattern. Rust compares
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the expression against the pattern and assigns any names it finds. So, in the
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<code>let x = 5;</code> example, <code>x</code> is a pattern that means “bind what matches here to
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the variable <code>x</code>.” Because the name <code>x</code> is the whole pattern, this pattern
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effectively means “bind everything to the variable <code>x</code>, whatever the value is.”</p>
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<p>To see the pattern-matching aspect of <code>let</code> more clearly, consider Listing
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19-1, which uses a pattern with <code>let</code> to destructure a tuple.</p>
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<figure class="listing" id="listing-19-1">
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<pre class="playground"><code class="language-rust edition2024"><span class="boring">fn main() {
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</span> let (x, y, z) = (1, 2, 3);
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<span class="boring">}</span></code></pre>
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<figcaption><a href="#listing-19-1">Listing 19-1</a>: Using a pattern to destructure a tuple and create three variables at once</figcaption>
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</figure>
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<p>Here, we match a tuple against a pattern. Rust compares the value <code>(1, 2, 3)</code>
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to the pattern <code>(x, y, z)</code> and sees that the value matches the pattern—that is,
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it sees that the number of elements is the same in both—so Rust binds <code>1</code> to
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<code>x</code>, <code>2</code> to <code>y</code>, and <code>3</code> to <code>z</code>. You can think of this tuple pattern as nesting
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three individual variable patterns inside it.</p>
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<p>If the number of elements in the pattern doesn’t match the number of elements
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in the tuple, the overall type won’t match and we’ll get a compiler error. For
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example, Listing 19-2 shows an attempt to destructure a tuple with three
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elements into two variables, which won’t work.</p>
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<figure class="listing" id="listing-19-2">
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<pre><code class="language-rust ignore does_not_compile"><span class="boring">fn main() {
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</span> let (x, y) = (1, 2, 3);
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<span class="boring">}</span></code></pre>
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<figcaption><a href="#listing-19-2">Listing 19-2</a>: Incorrectly constructing a pattern whose variables don’t match the number of elements in the tuple</figcaption>
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</figure>
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<p>Attempting to compile this code results in this type error:</p>
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<pre><code class="language-console">$ cargo run
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Compiling patterns v0.1.0 (file:///projects/patterns)
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error[E0308]: mismatched types
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--> src/main.rs:2:9
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|
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2 | let (x, y) = (1, 2, 3);
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| ^^^^^^ --------- this expression has type `({integer}, {integer}, {integer})`
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| |
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| expected a tuple with 3 elements, found one with 2 elements
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= note: expected tuple `({integer}, {integer}, {integer})`
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found tuple `(_, _)`
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For more information about this error, try `rustc --explain E0308`.
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error: could not compile `patterns` (bin "patterns") due to 1 previous error
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</code></pre>
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<p>To fix the error, we could ignore one or more of the values in the tuple using
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<code>_</code> or <code>..</code>, as you’ll see in the <a href="ch19-03-pattern-syntax.html#ignoring-values-in-a-pattern">“Ignoring Values in a
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Pattern”</a><!-- ignore --> section. If the problem
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is that we have too many variables in the pattern, the solution is to make the
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types match by removing variables so that the number of variables equals the
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number of elements in the tuple.</p>
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<h3 id="conditional-if-let-expressions"><a class="header" href="#conditional-if-let-expressions">Conditional <code>if let</code> Expressions</a></h3>
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<p>In Chapter 6, we discussed how to use <code>if let</code> expressions mainly as a shorter
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way to write the equivalent of a <code>match</code> that only matches one case.
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Optionally, <code>if let</code> can have a corresponding <code>else</code> containing code to run if
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the pattern in the <code>if let</code> doesn’t match.</p>
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<p>Listing 19-3 shows that it’s also possible to mix and match <code>if let</code>, <code>else if</code>, and <code>else if let</code> expressions. Doing so gives us more flexibility than a
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<code>match</code> expression in which we can express only one value to compare with the
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patterns. Also, Rust doesn’t require that the conditions in a series of <code>if let</code>, <code>else if</code>, and <code>else if let</code> arms relate to each other.</p>
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<p>The code in Listing 19-3 determines what color to make your background based on
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a series of checks for several conditions. For this example, we’ve created
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variables with hardcoded values that a real program might receive from user
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input.</p>
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<figure class="listing" id="listing-19-3">
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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 main() {
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let favorite_color: Option<&str> = None;
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let is_tuesday = false;
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let age: Result<u8, _> = "34".parse();
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if let Some(color) = favorite_color {
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println!("Using your favorite color, {color}, as the background");
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} else if is_tuesday {
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println!("Tuesday is green day!");
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} else if let Ok(age) = age {
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if age > 30 {
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println!("Using purple as the background color");
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} else {
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println!("Using orange as the background color");
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}
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} else {
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println!("Using blue as the background color");
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}
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}</code></pre>
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<figcaption><a href="#listing-19-3">Listing 19-3</a>: Mixing <code>if let</code>, <code>else if</code>, <code>else if let</code>, and <code>else</code></figcaption>
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</figure>
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<p>If the user specifies a favorite color, that color is used as the background.
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If no favorite color is specified and today is Tuesday, the background color is
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green. Otherwise, if the user specifies their age as a string and we can parse
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it as a number successfully, the color is either purple or orange depending on
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the value of the number. If none of these conditions apply, the background
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color is blue.</p>
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<p>This conditional structure lets us support complex requirements. With the
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hardcoded values we have here, this example will print <code>Using purple as the background color</code>.</p>
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<p>You can see that <code>if let</code> can also introduce new variables that shadow existing
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variables in the same way that <code>match</code> arms can: The line <code>if let Ok(age) = age</code>
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introduces a new <code>age</code> variable that contains the value inside the <code>Ok</code> variant,
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shadowing the existing <code>age</code> variable. This means we need to place the <code>if age > 30</code> condition within that block: We can’t combine these two conditions into <code>if let Ok(age) = age && age > 30</code>. The new <code>age</code> we want to compare to 30 isn’t
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valid until the new scope starts with the curly bracket.</p>
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<p>The downside of using <code>if let</code> expressions is that the compiler doesn’t check
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for exhaustiveness, whereas with <code>match</code> expressions it does. If we omitted the
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last <code>else</code> block and therefore missed handling some cases, the compiler would
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not alert us to the possible logic bug.</p>
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<h3 id="while-let-conditional-loops"><a class="header" href="#while-let-conditional-loops"><code>while let</code> Conditional Loops</a></h3>
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<p>Similar in construction to <code>if let</code>, the <code>while let</code> conditional loop allows a
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<code>while</code> loop to run for as long as a pattern continues to match. In Listing
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19-4, we show a <code>while let</code> loop that waits on messages sent between threads,
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but in this case checking a <code>Result</code> instead of an <code>Option</code>.</p>
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<figure class="listing" id="listing-19-4">
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<pre class="playground"><code class="language-rust edition2024"><span class="boring">fn main() {
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</span> let (tx, rx) = std::sync::mpsc::channel();
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std::thread::spawn(move || {
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for val in [1, 2, 3] {
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tx.send(val).unwrap();
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}
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});
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while let Ok(value) = rx.recv() {
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println!("{value}");
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}
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<span class="boring">}</span></code></pre>
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<figcaption><a href="#listing-19-4">Listing 19-4</a>: Using a <code>while let</code> loop to print values for as long as <code>rx.recv()</code> returns <code>Ok</code></figcaption>
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</figure>
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<p>This example prints <code>1</code>, <code>2</code>, and then <code>3</code>. The <code>recv</code> method takes the first
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message out of the receiver side of the channel and returns an <code>Ok(value)</code>. When
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we first saw <code>recv</code> back in Chapter 16, we unwrapped the error directly, or
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we interacted with it as an iterator using a <code>for</code> loop. As Listing 19-4 shows,
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though, we can also use <code>while let</code>, because the <code>recv</code> method returns an <code>Ok</code>
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each time a message arrives, as long as the sender exists, and then produces an
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<code>Err</code> once the sender side disconnects.</p>
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<h3 id="for-loops"><a class="header" href="#for-loops"><code>for</code> Loops</a></h3>
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<p>In a <code>for</code> loop, the value that directly follows the keyword <code>for</code> is a
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pattern. For example, in <code>for x in y</code>, the <code>x</code> is the pattern. Listing 19-5
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demonstrates how to use a pattern in a <code>for</code> loop to destructure, or break
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apart, a tuple as part of the <code>for</code> loop.</p>
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<figure class="listing" id="listing-19-5">
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<pre class="playground"><code class="language-rust edition2024"><span class="boring">fn main() {
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</span> let v = vec!['a', 'b', 'c'];
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for (index, value) in v.iter().enumerate() {
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println!("{value} is at index {index}");
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}
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<span class="boring">}</span></code></pre>
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<figcaption><a href="#listing-19-5">Listing 19-5</a>: Using a pattern in a <code>for</code> loop to destructure a tuple</figcaption>
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</figure>
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<p>The code in Listing 19-5 will print the following:</p>
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<pre><code class="language-console">$ cargo run
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Compiling patterns v0.1.0 (file:///projects/patterns)
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Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.52s
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Running `target/debug/patterns`
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a is at index 0
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b is at index 1
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c is at index 2
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</code></pre>
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<p>We adapt an iterator using the <code>enumerate</code> method so that it produces a value
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and the index for that value, placed into a tuple. The first value produced is
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the tuple <code>(0, 'a')</code>. When this value is matched to the pattern <code>(index, value)</code>, index will be <code>0</code> and value will be <code>'a'</code>, printing the first line of
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the output.</p>
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<h3 id="function-parameters"><a class="header" href="#function-parameters">Function Parameters</a></h3>
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<p>Function parameters can also be patterns. The code in Listing 19-6, which
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declares a function named <code>foo</code> that takes one parameter named <code>x</code> of type
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<code>i32</code>, should by now look familiar.</p>
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<figure class="listing" id="listing-19-6">
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<pre class="playground"><code class="language-rust edition2024">fn foo(x: i32) {
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// code goes here
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}
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<span class="boring">
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</span><span class="boring">fn main() {}</span></code></pre>
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<figcaption><a href="#listing-19-6">Listing 19-6</a>: A function signature using patterns in the parameters</figcaption>
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</figure>
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<p>The <code>x</code> part is a pattern! As we did with <code>let</code>, we could match a tuple in a
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function’s arguments to the pattern. Listing 19-7 splits the values in a tuple
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as we pass it to a function.</p>
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<figure class="listing" id="listing-19-7">
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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 print_coordinates(&(x, y): &(i32, i32)) {
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println!("Current location: ({x}, {y})");
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}
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fn main() {
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let point = (3, 5);
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print_coordinates(&point);
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}</code></pre>
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<figcaption><a href="#listing-19-7">Listing 19-7</a>: A function with parameters that destructure a tuple</figcaption>
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</figure>
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<p>This code prints <code>Current location: (3, 5)</code>. The values <code>&(3, 5)</code> match the
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pattern <code>&(x, y)</code>, so <code>x</code> is the value <code>3</code> and <code>y</code> is the value <code>5</code>.</p>
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<p>We can also use patterns in closure parameter lists in the same way as in
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function parameter lists because closures are similar to functions, as
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discussed in Chapter 13.</p>
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<p>At this point, you’ve seen several ways to use patterns, but patterns don’t
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work the same in every place we can use them. In some places, the patterns must
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be irrefutable; in other circumstances, they can be refutable. We’ll discuss
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these two concepts next.</p>
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</body>
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</html>
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