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<h2><a class="header" href="#the-match-control-flow-operator" id="the-match-control-flow-operator">The <code>match</code> Control Flow Operator</a></h2>
<p>Rust has an extremely powerful control flow operator called <code>match</code> that allows
you to compare a value against a series of patterns and then execute code based
on which pattern matches. Patterns can be made up of literal values, variable
names, wildcards, and many other things; Chapter 18 covers all the different
kinds of patterns and what they do. The power of <code>match</code> comes from the
expressiveness of the patterns and the fact that the compiler confirms that all
possible cases are handled.</p>
<p>Think of a <code>match</code> expression as being like a coin-sorting machine: coins slide
down a track with variously sized holes along it, and each coin falls through
the first hole it encounters that it fits into. In the same way, values go
through each pattern in a <code>match</code>, and at the first pattern the value “fits,”
the value falls into the associated code block to be used during execution.</p>
<p>Because we just mentioned coins, let’s use them as an example using <code>match</code>! We
can write a function that can take an unknown United States coin and, in a
similar way as the counting machine, determine which coin it is and return its
value in cents, as shown here in Listing 6-3.</p>
<pre><pre class="playpen"><code class="language-rust">
<span class="boring">#![allow(unused_variables)]
</span><span class="boring">fn main() {
</span>enum Coin {
Penny,
Nickel,
Dime,
Quarter,
}
fn value_in_cents(coin: Coin) -&gt; u8 {
match coin {
Coin::Penny =&gt; 1,
Coin::Nickel =&gt; 5,
Coin::Dime =&gt; 10,
Coin::Quarter =&gt; 25,
}
}
<span class="boring">}
</span></code></pre></pre>
<p><span class="caption">Listing 6-3: An enum and a <code>match</code> expression that has
the variants of the enum as its patterns</span></p>
<p>Let’s break down the <code>match</code> in the <code>value_in_cents</code> function. First, we list
the <code>match</code> keyword followed by an expression, which in this case is the value
<code>coin</code>. This seems very similar to an expression used with <code>if</code>, but there’s a
big difference: with <code>if</code>, the expression needs to return a Boolean value, but
here, it can be any type. The type of <code>coin</code> in this example is the <code>Coin</code> enum
that we defined on line 1.</p>
<p>Next are the <code>match</code> arms. An arm has two parts: a pattern and some code. The
first arm here has a pattern that is the value <code>Coin::Penny</code> and then the <code>=&gt;</code>
operator that separates the pattern and the code to run. The code in this case
is just the value <code>1</code>. Each arm is separated from the next with a comma.</p>
<p>When the <code>match</code> expression executes, it compares the resulting value against
the pattern of each arm, in order. If a pattern matches the value, the code
associated with that pattern is executed. If that pattern doesn’t match the
value, execution continues to the next arm, much as in a coin-sorting machine.
We can have as many arms as we need: in Listing 6-3, our <code>match</code> has four arms.</p>
<p>The code associated with each arm is an expression, and the resulting value of
the expression in the matching arm is the value that gets returned for the
entire <code>match</code> expression.</p>
<p>Curly brackets typically aren’t used if the match arm code is short, as it is
in Listing 6-3 where each arm just returns a value. If you want to run multiple
lines of code in a match arm, you can use curly brackets. For example, the
following code would print “Lucky penny!” every time the method was called with
a <code>Coin::Penny</code> but would still return the last value of the block, <code>1</code>:</p>
<pre><pre class="playpen"><code class="language-rust">
<span class="boring">#![allow(unused_variables)]
</span><span class="boring">fn main() {
</span><span class="boring">enum Coin {
</span><span class="boring"> Penny,
</span><span class="boring"> Nickel,
</span><span class="boring"> Dime,
</span><span class="boring"> Quarter,
</span><span class="boring">}
</span><span class="boring">
</span>fn value_in_cents(coin: Coin) -&gt; u8 {
match coin {
Coin::Penny =&gt; {
println!(&quot;Lucky penny!&quot;);
1
},
Coin::Nickel =&gt; 5,
Coin::Dime =&gt; 10,
Coin::Quarter =&gt; 25,
}
}
<span class="boring">}
</span></code></pre></pre>
<h3><a class="header" href="#patterns-that-bind-to-values" id="patterns-that-bind-to-values">Patterns that Bind to Values</a></h3>
<p>Another useful feature of match arms is that they can bind to the parts of the
values that match the pattern. This is how we can extract values out of enum
variants.</p>
<p>As an example, let’s change one of our enum variants to hold data inside it.
From 1999 through 2008, the United States minted quarters with different
designs for each of the 50 states on one side. No other coins got state
designs, so only quarters have this extra value. We can add this information to
our <code>enum</code> by changing the <code>Quarter</code> variant to include a <code>UsState</code> value stored
inside it, which we’ve done here in Listing 6-4.</p>
<pre><pre class="playpen"><code class="language-rust">
<span class="boring">#![allow(unused_variables)]
</span><span class="boring">fn main() {
</span>#[derive(Debug)] // so we can inspect the state in a minute
enum UsState {
Alabama,
Alaska,
// --snip--
}
enum Coin {
Penny,
Nickel,
Dime,
Quarter(UsState),
}
<span class="boring">}
</span></code></pre></pre>
<p><span class="caption">Listing 6-4: A <code>Coin</code> enum in which the <code>Quarter</code> variant
also holds a <code>UsState</code> value</span></p>
<p>Let’s imagine that a friend of ours is trying to collect all 50 state quarters.
While we sort our loose change by coin type, we’ll also call out the name of
the state associated with each quarter so if it’s one our friend doesn’t have,
they can add it to their collection.</p>
<p>In the match expression for this code, we add a variable called <code>state</code> to the
pattern that matches values of the variant <code>Coin::Quarter</code>. When a
<code>Coin::Quarter</code> matches, the <code>state</code> variable will bind to the value of that
quarter’s state. Then we can use <code>state</code> in the code for that arm, like so:</p>
<pre><pre class="playpen"><code class="language-rust">
<span class="boring">#![allow(unused_variables)]
</span><span class="boring">fn main() {
</span><span class="boring">#[derive(Debug)]
</span><span class="boring">enum UsState {
</span><span class="boring"> Alabama,
</span><span class="boring"> Alaska,
</span><span class="boring">}
</span><span class="boring">
</span><span class="boring">enum Coin {
</span><span class="boring"> Penny,
</span><span class="boring"> Nickel,
</span><span class="boring"> Dime,
</span><span class="boring"> Quarter(UsState),
</span><span class="boring">}
</span><span class="boring">
</span>fn value_in_cents(coin: Coin) -&gt; u8 {
match coin {
Coin::Penny =&gt; 1,
Coin::Nickel =&gt; 5,
Coin::Dime =&gt; 10,
Coin::Quarter(state) =&gt; {
println!(&quot;State quarter from {:?}!&quot;, state);
25
},
}
}
<span class="boring">}
</span></code></pre></pre>
<p>If we were to call <code>value_in_cents(Coin::Quarter(UsState::Alaska))</code>, <code>coin</code>
would be <code>Coin::Quarter(UsState::Alaska)</code>. When we compare that value with each
of the match arms, none of them match until we reach <code>Coin::Quarter(state)</code>. At
that point, the binding for <code>state</code> will be the value <code>UsState::Alaska</code>. We can
then use that binding in the <code>println!</code> expression, thus getting the inner
state value out of the <code>Coin</code> enum variant for <code>Quarter</code>.</p>
<h3><a class="header" href="#matching-with-optiont" id="matching-with-optiont">Matching with <code>Option&lt;T&gt;</code></a></h3>
<p>In the previous section, we wanted to get the inner <code>T</code> value out of the <code>Some</code>
case when using <code>Option&lt;T&gt;</code>; we can also handle <code>Option&lt;T&gt;</code> using <code>match</code> as we
did with the <code>Coin</code> enum! Instead of comparing coins, we’ll compare the
variants of <code>Option&lt;T&gt;</code>, but the way that the <code>match</code> expression works remains
the same.</p>
<p>Let’s say we want to write a function that takes an <code>Option&lt;i32&gt;</code> and, if
there’s a value inside, adds 1 to that value. If there isn’t a value inside,
the function should return the <code>None</code> value and not attempt to perform any
operations.</p>
<p>This function is very easy to write, thanks to <code>match</code>, and will look like
Listing 6-5.</p>
<pre><pre class="playpen"><code class="language-rust">
<span class="boring">#![allow(unused_variables)]
</span><span class="boring">fn main() {
</span>fn plus_one(x: Option&lt;i32&gt;) -&gt; Option&lt;i32&gt; {
match x {
None =&gt; None,
Some(i) =&gt; Some(i + 1),
}
}
let five = Some(5);
let six = plus_one(five);
let none = plus_one(None);
<span class="boring">}
</span></code></pre></pre>
<p><span class="caption">Listing 6-5: A function that uses a <code>match</code> expression on
an <code>Option&lt;i32&gt;</code></span></p>
<p>Let’s examine the first execution of <code>plus_one</code> in more detail. When we call
<code>plus_one(five)</code>, the variable <code>x</code> in the body of <code>plus_one</code> will have the
value <code>Some(5)</code>. We then compare that against each match arm.</p>
<pre><code class="language-rust ignore">None =&gt; None,
</code></pre>
<p>The <code>Some(5)</code> value doesn’t match the pattern <code>None</code>, so we continue to the
next arm.</p>
<pre><code class="language-rust ignore">Some(i) =&gt; Some(i + 1),
</code></pre>
<p>Does <code>Some(5)</code> match <code>Some(i)</code>? Why yes it does! We have the same variant. The
<code>i</code> binds to the value contained in <code>Some</code>, so <code>i</code> takes the value <code>5</code>. The
code in the match arm is then executed, so we add 1 to the value of <code>i</code> and
create a new <code>Some</code> value with our total <code>6</code> inside.</p>
<p>Now let’s consider the second call of <code>plus_one</code> in Listing 6-5, where <code>x</code> is
<code>None</code>. We enter the <code>match</code> and compare to the first arm.</p>
<pre><code class="language-rust ignore">None =&gt; None,
</code></pre>
<p>It matches! There’s no value to add to, so the program stops and returns the
<code>None</code> value on the right side of <code>=&gt;</code>. Because the first arm matched, no other
arms are compared.</p>
<p>Combining <code>match</code> and enums is useful in many situations. You’ll see this
pattern a lot in Rust code: <code>match</code> against an enum, bind a variable to the
data inside, and then execute code based on it. It’s a bit tricky at first, but
once you get used to it, you’ll wish you had it in all languages. It’s
consistently a user favorite.</p>
<h3><a class="header" href="#matches-are-exhaustive" id="matches-are-exhaustive">Matches Are Exhaustive</a></h3>
<p>There’s one other aspect of <code>match</code> we need to discuss. Consider this version
of our <code>plus_one</code> function that has a bug and won’t compile:</p>
<pre><code class="language-rust ignore does_not_compile">fn plus_one(x: Option&lt;i32&gt;) -&gt; Option&lt;i32&gt; {
match x {
Some(i) =&gt; Some(i + 1),
}
}
</code></pre>
<p>We didn’t handle the <code>None</code> case, so this code will cause a bug. Luckily, it’s
a bug Rust knows how to catch. If we try to compile this code, we’ll get this
error:</p>
<pre><code class="language-text">error[E0004]: non-exhaustive patterns: `None` not covered
--&gt;
|
6 | match x {
| ^ pattern `None` not covered
</code></pre>
<p>Rust knows that we didn’t cover every possible case and even knows which
pattern we forgot! Matches in Rust are <em>exhaustive</em>: we must exhaust every last
possibility in order for the code to be valid. Especially in the case of
<code>Option&lt;T&gt;</code>, when Rust prevents us from forgetting to explicitly handle the
<code>None</code> case, it protects us from assuming that we have a value when we might
have null, thus making the billion-dollar mistake discussed earlier.</p>
<h3><a class="header" href="#the-_-placeholder" id="the-_-placeholder">The <code>_</code> Placeholder</a></h3>
<p>Rust also has a pattern we can use when we don’t want to list all possible
values. For example, a <code>u8</code> can have valid values of 0 through 255. If we only
care about the values 1, 3, 5, and 7, we don’t want to have to list out 0, 2,
4, 6, 8, 9 all the way up to 255. Fortunately, we don’t have to: we can use the
special pattern <code>_</code> instead:</p>
<pre><pre class="playpen"><code class="language-rust">
<span class="boring">#![allow(unused_variables)]
</span><span class="boring">fn main() {
</span>let some_u8_value = 0u8;
match some_u8_value {
1 =&gt; println!(&quot;one&quot;),
3 =&gt; println!(&quot;three&quot;),
5 =&gt; println!(&quot;five&quot;),
7 =&gt; println!(&quot;seven&quot;),
_ =&gt; (),
}
<span class="boring">}
</span></code></pre></pre>
<p>The <code>_</code> pattern will match any value. By putting it after our other arms, the
<code>_</code> will match all the possible cases that aren’t specified before it. The <code>()</code>
is just the unit value, so nothing will happen in the <code>_</code> case. As a result, we
can say that we want to do nothing for all the possible values that we don’t
list before the <code>_</code> placeholder.</p>
<p>However, the <code>match</code> expression can be a bit wordy in a situation in which we
care about only <em>one</em> of the cases. For this situation, Rust provides <code>if let</code>.</p>
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