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<h2><a class="header" href="#concise-control-flow-with-if-let" id="concise-control-flow-with-if-let">Concise Control Flow with <code>if let</code></a></h2>
<p>The <code>if let</code> syntax lets you combine <code>if</code> and <code>let</code> into a less verbose way to
handle values that match one pattern while ignoring the rest. Consider the
program in Listing 6-6 that matches on an <code>Option&lt;u8&gt;</code> value but only wants to
execute code if the value is 3.</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 = Some(0u8);
match some_u8_value {
Some(3) =&gt; println!(&quot;three&quot;),
_ =&gt; (),
}
<span class="boring">}
</span></code></pre></pre>
<p><span class="caption">Listing 6-6: A <code>match</code> that only cares about executing
code when the value is <code>Some(3)</code></span></p>
<p>We want to do something with the <code>Some(3)</code> match but do nothing with any other
<code>Some&lt;u8&gt;</code> value or the <code>None</code> value. To satisfy the <code>match</code> expression, we
have to add <code>_ =&gt; ()</code> after processing just one variant, which is a lot of
boilerplate code to add.</p>
<p>Instead, we could write this in a shorter way using <code>if let</code>. The following
code behaves the same as the <code>match</code> in Listing 6-6:</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">let some_u8_value = Some(0u8);
</span>if let Some(3) = some_u8_value {
println!(&quot;three&quot;);
}
<span class="boring">}
</span></code></pre></pre>
<p>The syntax <code>if let</code> takes a pattern and an expression separated by an equal
sign. It works the same way as a <code>match</code>, where the expression is given to the
<code>match</code> and the pattern is its first arm.</p>
<p>Using <code>if let</code> means less typing, less indentation, and less boilerplate code.
However, you lose the exhaustive checking that <code>match</code> enforces. Choosing
between <code>match</code> and <code>if let</code> depends on what you’re doing in your particular
situation and whether gaining conciseness is an appropriate trade-off for
losing exhaustive checking.</p>
<p>In other words, you can think of <code>if let</code> as syntax sugar for a <code>match</code> that
runs code when the value matches one pattern and then ignores all other values.</p>
<p>We can include an <code>else</code> with an <code>if let</code>. The block of code that goes with the
<code>else</code> is the same as the block of code that would go with the <code>_</code> case in the
<code>match</code> expression that is equivalent to the <code>if let</code> and <code>else</code>. Recall the
<code>Coin</code> enum definition in Listing 6-4, where the <code>Quarter</code> variant also held a
<code>UsState</code> value. If we wanted to count all non-quarter coins we see while also
announcing the state of the quarters, we could do that with a <code>match</code>
expression like this:</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">let coin = Coin::Penny;
</span>let mut count = 0;
match coin {
Coin::Quarter(state) =&gt; println!(&quot;State quarter from {:?}!&quot;, state),
_ =&gt; count += 1,
}
<span class="boring">}
</span></code></pre></pre>
<p>Or we could use an <code>if let</code> and <code>else</code> expression like this:</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">let coin = Coin::Penny;
</span>let mut count = 0;
if let Coin::Quarter(state) = coin {
println!(&quot;State quarter from {:?}!&quot;, state);
} else {
count += 1;
}
<span class="boring">}
</span></code></pre></pre>
<p>If you have a situation in which your program has logic that is too verbose to
express using a <code>match</code>, remember that <code>if let</code> is in your Rust toolbox as well.</p>
<h2><a class="header" href="#summary" id="summary">Summary</a></h2>
<p>We’ve now covered how to use enums to create custom types that can be one of a
set of enumerated values. We’ve shown how the standard library’s <code>Option&lt;T&gt;</code>
type helps you use the type system to prevent errors. When enum values have
data inside them, you can use <code>match</code> or <code>if let</code> to extract and use those
values, depending on how many cases you need to handle.</p>
<p>Your Rust programs can now express concepts in your domain using structs and
enums. Creating custom types to use in your API ensures type safety: the
compiler will make certain your functions get only values of the type each
function expects.</p>
<p>In order to provide a well-organized API to your users that is straightforward
to use and only exposes exactly what your users will need, let’s now turn to
Rust’s modules.</p>
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