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<ol class="chapter"><li class="chapter-item expanded "><a href="Chapter_0.html"><strong aria-hidden="true">1.</strong> Update</a></li><li class="chapter-item expanded "><a href="Chapter_1.html"><strong aria-hidden="true">2.</strong> Introduction</a></li><li class="chapter-item expanded "><a href="Chapter_2.html"><strong aria-hidden="true">3.</strong> Who am I?</a></li><li class="chapter-item expanded "><a href="Chapter_3.html"><strong aria-hidden="true">4.</strong> Writing Rust in Easy English</a></li><li class="chapter-item expanded "><a href="Chapter_4.html"><strong aria-hidden="true">5.</strong> Rust Playground</a></li><li class="chapter-item expanded "><a href="Chapter_5.html"><strong aria-hidden="true">6.</strong> 🚧 and ⚠️</a></li><li class="chapter-item expanded "><a href="Chapter_6.html"><strong aria-hidden="true">7.</strong> Comments</a></li><li class="chapter-item expanded "><a href="Chapter_7.html"><strong aria-hidden="true">8.</strong> Types</a></li><li class="chapter-item expanded "><a href="Chapter_8.html"><strong aria-hidden="true">9.</strong> Type inference</a></li><li class="chapter-item expanded "><a href="Chapter_9.html"><strong aria-hidden="true">10.</strong> Printing 'hello, world!'</a></li><li class="chapter-item expanded "><a href="Chapter_10.html"><strong aria-hidden="true">11.</strong> Display and debug</a></li><li class="chapter-item expanded "><a href="Chapter_11.html"><strong aria-hidden="true">12.</strong> Mutability (changing)</a></li><li class="chapter-item expanded "><a href="Chapter_12.html"><strong aria-hidden="true">13.</strong> The stack, the heap, and pointers</a></li><li class="chapter-item expanded "><a href="Chapter_13.html"><strong aria-hidden="true">14.</strong> More about printing</a></li><li class="chapter-item expanded "><a href="Chapter_14.html"><strong aria-hidden="true">15.</strong> Strings</a></li><li class="chapter-item expanded "><a href="Chapter_15.html"><strong aria-hidden="true">16.</strong> const and static</a></li><li class="chapter-item expanded "><a href="Chapter_16.html"><strong aria-hidden="true">17.</strong> More on references</a></li><li class="chapter-item expanded "><a href="Chapter_17.html"><strong aria-hidden="true">18.</strong> Mutable references</a></li><li class="chapter-item expanded "><a href="Chapter_18.html"><strong aria-hidden="true">19.</strong> Giving references to functions</a></li><li class="chapter-item expanded "><a href="Chapter_19.html"><strong aria-hidden="true">20.</strong> Copy types</a></li><li class="chapter-item expanded "><a href="Chapter_20.html"><strong aria-hidden="true">21.</strong> Collection types</a></li><li class="chapter-item expanded "><a href="Chapter_21.html"><strong aria-hidden="true">22.</strong> Vectors</a></li><li class="chapter-item expanded "><a href="Chapter_22.html"><strong aria-hidden="true">23.</strong> Tuples</a></li><li class="chapter-item expanded "><a href="Chapter_23.html"><strong aria-hidden="true">24.</strong> Control flow</a></li><li class="chapter-item expanded "><a href="Chapter_24.html"><strong aria-hidden="true">25.</strong> Structs</a></li><li class="chapter-item expanded "><a href="Chapter_25.html"><strong aria-hidden="true">26.</strong> Enums</a></li><li class="chapter-item expanded "><a href="Chapter_26.html"><strong aria-hidden="true">27.</strong> Loops</a></li><li class="chapter-item expanded "><a href="Chapter_27.html"><strong aria-hidden="true">28.</strong> Implementing structs and enums</a></li><li class="chapter-item expanded "><a href="Chapter_28.html"><strong aria-hidden="true">29.</strong> Destructuring</a></li><li class="chapter-item expanded "><a href="Chapter_29.html"><strong aria-hidden="true">30.</strong> References and the dot operator</a></li><li class="chapter-item expanded "><a href="Chapter_30.html"><strong aria-hidden="true">31.</strong> Generics</a></li><li class="chapter-item expanded "><a href="Chapter_31.html"><strong aria-hidden="true">32.</strong> Option and Result</a></li><li class="chapter-item expanded "><a href="Chapter_32.html"><strong aria-hidden="true">33.</strong> Other collections</a></li><li class="chapter-item expanded "><a href="Chapter_33.html"><strong aria-hidden="true">34.</strong> The ? operator</a></li><li class="chapter-item expanded "><a href="Chapter_34.html"><strong aria-hidden="true">35.</strong> Traits</a></li><li class="chapter-item expanded "><a href="Chapter_35.html"><strong aria-hidden="true">36.</strong> Chaining methods</a></li><li class="chapter-item expanded "><a href="Chapter_36.html"><strong aria-hidden="true">37.</strong> Iterators</a></li><li class="chapter-item expanded "><a href="Chapter_37.html"><strong aria-hidden="true">38.</strong> Closures</a></li><li class="chapter-item expanded "><a href="Chapter_38.html"><strong aria-hidden="true">39.</strong> The dbg! macro and .inspect</a></li><li class="chapter-item expanded "><a href="Chapter_39.html"><strong aria-hidden="true">40.</strong> Types of &amp;str</a></li><li class="chapter-item expanded "><a href="Chapter_40.html"><strong aria-hidden="true">41.</strong> Lifetimes</a></li><li class="chapter-item expanded "><a href="Chapter_41.html"><strong aria-hidden="true">42.</strong> Interior mutability</a></li><li class="chapter-item expanded "><a href="Chapter_42.html"><strong aria-hidden="true">43.</strong> Cow</a></li><li class="chapter-item expanded "><a href="Chapter_43.html"><strong aria-hidden="true">44.</strong> Type aliases</a></li><li class="chapter-item expanded "><a href="Chapter_44.html"><strong aria-hidden="true">45.</strong> The todo! macro</a></li><li class="chapter-item expanded "><a href="Chapter_45.html"><strong aria-hidden="true">46.</strong> Rc</a></li><li class="chapter-item expanded "><a href="Chapter_46.html"><strong aria-hidden="true">47.</strong> Multiple threads</a></li><li class="chapter-item expanded "><a href="Chapter_47.html" class="active"><strong aria-hidden="true">48.</strong> Closures in functions</a></li><li class="chapter-item expanded "><a href="Chapter_48.html"><strong aria-hidden="true">49.</strong> impl Trait</a></li><li class="chapter-item expanded "><a href="Chapter_49.html"><strong aria-hidden="true">50.</strong> Arc</a></li><li class="chapter-item expanded "><a href="Chapter_50.html"><strong aria-hidden="true">51.</strong> Channels</a></li><li class="chapter-item expanded "><a href="Chapter_51.html"><strong aria-hidden="true">52.</strong> Reading Rust documentation</a></li><li class="chapter-item expanded "><a href="Chapter_52.html"><strong aria-hidden="true">53.</strong> Attributes</a></li><li class="chapter-item expanded "><a href="Chapter_53.html"><strong aria-hidden="true">54.</strong> Box</a></li><li class="chapter-item expanded "><a href="Chapter_54.html"><strong aria-hidden="true">55.</strong> Box around traits</a></li><li class="chapter-item expanded "><a href="Chapter_55.html"><strong aria-hidden="true">56.</strong> Default and the builder pattern</a></li><li class="chapter-item expanded "><a href="Chapter_56.html"><strong aria-hidden="true">57.</strong> Deref and DerefMut</a></li><li class="chapter-item expanded "><a href="Chapter_57.html"><strong aria-hidden="true">58.</strong> Crates and modules</a></li><li class="chapter-item expanded "><a href="Chapter_58.html"><strong aria-hidden="true">59.</strong> Testing</a></li><li class="chapter-item expanded "><a href="Chapter_59.html"><strong aria-hidden="true">60.</strong> External crates</a></li><li class="chapter-item expanded "><a href="Chapter_60.html"><strong aria-hidden="true">61.</strong> A tour of the standard library</a></li><li class="chapter-item expanded "><a href="Chapter_61.html"><strong aria-hidden="true">62.</strong> Writing macros</a></li><li class="chapter-item expanded "><a href="Chapter_62.html"><strong aria-hidden="true">63.</strong> cargo</a></li><li class="chapter-item expanded "><a href="Chapter_63.html"><strong aria-hidden="true">64.</strong> Taking user input</a></li><li class="chapter-item expanded "><a href="Chapter_64.html"><strong aria-hidden="true">65.</strong> Using files</a></li><li class="chapter-item expanded "><a href="Chapter_65.html"><strong aria-hidden="true">66.</strong> cargo doc</a></li><li class="chapter-item expanded "><a href="Chapter_66.html"><strong aria-hidden="true">67.</strong> The end?</a></li></ol>
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<h2 id="closures-in-functions"><a class="header" href="#closures-in-functions">Closures in functions</a></h2>
<p>Closures are great. So how do we put them into our own functions?</p>
<p>You can make your own functions that take closures, but inside them it is less free and you have to decide the type. Outside a function a closure can decide by itself between <code>Fn</code>, <code>FnMut</code> and <code>FnOnce</code>, but inside you have to choose one. The best way to understand is to look at a few function signatures. Here is the one for <code>.all()</code>. We remember that it checks an iterator to see if everything is <code>true</code> (depending on what you decide is <code>true</code> or <code>false</code>). Part of its signature says this:</p>
<pre><pre class="playground"><code class="language-rust">
<span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span> fn all&lt;F&gt;(&amp;mut self, f: F) -&gt; bool // 🚧
where
F: FnMut(Self::Item) -&gt; bool,
<span class="boring">}
</span></code></pre></pre>
<p><code>fn all&lt;F&gt;</code>: this tells you that there is a generic type <code>F</code>. A closure is always generic because every time it is a different type.</p>
<p><code>(&amp;mut self, f: F)</code>: <code>&amp;mut self</code> tells you that it's a method. <code>f: F</code> is usually what you see for a closure: this is the variable name and the type. Of course, there is nothing special about <code>f</code> and <code>F</code> and they could be different names. You could write <code>my_closure: Closure</code> if you wanted - it doesn't matter. But in signatures you almost always see <code>f: F</code>.</p>
<p>Next is the part about the closure: <code>F: FnMut(Self::Item) -&gt; bool</code>. Here it decides that the closure is <code>FnMut</code>, so it can change the values. It changes the values of <code>Self::Item</code>, which is the iterator that it takes. And it has to return <code>true</code> or <code>false</code>.</p>
<p>Here is a much simpler signature with a closure:</p>
<pre><pre class="playground"><code class="language-rust">
<span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>fn do_something&lt;F&gt;(f: F) // 🚧
where
F: FnOnce(),
{
f();
}
<span class="boring">}
</span></code></pre></pre>
<p>This just says that it takes a closure, takes the value (<code>FnOnce</code> = takes the value), and doesn't return anything. So now we can call this closure that takes nothing and do whatever we like. We will create a <code>Vec</code> and then iterate over it just to show what we can do now.</p>
<pre><pre class="playground"><code class="language-rust">fn do_something&lt;F&gt;(f: F)
where
F: FnOnce(),
{
f();
}
fn main() {
do_something(|| {
let some_vec = vec![9, 8, 10];
some_vec
.iter()
.for_each(|x| println!(&quot;The number is: {}&quot;, x));
})
}
</code></pre></pre>
<p>For a more real example, we will create a <code>City</code> struct again. This time the <code>City</code> struct has more data about years and populations. It has a <code>Vec&lt;u32&gt;</code> for all the years, and another <code>Vec&lt;u32&gt;</code> for all the populations.</p>
<p><code>City</code> has two functions: <code>new()</code> to create a new <code>City</code>, and <code>.city_data()</code> which has a closure. When we use <code>.city_data()</code>, it gives us the years and the populations and a closure, so we can do what we want with the data. The closure type is <code>FnMut</code> so we can change the data. It looks like this:</p>
<pre><pre class="playground"><code class="language-rust">#[derive(Debug)]
struct City {
name: String,
years: Vec&lt;u32&gt;,
populations: Vec&lt;u32&gt;,
}
impl City {
fn new(name: &amp;str, years: Vec&lt;u32&gt;, populations: Vec&lt;u32&gt;) -&gt; Self {
Self {
name: name.to_string(),
years,
populations,
}
}
fn city_data&lt;F&gt;(&amp;mut self, mut f: F) // We bring in self, but only f is generic F. f is the closure
where
F: FnMut(&amp;mut Vec&lt;u32&gt;, &amp;mut Vec&lt;u32&gt;), // The closure takes mutable vectors of u32
// which are the year and population data
{
f(&amp;mut self.years, &amp;mut self.populations) // Finally this is the actual function. It says
// &quot;use a closure on self.years and self.populations&quot;
// We can do whatever we want with the closure
}
}
fn main() {
let years = vec![
1372, 1834, 1851, 1881, 1897, 1925, 1959, 1989, 2000, 2005, 2010, 2020,
];
let populations = vec![
3_250, 15_300, 24_000, 45_900, 58_800, 119_800, 283_071, 478_974, 400_378, 401_694,
406_703, 437_619,
];
// Now we can create our city
let mut tallinn = City::new(&quot;Tallinn&quot;, years, populations);
// Now we have a .city_data() method that has a closure. We can do anything we want.
// First let's put the data for 5 years together and print it.
tallinn.city_data(|city_years, city_populations| { // We can call the input anything we want
let new_vec = city_years
.into_iter()
.zip(city_populations.into_iter()) // Zip the two together
.take(5) // but only take the first 5
.collect::&lt;Vec&lt;(_, _)&gt;&gt;(); // Tell Rust to decide the type inside the tuple
println!(&quot;{:?}&quot;, new_vec);
});
// Now let's add some data for the year 2030
tallinn.city_data(|x, y| { // This time we just call the input x and y
x.push(2030);
y.push(500_000);
});
// We don't want the 1834 data anymore
tallinn.city_data(|x, y| {
let position_option = x.iter().position(|x| *x == 1834);
if let Some(position) = position_option {
println!(
&quot;Going to delete {} at position {:?} now.&quot;,
x[position], position
); // Confirm that we delete the right item
x.remove(position);
y.remove(position);
}
});
println!(
&quot;Years left are {:?}\nPopulations left are {:?}&quot;,
tallinn.years, tallinn.populations
);
}
</code></pre></pre>
<p>This will print the result of all the times we called <code>.city_data().</code> It is:</p>
<pre><code class="language-text">[(1372, 3250), (1834, 15300), (1851, 24000), (1881, 45900), (1897, 58800)]
Going to delete 1834 at position 1 now.
Years left are [1372, 1851, 1881, 1897, 1925, 1959, 1989, 2000, 2005, 2010, 2020, 2030]
Populations left are [3250, 24000, 45900, 58800, 119800, 283071, 478974, 400378, 401694, 406703, 437619, 500000]
</code></pre>
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