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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-hi
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<main>
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<h2><a class="header" href="#closures-in-functions" id="closures-in-functions">Closures in functions</a></h2>
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<p>Closures are great. So how do we put them into our own functions?</p>
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<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>
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<pre><pre class="playground"><code class="language-rust">
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<span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span> fn all<F>(&mut self, f: F) -> bool // 🚧
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where
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F: FnMut(Self::Item) -> bool,
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<span class="boring">}
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</span></code></pre></pre>
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<p><code>fn all<F></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>
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<p><code>(&mut self, f: F)</code>: <code>&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>
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<p>Next is the part about the closure: <code>F: FnMut(Self::Item) -> 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>
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<p>Here is a much simpler signature with a closure:</p>
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<pre><pre class="playground"><code class="language-rust">
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<span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>fn do_something<F>(f: F) // 🚧
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where
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F: FnOnce(),
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{
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f();
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}
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<span class="boring">}
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</span></code></pre></pre>
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<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>
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<pre><pre class="playground"><code class="language-rust">fn do_something<F>(f: F)
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where
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F: FnOnce(),
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{
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f();
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}
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fn main() {
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do_something(|| {
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let some_vec = vec![9, 8, 10];
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some_vec
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.iter()
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.for_each(|x| println!("The number is: {}", x));
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})
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}
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</code></pre></pre>
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<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<u32></code> for all the years, and another <code>Vec<u32></code> for all the populations.</p>
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<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>
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<pre><pre class="playground"><code class="language-rust">#[derive(Debug)]
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struct City {
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name: String,
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years: Vec<u32>,
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populations: Vec<u32>,
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}
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impl City {
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fn new(name: &str, years: Vec<u32>, populations: Vec<u32>) -> Self {
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Self {
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name: name.to_string(),
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years,
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populations,
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}
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}
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fn city_data<F>(&mut self, mut f: F) // We bring in self, but only f is generic F. f is the closure
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where
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F: FnMut(&mut Vec<u32>, &mut Vec<u32>), // The closure takes mutable vectors of u32
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// which are the year and population data
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{
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f(&mut self.years, &mut self.populations) // Finally this is the actual function. It says
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// "use a closure on self.years and self.populations"
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// We can do whatever we want with the closure
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}
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}
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fn main() {
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let years = vec![
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1372, 1834, 1851, 1881, 1897, 1925, 1959, 1989, 2000, 2005, 2010, 2020,
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];
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let populations = vec![
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3_250, 15_300, 24_000, 45_900, 58_800, 119_800, 283_071, 478_974, 400_378, 401_694,
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406_703, 437_619,
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];
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// Now we can create our city
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let mut tallinn = City::new("Tallinn", years, populations);
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// Now we have a .city_data() method that has a closure. We can do anything we want.
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// First let's put the data for 5 years together and print it.
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tallinn.city_data(|city_years, city_populations| { // We can call the input anything we want
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let new_vec = city_years
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.into_iter()
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.zip(city_populations.into_iter()) // Zip the two together
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.take(5) // but only take the first 5
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.collect::<Vec<(_, _)>>(); // Tell Rust to decide the type inside the tuple
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println!("{:?}", new_vec);
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});
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// Now let's add some data for the year 2030
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tallinn.city_data(|x, y| { // This time we just call the input x and y
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x.push(2030);
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y.push(500_000);
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});
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// We don't want the 1834 data anymore
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tallinn.city_data(|x, y| {
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let position_option = x.iter().position(|x| *x == 1834);
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if let Some(position) = position_option {
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println!(
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"Going to delete {} at position {:?} now.",
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x[position], position
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||
|
|
); // Confirm that we delete the right item
|
||
|
|
x.remove(position);
|
||
|
|
y.remove(position);
|
||
|
|
}
|
||
|
|
});
|
||
|
|
|
||
|
|
println!(
|
||
|
|
"Years left are {:?}\nPopulations left are {:?}",
|
||
|
|
tallinn.years, tallinn.populations
|
||
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|
);
|
||
|
|
}
|
||
|
|
</code></pre></pre>
|
||
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|
<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]
|
||
|
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</code></pre>
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