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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< F> (& mut self, f: F) -> bool // 🚧
where
F: FnMut(Self::Item) -> bool,
< span class = "boring" > }
< / span > < / code > < / pre > < / pre >
< 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 >
< 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 >
< 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 >
< 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< F> (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< F> (f: F)
where
F: FnOnce(),
{
f();
}
fn main() {
let some_vec = vec![9, 8, 10];
do_something(|| {
some_vec
.into_iter()
.for_each(|x| println!(" The number is: {}" , 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< u32> < / code > for all the years, and another < code > Vec< u32> < / 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< u32> ,
populations: Vec< u32> ,
}
impl City {
fn new(name: & str, years: Vec< u32> , populations: Vec< u32> ) -> Self {
Self {
name: name.to_string(),
years,
populations,
}
}
fn city_data< F> (& mut self, mut f: F) // We bring in self, but only f is generic F. f is the closure
where
F: FnMut(& mut Vec< u32> , & mut Vec< u32> ), // The closure takes mutable vectors of u32
// which are the year and population data
{
f(& mut self.years, & mut self.populations) // Finally this is the actual function. It says
// " use a closure on self.years and self.populations"
// 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(" Tallinn" , 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::< Vec< (_, _)> > (); // Tell Rust to decide the type inside the tuple
println!(" {:?}" , 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!(
" Going to delete {} at position {:?} now." ,
x[position], position
); // Confirm that we delete the right item
x.remove(position);
y.remove(position);
}
});
println!(
" Years left are {:?}\nPopulations left are {:?}" ,
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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