@ -6987,6 +6988,161 @@ Years left are [1372, 1851, 1881, 1897, 1925, 1959, 1989, 2000, 2005, 2010, 2020
Populations left are [3250, 24000, 45900, 58800, 119800, 283071, 478974, 400378, 401694, 406703, 437619, 500000]
```
## impl Trait
`impl Trait` is similar to generics. You remember that generics use a type `T` (or any other name) which then gets decided when the program compiles. First a concrete type:
```rust
fn gives_higher_i32(one: i32, two: i32) {
let higher = if one > two { one } else { two };
println!("{} is higher.", higher);
}
fn main() {
gives_higher_i32(8, 10);
}
```
This prints: `10 is higher.`.
But this only takes `i32`, so now we will make it generic. We need to compare and we need to print with `{}`, so our type T needs `PartialOrd` and `Display`. Remember, this means "only take types that already have `PartialOrd` and `Display`".
Now let's look at `impl Trait`, which is similar. Instead of a type `T`, we can bring in a type `impl Trait`. Then it will take in a type that implements that trait. It is almost the same:
```rust
fn prints_it(input: impl Into<String> + std::fmt::Display) { // Takes anything that can turn into a String and has Display
println!("You can print many things, including {}", input);
}
fn main() {
let name = "Tuon";
let string_name = String::from("Tuon");
prints_it(name);
prints_it(string_name);
}
```
However, the more interesting part is that we can return `impl Trait`, and that lets us return closures because their function signatures are traits. You can see this in the signatures for methods that have them. For example, this is the signature for `.map()`:
`fn map<B, F>(self, f: F)` mean that it takes two generic types. `B` is self and `F` is the closure. Then after the `where` we see the trait bounds. ("Trait bound" means "it must have this trait".) One is `Sized`, but the next is the closure signature. It must be an `FnMut`, and do the closure on `Self::Item`, which is the iterator that you give it. Then it returns `B`, which is self.
So we can do the same thing to return a closure. To return a closure, use `impl` and then the closure signature. Once you return it, you can use it just like a function. Here is a small example of a function that gives you a closure depending on the number you put in. If you put 2 or 40 in then it multiplies it, and otherwise it gives you the same number. Because it's a closure we can do anything we want, so we also print a message.
Here is a bit longer example. Let's imagine a game where your character is facing monsters that are stronger at night. We can make an enum called `TimeOfDay` to keep track of the day. Your character is named Simon and has a number called `character_fear`, which is an `f64`. It goes up at night and down during the day. We will create a function called `change_fear` that changes the fear, but also does some other things like write messages. It could look like this:
```rust
enum TimeOfDay { // just a simple enum
Dawn,
Day,
Sunset,
Night,
}
fn change_fear(input: TimeOfDay) -> impl FnMut(f64) -> f64 { // The function takes a TimeOfDay. It returns a closure.
// We use impl FnMut(64) -> f64 to say that it needs to
// change the value, and also gives the same type back.
use TimeOfDay::*; // So we only have to write Dawn, Day, Sunset, Night
// Instead of TimeOfDay::Dawn, TimeOfDay::Day, etc.
match input {
Dawn => |x| { // This is the variable character_fear that we give it later
println!("The morning sun has vanquished the horrible night. You no longer feel afraid.");
println!("Your fear is now {}", x * 0.5);
x * 0.5
},
Day => |x| {
println!("What a nice day. Maybe put your feet up and rest a bit.");
println!("Your fear is now {}", x * 0.2);
x * 0.2
},
Sunset => |x| {
println!("The sun is almost down! This is no good.");
println!("Your fear is now {}", x * 1.4);
x * 1.4
},
Night => |x| {
println!("What a horrible night to have a curse.");
println!("Your fear is now {}", x * 5.0);
x * 5.0
},
}
}
fn main() {
use TimeOfDay::*;
let mut character_fear = 10.0; // Start Simon with 10
let mut daytime = change_fear(Day); // Make four closures here to call every time we want to change Simon's fear.
let mut sunset = change_fear(Sunset);
let mut night = change_fear(Night);
let mut morning = change_fear(Dawn);
character_fear = daytime(character_fear); // Call the closures on Simon's fear. They give a message and change the fear number.
// In real life we would have a Character struct and use it as a method instead,
// like this: character_fear.daytime()
character_fear = sunset(character_fear);
character_fear = night(character_fear);
character_fear = morning(character_fear);
}
```
## Arc
You remember that we used an `Rc` to give a variable more than one owner. If we are doing the same thing in a thread, we need an `Arc`. `Arc` means "atomic reference counter". Atomic means that it uses the computer's processor so that data only gets written once each time. This is important because if two threads write data at the same time, you will get the wrong result. For example, imagine if you could do this in Rust:
Dhghomon
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