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 let's look at a concrete type:
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".
use std::fmt::Display; fn gives_higher_i32<T: PartialOrd + Display>(one: T, two: T) { let higher = if one > two { one } else { two }; println!("{} is higher.", higher); } fn main() { gives_higher_i32(8, 10); }
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:
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():
#![allow(unused)] fn main() { fn map<B, F>(self, f: F) -> Map<Self, F> // 🚧 where Self: Sized, F: FnMut(Self::Item) -> B, { Map::new(self, f) } }
fn map<B, F>(self, f: F) mean that it takes two generic types. F is a function that takes one item from the container implementing .map() and B is the return type of that function. Then after the where we see the trait bounds. ("Trait bound" means "it must have this trait".) One is Sized, and 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.
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 text you put in. If you put "double" or "triple" in then it multiplies it by 2 or 3, 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.
fn returns_a_closure(input: &str) -> impl FnMut(i32) -> i32 { match input { "double" => |mut number| { number *= 2; println!("Doubling number. Now it is {}", number); number }, "triple" => |mut number| { number *= 40; println!("Tripling number. Now it is {}", number); number }, _ => |number| { println!("Sorry, it's the same: {}.", number); number }, } } fn main() { let my_number = 10; // Make three closures let mut doubles = returns_a_closure("double"); let mut triples = returns_a_closure("triple"); let mut quadruples = returns_a_closure("quadruple"); doubles(my_number); triples(my_number); quadruples(my_number); }
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 make a change_fear function that changes his fear, but also does other things like write messages. It could look like this:
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); }
This prints:
What a nice day. Maybe put your feet up and rest a bit.
Your fear is now 2
The sun is almost down! This is no good.
Your fear is now 2.8
What a horrible night to have a curse.
Your fear is now 14
The morning sun has vanquished the horrible night. You no longer feel afraid.
Your fear is now 7