Use trait bounds and generic functions

4 minutes

Traits let us accept different types by how we define our functions, because when a type implements a trait it can be treated abstractly as that trait.

We can declare function arguments to be an anonymous type parameter where the callee must provide a type that has the bounds declared by the anonymous type parameter.

Let's imagine that we're writing a web application and want to have an interface for serializing values to the JSON format. We could write a trait like this:

trait AsJson {
    fn as_json(&self) -> String;
}

Then we could write a function that accepts any type that implements the AsJson trait. They're written as impl followed by a set of trait bounds.

fn send_data_as_json(value: &impl AsJson) {
    println!("Sending JSON data to server...");
    println!("-> {}", value.as_json());
    println!("Done!\n");
}

Here, we specify the trait name and the impl keyword. We specify with these values instead of using a concrete type for the value parameter. The value parameter accepts all types that use the defined trait. Because the function doesn't know anything about the concrete type it will receive, it can only use the methods available by the trait bounds of the anonymous type parameter.

Another way to write the same function, but with a little different syntax, explicitly states that T is a generic type that must implement the AsJson trait:

fn send_data_as_json<T: AsJson>(value: &T) { ... }

We can then declare our types and implement the AsJson trait for them:

struct Person {
    name: String,
    age: u8,
    favorite_fruit: String,
}

struct Dog {
    name: String,
    color: String,
    likes_petting: bool,
}

impl AsJson for Person {
    fn as_json(&self) -> String {
	    format!(
	        r#"{{ "type": "person", "name": "{}", "age": {}, "favoriteFruit": "{}" }}"#,
	        self.name, self.age, self.favorite_fruit
	    )
    }
}

impl AsJson for Dog {
    fn as_json(&self) -> String {
	    format!(
	        r#"{{ "type": "dog", "name": "{}", "color": "{}", "likesPetting": {} }}"#,
	        self.name, self.color, self.likes_petting
	    )
    }
}

Now that both Person and Dog implement the AsJson trait, we can use them as input parameters for our send_data_as_json function.

fn main() {
    let laura = Person {
    	name: String::from("Laura"),
	    age: 31,
	    favorite_fruit: String::from("apples"),
    };

    let fido = Dog {
	    name: String::from("Fido"),
	    color: String::from("Black"),
	    likes_petting: true,
    };

    send_data_as_json(&laura);
    send_data_as_json(&fido);
}

But what happens when we pass a type that doesn't implement the expected trait to the function? Let's create a new struct and see what happens:

struct Cat {
    name: String,
    sharp_claws: bool,
}

let kitty = Cat {
    name: String::from("Kitty"),
    sharp_claws: false,
};

send_data_as_json(&kitty);

The compiler raises the following error:

    error[E0277]: the trait bound `Cat: AsJson` is not satisfied
      --> src/main.rs:70:23
       |
    5  | fn send_data_as_json(value: &impl AsJson) {
       |                                   ------ required by this bound in `send_data_as_json`
    ...
    70 |     send_data_as_json(&kitty);
       |                       ^^^^^^ the trait `AsJson` is not implemented for `Cat`

This error happened because we tried to use a type that doesn't implement the AsJson trait in a place that expected that trait: the send_data_as_json function.

To view the code used in this unit, see this Rust Playground link.

As an optional challenge, you can try to implement the AsJson trait for the Cat type.

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