rust/src/doc/trpl/generics.md

% Generics

Sometimes, when writing a function or data type, we may want it to work for multiple types of arguments. Luckily, Rust has a feature that gives us a better way: generics. Generics are called ‘parametric polymorphism’ in type theory, which means that they are types or functions that have multiple forms (‘poly’ is multiple, ‘morph’ is form) over a given parameter (‘parametric’).

Anyway, enough with type theory, let’s check out some generic code. Rust’s standard library provides a type, Option<T>, that’s generic:

enum Option<T> {
    Some(T),
    None,
}

The <T> part, which you’ve seen a few times before, indicates that this is a generic data type. Inside the declaration of our enum, wherever we see a T, we substitute that type for the same type used in the generic. Here’s an example of using Option<T>, with some extra type annotations:

let x: Option<i32> = Some(5);

In the type declaration, we say Option<i32>. Note how similar this looks to Option<T>. So, in this particular Option, T has the value of i32. On the right-hand side of the binding, we do make a Some(T), where T is 5. Since that’s an i32, the two sides match, and Rust is happy. If they didn’t match, we’d get an error:

let x: Option<f64> = Some(5);
// error: mismatched types: expected `core::option::Option<f64>`,
// found `core::option::Option<_>` (expected f64 but found integral variable)

That doesn’t mean we can’t make Option<T>s that hold an f64! They just have to match up:

let x: Option<i32> = Some(5);
let y: Option<f64> = Some(5.0f64);

This is just fine. One definition, multiple uses.

Generics don’t have to only be generic over one type. Consider another type from Rust’s standard library that’s similar, Result<T, E>:

enum Result<T, E> {
    Ok(T),
    Err(E),
}

This type is generic over two types: T and E. By the way, the capital letters can be any letter you’d like. We could define Result<T, E> as:

enum Result<A, Z> {
    Ok(A),
    Err(Z),
}

if we wanted to. Convention says that the first generic parameter should be T, for ‘type’, and that we use E for ‘error’. Rust doesn’t care, however.

The Result<T, E> type is intended to be used to return the result of a computation, and to have the ability to return an error if it didn’t work out.

Generic functions

We can write functions that take generic types with a similar syntax:

fn takes_anything<T>(x: T) {
    // do something with x
}

The syntax has two parts: the <T> says “this function is generic over one type, T”, and the x: T says “x has the type T.”

Multiple arguments can have the same generic type:

fn takes_two_of_the_same_things<T>(x: T, y: T) {
    // ...
}

We could write a version that takes multiple types:

fn takes_two_things<T, U>(x: T, y: U) {
    // ...
}

Generic functions are most useful with ‘trait bounds’, which we’ll cover in the section on traits.

Generic structs

You can store a generic type in a struct as well:

struct Point<T> {
    x: T,
    y: T,
}

let int_origin = Point { x: 0, y: 0 };
let float_origin = Point { x: 0.0, y: 0.0 };

Similarly to functions, the <T> is where we declare the generic parameters, and we then use x: T in the type declaration, too.