% Generics

Sometimes, when writing a function or data type, we may want it to work for
multiple types of arguments. In Rust, we can do this with 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 type theory, let’s check out some generic code. Rust’s
standard library provides a type, `Option<T>`, that’s generic:

```rust
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:

```rust
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 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:

```rust,ignore
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 have
to match up:

```rust
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>`:

```rust
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:

```rust
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:

```rust
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:

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

We could write a version that takes multiple types:

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

## Generic structs

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

```rust
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 };
```

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

When you want to add an implementation for the generic `struct`, you
declare the type parameter after the `impl`:

```rust
# struct Point<T> {
#     x: T,
#     y: T,
# }
#
impl<T> Point<T> {
    fn swap(&mut self) {
        std::mem::swap(&mut self.x, &mut self.y);
    }
}
```

So far you’ve seen generics that take absolutely any type. These are useful in
many cases: you’ve already seen `Option<T>`, and later you’ll meet universal
container types like [`Vec<T>`][Vec]. On the other hand, often you want to
trade that flexibility for increased expressive power. Read about [trait
bounds][traits] to see why and how.

[traits]: traits.html
[Vec]: ../std/vec/struct.Vec.html
