TRPL edits: generics
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% Generics
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Sometimes, when writing a function or data type, we may want it to work for
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multiple types of arguments. For example, remember our `OptionalInt` type?
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multiple types of arguments. Luckily, Rust has a feature that gives us a better
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way: generics. Generics are called ‘parametric polymorphism’ in type theory,
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which means that they are types or functions that have multiple forms (‘poly’
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is multiple, ‘morph’ is form) over a given parameter (‘parametric’).
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```{rust}
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enum OptionalInt {
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Value(i32),
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Missing,
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}
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```
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If we wanted to also have an `OptionalFloat64`, we would need a new enum:
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```{rust}
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enum OptionalFloat64 {
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Valuef64(f64),
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Missingf64,
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}
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```
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This is really unfortunate. Luckily, Rust has a feature that gives us a better
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way: generics. Generics are called *parametric polymorphism* in type theory,
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which means that they are types or functions that have multiple forms (*poly*
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is multiple, *morph* is form) over a given parameter (*parametric*).
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Anyway, enough with type theory declarations, let's check out the generic form
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of `OptionalInt`. It is actually provided by Rust itself, and looks like this:
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Anyway, enough with type theory, let’s check out some generic code. Rust’s
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standard library provides a type, `Option<T>`, that’s generic:
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```rust
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enum Option<T> {
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@ -34,41 +16,40 @@ enum Option<T> {
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}
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```
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The `<T>` part, which you've seen a few times before, indicates that this is
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The `<T>` part, which you’ve seen a few times before, indicates that this is
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a generic data type. Inside the declaration of our enum, wherever we see a `T`,
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we substitute that type for the same type used in the generic. Here's an
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we substitute that type for the same type used in the generic. Here’s an
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example of using `Option<T>`, with some extra type annotations:
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```{rust}
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```rust
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let x: Option<i32> = Some(5);
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```
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In the type declaration, we say `Option<i32>`. Note how similar this looks to
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`Option<T>`. So, in this particular `Option`, `T` has the value of `i32`. On
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the right-hand side of the binding, we do make a `Some(T)`, where `T` is `5`.
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Since that's an `i32`, the two sides match, and Rust is happy. If they didn't
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match, we'd get an error:
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Since that’s an `i32`, the two sides match, and Rust is happy. If they didn’t
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match, we’d get an error:
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```{rust,ignore}
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```rust,ignore
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let x: Option<f64> = Some(5);
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// error: mismatched types: expected `core::option::Option<f64>`,
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// found `core::option::Option<_>` (expected f64 but found integral variable)
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```
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That doesn't mean we can't make `Option<T>`s that hold an `f64`! They just have to
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match up:
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That doesn’t mean we can’t make `Option<T>`s that hold an `f64`! They just have
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to match up:
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```{rust}
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```rust
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let x: Option<i32> = Some(5);
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let y: Option<f64> = Some(5.0f64);
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```
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This is just fine. One definition, multiple uses.
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Generics don't have to only be generic over one type. Consider Rust's built-in
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`Result<T, E>` type:
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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>`:
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```{rust}
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```rust
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enum Result<T, E> {
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Ok(T),
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Err(E),
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```
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This type is generic over _two_ types: `T` and `E`. By the way, the capital letters
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can be any letter you'd like. We could define `Result<T, E>` as:
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can be any letter you’d like. We could define `Result<T, E>` as:
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```{rust}
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```rust
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enum Result<A, Z> {
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Ok(A),
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Err(Z),
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```
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if we wanted to. Convention says that the first generic parameter should be
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`T`, for 'type,' and that we use `E` for 'error.' Rust doesn't care, however.
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`T`, for ‘type’, and that we use `E` for ‘error’. Rust doesn’t care, however.
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The `Result<T, E>` type is intended to be used to return the result of a
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computation, and to have the ability to return an error if it didn't work out.
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computation, and to have the ability to return an error if it didn’t work out.
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## Generic functions
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We can write functions that take generic types with a similar syntax:
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```rust
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fn takes_anything<T>(x: T) {
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// do something with x
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}
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```
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The syntax has two parts: the `<T>` says “this function is generic over one
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type, `T`”, and the `x: T` says “x has the type `T`.”
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Multiple arguments can have the same generic type:
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```rust
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fn takes_two_of_the_same_things<T>(x: T, y: T) {
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// ...
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}
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```
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We could write a version that takes multiple types:
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```rust
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fn takes_two_things<T, U>(x: T, y: U) {
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// ...
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}
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```
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Generic functions are most useful with ‘trait bounds’, which we’ll cover in the
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[section on traits][traits].
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[traits]: traits.html
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## Generic structs
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You can store a generic type in a `struct` as well:
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```
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struct Point<T> {
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x: T,
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y: T,
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}
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let int_origin = Point { x: 0, y: 0 };
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let float_origin = Point { x: 0.0, y: 0.0 };
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```
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Similarly to functions, the `<T>` is where we declare the generic parameters,
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and we then use `x: T` in the type declaration, too.
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