## Advanced Types

The Rust type system has some features that we've mentioned or used without discussing. We started talking about the newtype pattern in regards to traits; we'll start with a more general discussion about why newtypes are useful as types. We'll then move to type aliases, a feature that is similar to newtypes but has slightly different semantics. We'll also discuss the `!` type and dynamically sized types.

### Using the Newtype Pattern for Type Safety and Abstraction

The newtype pattern that we started discussing at the end of the "Advanced Traits" section, where we create a new type as a tuple struct with one field that wraps a type can also be useful for statically enforcing that values are never confused, and is often used to indicate the units of a value. We actually had an example of this in Listing 19-26: the `Millimeters` and `Meters` structs both wrap `u32` values in a new type. If we write a function with a parameter of type `Millimeters`, we won't be able to compile a program that accidentally tries to call that function with a value of type `Meters` or a plain `u32`.

Another reason to use the newtype pattern is to abstract away some implementation details of a type: the wrapper type can expose a different public API than the private inner type would if we used it directly in order to restrict the functionality that is available, for example. New types can also hide internal generic types. For example, we could provide a `People` type that wraps a `HashMap<i32, String>` that stores a person's ID associated with their name. Code using `People` would only interact with the public API we provide, such as a method to add a name string to the `People` collection, and that code wouldn't need to know that we assign an `i32` ID to names internally. The newtype pattern is a lightweight way to achieve encapsulation to hide implementation details that we discussed in Chapter 17.

### Type Aliases Create Type Synonyms

The newtype pattern involves creating a new struct to be a new, separate type. Rust also provides the ability to declare a type alias with the `type` keyword to give an existing type another name. For example, we can create the alias `Kilometers` to `i32` like so:

    # #![allow(unused_variables)]
    #fn main() {
    type Kilometers = i32;

    #}

This means `Kilometers` is a synonym for `i32`; unlike the `Millimeters` and `Meters` types we created in Listing 19-26, `Kilometers` is not a separate, new type. Values that have the type `Kilometers` will be treated exactly the same as values of type `i32`:

    # #![allow(unused_variables)]
    #fn main() {
    type Kilometers = i32;

    let x: i32 = 5;
    let y: Kilometers = 5;

    println!("x + y = {}", x + y);

    #}

Since `Kilometers` is an alias for `i32`, they're the same type. We can add values of type `i32` and `Kilometers` together, and we can pass `Kilometers` values to functions that take `i32` parameters. We don't get the type checking benefits that we get from the newtype pattern that we discussed in the previous section.

The main use case for type synonyms is to reduce repetition. For example, we may have a lengthy type like this:

    Box<FnOnce() + Send + 'static>

Writing this out in function signatures and as type annotations all over the place can be tiresome and error-prone. Imagine having a project full of code like that in Listing 19-31:

    # #![allow(unused_variables)]
    #fn main() {
    let f: Box<FnOnce() + Send + 'static> = Box::new(|| println!("hi"));

    fn takes_long_type(f: Box<FnOnce() + Send + 'static>) {
        // ...
    }

    fn returns_long_type() -> Box<FnOnce() + Send + 'static> {
        // ...
    #     Box::new(|| ())
    }

    #}

Listing 19-31: Using a long type in many places

A type alias makes this code more manageable by reducing the amount of repetition this project has. Here, we've introduced an alias named `Thunk` for the verbose type, and we can replace all uses of the type with the shorter `Thunk` as shown in Listing 19-32:

    # #![allow(unused_variables)]
    #fn main() {
    type Thunk = Box<FnOnce() + Send + 'static>;

    let f: Thunk = Box::new(|| println!("hi"));

    fn takes_long_type(f: Thunk) {
        // ...
    }

    fn returns_long_type() -> Thunk {
        // ...
    #     Box::new(|| ())
    }

    #}

Listing 19-32: Introducing a type alias `Thunk` to reduce repetition

Much easier to read and write! Choosing a good name for a type alias can help communicate your intent as well (thunk is a word for code to be evaluated at a later time, so it's an appropriate name for a closure that gets stored).

Another common use of type aliases is with the `Result<T, E>` type. Consider the `std::io` module in the standard library. I/O operations often return a `Result<T, E>`, since their operations may fail to work. There's a `std::io::Error` struct that represents all of the possible I/O errors. Many of the functions in `std::io` will be returning `Result<T, E>` where the `E` is `std::io::Error`, such as these functions in the `Write` trait:

    # #![allow(unused_variables)]
    #fn main() {
    use std::io::Error;
    # use std::fmt::Arguments;

    pub trait Write {
        fn write(&mut self, buf: &[u8]) -> Result<usize, Error>;
        fn flush(&mut self) -> Result<(), Error>;

        fn write_all(&mut self, buf: &[u8]) -> Result<(), Error>;
        fn write_fmt(&mut self, fmt: Arguments) -> Result<(), Error>;
    }

    #}

We're writing `Result<..., Error>` a lot. As such, `std::io` has this type alias declaration:

    type Result<T> = Result<T, std::io::Error>;

Because this is in the `std::io` module, the fully qualified alias that we can use is `std::io::Result<T>`; that is, a `Result<T, E>` with the `E` filled in as `std::io::Error`. The `Write` trait function signatures end up looking like this:

    pub trait Write {
        fn write(&mut self, buf: &[u8]) -> Result<usize>;
        fn flush(&mut self) -> Result<()>;

        fn write_all(&mut self, buf: &[u8]) -> Result<()>;
        fn write_fmt(&mut self, fmt: Arguments) -> Result<()>;
    }

The type alias helps in two ways: this is easier to write and it gives us a consistent interface across all of `std::io`. Because it's an alias, it is just another `Result<T, E>`, which means we can use any methods that work on `Result<T, E>` with it, and special syntax like `?`.

### The Never Type, `!`, that Never Returns

Rust has a special type named `!`. In type theory lingo, it's called the bottom type, but we prefer the name never. The name describes what it does: it stands in the place of the return type when a function will never return. For example:

    fn bar() -> ! {

This is read as "the function `bar` returns never," and functions that return never are called diverging functions. We can't create values of the type `!`, so `bar` can never possibly return. What use is a type you can never create values for? If you think all the way back to Chapter 2, we had some code that looked like this, reproduced here in Listing 19-33:

    # #![allow(unused_variables)]
    #fn main() {
    # let guess = "3";
    # loop {
    let guess: u32 = match guess.trim().parse() {
        Ok(num) => num,
        Err(_) => continue,
    };
    # break;
    # }

    #}

Listing 19-33: A `match` with an arm that ends in `continue`

At the time, we skipped over some details in this code. In Chapter 6, we learned that `match` arms must return the same type. This doesn't work:

    let guess = match guess.trim().parse()  {
        Ok(_) => 5,
        Err(_) => "hello",
    }

What would the type of `guess` be here? It'd have to be both an integer and a string, and Rust requires that `guess` can only have one type. So what `continue` return? Why are we allowed to return a `u32` from one arm in Listing 19-33 and have another arm that ends with `continue`?

As you may have guessed, `continue` has a value of `!`. That is, when Rust goes to compute the type of `guess`, it looks at both of the match arms. The former has a value of `u32`, and the latter has a value of `!`. Since `!` can never have a value, Rust is okay with this, and decides that the type of `guess` is `u32`. The formal way of describing this behavior of `!` is that the never type unifies with all other types. We're allowed to end this `match` arm with `continue` because `continue` doesn't actually return a value; it instead moves control back to the top of the loop, so in the `Err` case, we never actually assign a value to `guess`.

Another use of the never type is `panic!`. Remember the `unwrap` function that we call on `Option<T>` values to produce a value or panic? Here's its definition:

    impl<T> Option<T> {
        pub fn unwrap(self) -> T {
            match self {
                Some(val) => val,
                None => panic!("called `Option::unwrap()` on a `None` value"),
            }
        }
    }

Here, the same thing happens as in the `match` in Listing 19-33: we know that `val` has the type `T`, and `panic!` has the type `!`, so the result of the overall `match` expression is `T`. This works because `panic!` doesn't produce a value; it ends the program. In the `None` case, we won't be returning a value from `unwrap`, so this code is valid.

One final expression that has the type `!` is a `loop`:

    print!("forever ");

    loop {
        print!("and ever ");
    }

Here, the loop never ends, so the value of the expression is `!`. This wouldn't be true if we included a `break`, however, as the loop would terminate when it gets to the `break`.

### Dynamically Sized Types & `Sized`

Because Rust needs to know things like memory layout, there's a particular corner of its type system that can be confusing, and that's the concept of dynamically sized types. Sometimes referred to as 'DSTs' or 'unsized types', these types let us talk about types whose size we can only know at runtime.

Let's dig into the details of a dynamically sized type that we've been using this whole book: `str`. That's right, not `&str`, but `str` on its own. `str` is a DST; we can't know how long the string is until runtime. Since we can't know that, we can't create a variable of type `str`, nor can we take an argument of type `str`. Consider this code, which does not work:

    let s1: str = "Hello there!";
    let s2: str = "How's it going?";

These two `str` values would need to have the exact same memory layout, but they have different lengths: `s1` needs 12 bytes of storage, and `s2` needs 15. This is why it's not possible to create a variable holding a dynamically sized type.

So what to do? Well, you already know the answer in this case: the types of `s1` and `s2` are `&str` rather than `str`. If you think back to Chapter 4, we said this about `&str`:

> ... it’s a reference to an internal position in the String and the number of elements that it refers to.

So while a `&T` is a single value that stores the memory address of where the `T` is located, a `&str` is two values: the address of the `str` and how long it is. As such, a `&str` has a size we can know at compile time: it's two times the size of a `usize` in length. That is, we always know the size of a `&str`, no matter how long the string it refers to is. This is the general way in which dynamically sized types are used in Rust; they have an extra bit of metadata that stores the size of the dynamic information. This leads us to the golden rule of dynamically sized types: we must always put values of dynamically sized types behind a pointer of some kind.

While we've talked a lot about `&str`, we can combine `str` with all kinds of pointers: `Box<str>`, for example, or `Rc<str>`. In fact, you've already seen this before, but with a different dynamically sized type: traits. Every trait is a dynamically sized type we can refer to by using the name of the trait. In Chapter 17, we mentioned that in order to use traits as trait objects, we have to put them behind a pointer like `&Trait` or `Box<Trait>` (`Rc<Trait>` would work too). Traits being dynamically sized is the reason we have to do that!

#### The `Sized` Trait

To work with DSTs, Rust has a trait that determines if a type's size is known at compile time or not, which is `Sized`. This trait is automatically implemented for everything the compiler knows the size of at compile time. In addition, Rust implicitly adds a bound on `Sized` to every generic function. That is, a generic function definition like this:

    fn generic<T>(t: T) {

is actually treated as if we had written this:

    fn generic<T: Sized>(t: T) {

By default, generic functions will only work on types that have a known size at compile time. There is, however, special syntax you can use to relax this restriction:

    fn generic<T: ?Sized>(t: &T) {

A trait bound on `?Sized` is the opposite of a trait bound on `Sized`; that is, we would read this as "`T` may or may not be `Sized`". This syntax is only available for `Sized`, no other traits.

Also note we switched the type of the `t` parameter from `T` to `&T`: since the type might not be `Sized`, we need to use it behind some kind of pointer. In this case, we've chosen a reference.

Next let's talk about functions and closures!