path: root/src/doc/book/nostarch/chapter06.md

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[TOC]

# Enums and Pattern Matching

In this chapter we’ll look at *enumerations*, also referred to as *enums*.
Enums allow you to define a type by enumerating its possible *variants*. First,
we’ll define and use an enum to show how an enum can encode meaning along with
data. Next, we’ll explore a particularly useful enum, called `Option`, which
expresses that a value can be either something or nothing. Then we’ll look at
how pattern matching in the `match` expression makes it easy to run different
code for different values of an enum. Finally, we’ll cover how the `if let`
construct is another convenient and concise idiom available to handle enums in
your code.

<!--- The above about algebraic data types feels pretty niche. Should it get
the "expert aside" treatment that some of the early texts gets? /JT --->
<!-- I decided to just remove the paragraph this comment was about. /Carol -->

## Defining an Enum

<!--- I added this first line, it seems like this is what we're saying? Maybe
summarize what enums are better suited for: when you know all possible outcomes
and that the outcomes must be distinct from each other? I was hoping to
generalize their usage early. Edit: reading on, I can see that might be tricky,
so ignore this if so! /LC --->
<!-- I made a slight edit to the first line here, what do you think? I don't
think "enums are an alternative to structs" was quite right, because that
sounded like in any situation, you could choose either enum or struct according
to your preferences, but what I'd like the reader to come away with is that
some situations are better expressed with enums; others with structs. /Carol -->
<!-- I think this makes sense! I wonder if there's more we could add to give an
idea of why we're contrasting them with structs, to give the reader a point of
reference. What do you think JT? Would more explanation here be redundant? /LC
-->
<!--- Here's my try for a framing, using our earlier Rectangle example:
Where structs give you a way of grouping together related fields and data, like
a `Rectangle` with its `width` and `height`, we don't yet have a way of saying
a values is one of a possible set of values. For example, we may want to say
that Rectangle is one of a set of possible shapes. To do this, Rust allows us
to encode these possibilities as an enum. Let's look at...
/JT --->
<!-- I've generally taken JT's suggestion with a few edits. I'm a little
concerned that we won't ever actually make a `Shape` enum with variants
`Rectangle`, `Circle`, and `Triangle`? Is that a problem, Liz? /Carol -->

Where structs give you a way of grouping together related fields and data, like
a `Rectangle` with its `width` and `height`, enums give you a way of saying a
value is one of a possible set of values. For example, we may want to say that
`Rectangle` is one of a set of possible shapes that also includes `Circle` and
`Triangle`. To do this, Rust allows us to encode these possibilities as an enum.

Let’s look at a situation we might want to express in code and see why enums
are useful and more appropriate than structs in this case. Say we need to work
with IP addresses. Currently, two major standards are used for IP addresses:
version four and version six. Because these are the only possibilities for an
IP address that our program will come across, we can *enumerate* all possible
variants, which is where enumeration gets its name.

Any IP address can be either a version four or a version six address, but not
both at the same time. That property of IP addresses makes the enum data
structure appropriate, because an enum value can only be one of its variants.
Both version four and version six addresses are still fundamentally IP
addresses, so they should be treated as the same type when the code is handling
situations that apply to any kind of IP address.

We can express this concept in code by defining an `IpAddrKind` enumeration and
listing the possible kinds an IP address can be, `V4` and `V6`. These are the
variants of the enum:

```
enum IpAddrKind {
    V4,
    V6,
}
```

`IpAddrKind` is now a custom data type that we can use elsewhere in our code.

### Enum Values

We can create instances of each of the two variants of `IpAddrKind` like this:

```
let four = IpAddrKind::V4;
let six = IpAddrKind::V6;
```

Note that the variants of the enum are namespaced under its identifier, and we
use a double colon to separate the two. This is useful because now both values
`IpAddrKind::V4` and `IpAddrKind::V6` are of the same type: `IpAddrKind`. We
can then, for instance, define a function that takes any `IpAddrKind`:

```
fn route(ip_kind: IpAddrKind) {}
```

And we can call this function with either variant:

```
route(IpAddrKind::V4);
route(IpAddrKind::V6);
```

Using enums has even more advantages. Thinking more about our IP address type,
at the moment we don’t have a way to store the actual IP address *data*; we
only know what *kind* it is. Given that you just learned about structs in
Chapter 5, you might be tempted to tackle this problem with structs as shown in
Listing 6-1.

```
enum IpAddrKind {
    V4,
    V6,
}

struct IpAddr {
    kind: IpAddrKind,
    address: String,
}

let home = IpAddr {
    kind: IpAddrKind::V4,
    address: String::from("127.0.0.1"),
};

let loopback = IpAddr {
    kind: IpAddrKind::V6,
    address: String::from("::1"),
};
```

Listing 6-1: Storing the data and `IpAddrKind` variant of an IP address using a
`struct`

Here, we’ve defined a struct `IpAddr` that has two fields: a `kind` field that
is of type `IpAddrKind` (the enum we defined previously) and an `address` field
of type `String`. We have two instances of this struct. The first is `home`,
and it has the value `IpAddrKind::V4` as its `kind` with associated address
data of `127.0.0.1`. The second instance is `loopback`. It has the other
variant of `IpAddrKind` as its `kind` value, `V6`, and has address `::1`
associated with it. We’ve used a struct to bundle the `kind` and `address`
values together, so now the variant is associated with the value.

However, representing the same concept using just an enum is more concise:
rather than an enum inside a struct, we can put data directly into each enum
variant. This new definition of the `IpAddr` enum says that both `V4` and `V6`
variants will have associated `String` values:

```
enum IpAddr {
    V4(String),
    V6(String),
}

let home = IpAddr::V4(String::from("127.0.0.1"));

let loopback = IpAddr::V6(String::from("::1"));
```

We attach data to each variant of the enum directly, so there is no need for an
extra struct. Here it’s also easier to see another detail of how enums work:
the name of each enum variant that we define also becomes a function that
constructs an instance of the enum. That is, `IpAddr::V4()` is a function call
that takes a `String` argument and returns an instance of the `IpAddr` type. We
automatically get this constructor function defined as a result of defining the
enum.

There’s another advantage to using an enum rather than a struct: each variant
can have different types and amounts of associated data. Version four type IP
addresses will always have four numeric components that will have values
between 0 and 255. If we wanted to store `V4` addresses as four `u8` values but
still express `V6` addresses as one `String` value, we wouldn’t be able to with
a struct. Enums handle this case with ease:

```
enum IpAddr {
    V4(u8, u8, u8, u8),
    V6(String),
}

let home = IpAddr::V4(127, 0, 0, 1);

let loopback = IpAddr::V6(String::from("::1"));
```

We’ve shown several different ways to define data structures to store version
four and version six IP addresses. However, as it turns out, wanting to store
IP addresses and encode which kind they are is so common that the standard
library has a definition we can use! Let’s look at how the standard library
defines `IpAddr`: it has the exact enum and variants that we’ve defined and
used, but it embeds the address data inside the variants in the form of two
different structs, which are defined differently for each variant:

```
struct Ipv4Addr {
    // --snip--
}

struct Ipv6Addr {
    // --snip--
}

enum IpAddr {
    V4(Ipv4Addr),
    V6(Ipv6Addr),
}
```

This code illustrates that you can put any kind of data inside an enum variant:
strings, numeric types, or structs, for example. You can even include another
enum! Also, standard library types are often not much more complicated than
what you might come up with.

Note that even though the standard library contains a definition for `IpAddr`,
we can still create and use our own definition without conflict because we
haven’t brought the standard library’s definition into our scope. We’ll talk
more about bringing types into scope in Chapter 7.

Let’s look at another example of an enum in Listing 6-2: this one has a wide
variety of types embedded in its variants.

```
enum Message {
    Quit,
    Move { x: i32, y: i32 },
    Write(String),
    ChangeColor(i32, i32, i32),
}
```

Listing 6-2: A `Message` enum whose variants each store different amounts and
types of values

This enum has four variants with different types:

* `Quit` has no data associated with it at all.
* `Move` has named fields like a struct does.
* `Write` includes a single `String`.
* `ChangeColor` includes three `i32` values.

Defining an enum with variants such as the ones in Listing 6-2 is similar to
defining different kinds of struct definitions, except the enum doesn’t use the
`struct` keyword and all the variants are grouped together under the `Message`
type. The following structs could hold the same data that the preceding enum
variants hold:

```
struct QuitMessage; // unit struct
struct MoveMessage {
    x: i32,
    y: i32,
}
struct WriteMessage(String); // tuple struct
struct ChangeColorMessage(i32, i32, i32); // tuple struct
```

But if we used the different structs, which each have their own type, we
couldn’t as easily define a function to take any of these kinds of messages as
we could with the `Message` enum defined in Listing 6-2, which is a single type.

<!--- We're also hinting at pattern matching complexity if we use the struct
method. Should we call it out and mention the pattern matching chapter?
/JT --->
<!-- If readers don't have experience with pattern matching, I don't think this
will resonate with them, so I'm not going to mention it here. /Carol -->

There is one more similarity between enums and structs: just as we’re able to
define methods on structs using `impl`, we’re also able to define methods on
enums. Here’s a method named `call` that we could define on our `Message` enum:

```
impl Message {
    fn call(&self) {
        // method body would be defined here
    }
}

let m = Message::Write(String::from("hello"));
m.call();
```

The body of the method would use `self` to get the value that we called the
method on. In this example, we’ve created a variable `m` that has the value
`Message::Write(String::from("hello"))`, and that is what `self` will be in the
body of the `call` method when `m.call()` runs.

Let’s look at another enum in the standard library that is very common and
useful: `Option`.

### The `Option` Enum and Its Advantages Over Null Values

This section explores a case study of `Option`, which is another enum defined
by the standard library. The `Option` type encodes the very common scenario in
which a value could be something or it could be nothing.

For example, if you request the first of a list containing items, you would get
a value. If you request the first item of an empty list, you would get nothing.
Expressing this concept in terms of the type system means the compiler can
check whether you’ve handled all the cases you should be handling; this
functionality can prevent bugs that are extremely common in other programming
languages.

Programming language design is often thought of in terms of which features you
include, but the features you exclude are important too. Rust doesn’t have the
null feature that many other languages have. *Null* is a value that means there
is no value there. In languages with null, variables can always be in one of
two states: null or not-null.

In his 2009 presentation “Null References: The Billion Dollar Mistake,” Tony
Hoare, the inventor of null, has this to say:

> I call it my billion-dollar mistake. At that time, I was designing the first
> comprehensive type system for references in an object-oriented language. My
> goal was to ensure that all use of references should be absolutely safe, with
> checking performed automatically by the compiler. But I couldn’t resist the
> temptation to put in a null reference, simply because it was so easy to
> implement. This has led to innumerable errors, vulnerabilities, and system
> crashes, which have probably caused a billion dollars of pain and damage in
> the last forty years.

The problem with null values is that if you try to use a null value as a
not-null value, you’ll get an error of some kind. Because this null or not-null
property is pervasive, it’s extremely easy to make this kind of error.

However, the concept that null is trying to express is still a useful one: a
null is a value that is currently invalid or absent for some reason.

The problem isn’t really with the concept but with the particular
implementation. As such, Rust does not have nulls, but it does have an enum
that can encode the concept of a value being present or absent. This enum is
`Option<T>`, and it is defined by the standard library
as follows:

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

The `Option<T>` enum is so useful that it’s even included in the prelude; you
don’t need to bring it into scope explicitly. Its variants are also included in
the prelude: you can use `Some` and `None` directly without the `Option::`
prefix. The `Option<T>` enum is still just a regular enum, and `Some(T)` and
`None` are still variants of type `Option<T>`.

The `<T>` syntax is a feature of Rust we haven’t talked about yet. It’s a
generic type parameter, and we’ll cover generics in more detail in Chapter 10.
For now, all you need to know is that `<T>` means the `Some` variant of the
`Option` enum can hold one piece of data of any type, and that each concrete
type that gets used in place of `T` makes the overall `Option<T>` type a
different type. Here are some examples of using `Option` values to hold number
types and string types:

```
let some_number = Some(5);
let some_char = Some('e');

let absent_number: Option<i32> = None;
```

<!--- I would maybe do the above more explicitly as:

"
```
let some_number = Some(5);

let some_number2: Option<i32> = Some(5);
```
The types of `some_number` and `some_number2` in the above are identical.
"

Using `Some("a string")` we're going to open the door to references in
generic positions (which we still need to build up to). We talk a little about
Option<&str> below, but I don't think it helps explain the enum concept.

/JT --->
<!-- I changed the above to use `char` rather than string slice, but we're
trying to show that options holding different types are themselves different
types below, so I didn't want to make them both `i32`. /Carol -->

The type of `some_number` is `Option<i32>`. The type of `some_char` is
`Option<char>`, which is a different type. Rust can infer these types because
we’ve specified a value inside the `Some` variant. For `absent_number`, Rust
requires us to annotate the overall `Option` type: the compiler can’t infer the
type that the corresponding `Some` variant will hold by looking only at a
`None` value. Here, we tell Rust that we mean for `absent_number` to be of type
`Option<i32>`.

When we have a `Some` value, we know that a value is present and the value is
held within the `Some`. When we have a `None` value, in some sense, it means
the same thing as null: we don’t have a valid value. So why is having
`Option<T>` any better than having null?

In short, because `Option<T>` and `T` (where `T` can be any type) are different
types, the compiler won’t let us use an `Option<T>` value as if it were
definitely a valid value. For example, this code won’t compile because it’s
trying to add an `i8` to an `Option<i8>`:

```
let x: i8 = 5;
let y: Option<i8> = Some(5);

let sum = x + y;
```

If we run this code, we get an error message like this:

```
$ cargo run
   Compiling enums v0.1.0 (file:///projects/enums)
error[E0277]: cannot add `Option<i8>` to `i8`
 --> src/main.rs:5:17
  |
5 |     let sum = x + y;
  |                 ^ no implementation for `i8 + Option<i8>`
  |
  = help: the trait `Add<Option<i8>>` is not implemented for `i8`
```

Intense! In effect, this error message means that Rust doesn’t understand how
to add an `i8` and an `Option<i8>`, because they’re different types. When we
have a value of a type like `i8` in Rust, the compiler will ensure that we
always have a valid value. We can proceed confidently without having to check
for null before using that value. Only when we have an `Option<i8>` (or
whatever type of value we’re working with) do we have to worry about possibly
not having a value, and the compiler will make sure we handle that case before
using the value.

In other words, you have to convert an `Option<T>` to a `T` before you can
perform `T` operations with it. Generally, this helps catch one of the most
common issues with null: assuming that something isn’t null when it actually
is.

Eliminating the risk of incorrectly assuming a not-null value helps you to be
more confident in your code. In order to have a value that can possibly be
null, you must explicitly opt in by making the type of that value `Option<T>`.
Then, when you use that value, you are required to explicitly handle the case
when the value is null. Everywhere that a value has a type that isn’t an
`Option<T>`, you *can* safely assume that the value isn’t null. This was a
deliberate design decision for Rust to limit null’s pervasiveness and increase
the safety of Rust code.

So, how do you get the `T` value out of a `Some` variant when you have a value
of type `Option<T>` so you can use that value? The `Option<T>` enum has a large
number of methods that are useful in a variety of situations; you can check
them out in its documentation. Becoming familiar with the methods on
`Option<T>` will be extremely useful in your journey with Rust.

In general, in order to use an `Option<T>` value, you want to have code that
will handle each variant. You want some code that will run only when you have a
`Some(T)` value, and this code is allowed to use the inner `T`. You want some
other code to run if you have a `None` value, and that code doesn’t have a `T`
value available. The `match` expression is a control flow construct that does
just this when used with enums: it will run different code depending on which
variant of the enum it has, and that code can use the data inside the matching
value.

## The `match` Control Flow Construct

Rust has an extremely powerful control flow construct called `match` that allows
you to compare a value against a series of patterns and then execute code based
on which pattern matches. Patterns can be made up of literal values, variable
names, wildcards, and many other things; Chapter 18 covers all the different
kinds of patterns and what they do. The power of `match` comes from the
expressiveness of the patterns and the fact that the compiler confirms that all
possible cases are handled.

Think of a `match` expression as being like a coin-sorting machine: coins slide
down a track with variously sized holes along it, and each coin falls through
the first hole it encounters that it fits into. In the same way, values go
through each pattern in a `match`, and at the first pattern the value “fits,”
the value falls into the associated code block to be used during execution.

Speaking of coins, let’s use them as an example using `match`! We can write a
function that takes an unknown United States coin and, in a similar way as the
counting machine, determines which coin it is and return its value in cents, as
shown here in Listing 6-3.

```
[1]enum Coin {
    Penny,
    Nickel,
    Dime,
    Quarter,
}

fn value_in_cents(coin: Coin) -> u8 {
    match coin {
        Coin::Penny => 1,
        Coin::Nickel => 5,
        Coin::Dime => 10,
        Coin::Quarter => 25,
    }
}
```

Listing 6-3: An enum and a `match` expression that has the variants of the enum
as its patterns

Let’s break down the `match` in the `value_in_cents` function. First, we list
the `match` keyword followed by an expression, which in this case is the value
`coin`. This seems very similar to an expression used with `if`, but there’s a
big difference: with `if`, the expression needs to return a Boolean value, but
here, it can return any type. The type of `coin` in this example is the `Coin`
enum that we defined at [1].

Next are the `match` arms. An arm has two parts: a pattern and some code. The
first arm here has a pattern that is the value `Coin::Penny` and then the `=>`
operator that separates the pattern and the code to run. The code in this case
is just the value `1`. Each arm is separated from the next with a comma.

<!--- Tiny nit, though not sure how to phrase it. Arms are separated by commas
in this example, though if you use blocks instead of simple values you won't use
commas. We see this happen in the next example.
/JT --->
<!-- I clarified in the paragraph before the next example. You *can* use commas
even if there's curly brackets, but people don't usually. /Carol -->

When the `match` expression executes, it compares the resulting value against
the pattern of each arm, in order. If a pattern matches the value, the code
associated with that pattern is executed. If that pattern doesn’t match the
value, execution continues to the next arm, much as in a coin-sorting machine.
We can have as many arms as we need: in Listing 6-3, our `match` has four arms.

The code associated with each arm is an expression, and the resulting value of
the expression in the matching arm is the value that gets returned for the
entire `match` expression.

We don't typically use curly brackets if the match arm code is short, as it is
in Listing 6-3 where each arm just returns a value. If you want to run multiple
lines of code in a match arm, you must use curly brackets, and the comma
following the arm is then optional. For example, the following code prints
“Lucky penny!” every time the method is called with a `Coin::Penny`, but still
returns the last value of the block, `1`:

```
fn value_in_cents(coin: Coin) -> u8 {
    match coin {
        Coin::Penny => {
            println!("Lucky penny!");
            1
        }
        Coin::Nickel => 5,
        Coin::Dime => 10,
        Coin::Quarter => 25,
    }
}
```

### Patterns that Bind to Values

Another useful feature of match arms is that they can bind to the parts of the
values that match the pattern. This is how we can extract values out of enum
variants.

As an example, let’s change one of our enum variants to hold data inside it.
From 1999 through 2008, the United States minted quarters with different
designs for each of the 50 states on one side. No other coins got state
designs, so only quarters have this extra value. We can add this information to
our `enum` by changing the `Quarter` variant to include a `UsState` value stored
inside it, which we’ve done here in Listing 6-4.

```
#[derive(Debug)] // so we can inspect the state in a minute
enum UsState {
    Alabama,
    Alaska,
    // --snip--
}

enum Coin {
    Penny,
    Nickel,
    Dime,
    Quarter(UsState),
}
```

Listing 6-4: A `Coin` enum in which the `Quarter` variant also holds a
`UsState` value

Let’s imagine that a friend is trying to collect all 50 state quarters. While
we sort our loose change by coin type, we’ll also call out the name of the
state associated with each quarter so if it’s one our friend doesn’t have, they
can add it to their collection.

In the match expression for this code, we add a variable called `state` to the
pattern that matches values of the variant `Coin::Quarter`. When a
`Coin::Quarter` matches, the `state` variable will bind to the value of that
quarter’s state. Then we can use `state` in the code for that arm, like so:

```
fn value_in_cents(coin: Coin) -> u8 {
    match coin {
        Coin::Penny => 1,
        Coin::Nickel => 5,
        Coin::Dime => 10,
        Coin::Quarter(state) => {
            println!("State quarter from {:?}!", state);
            25
        }
    }
}
```

If we were to call `value_in_cents(Coin::Quarter(UsState::Alaska))`, `coin`
would be `Coin::Quarter(UsState::Alaska)`. When we compare that value with each
of the match arms, none of them match until we reach `Coin::Quarter(state)`. At
that point, the binding for `state` will be the value `UsState::Alaska`. We can
then use that binding in the `println!` expression, thus getting the inner
state value out of the `Coin` enum variant for `Quarter`.

### Matching with `Option<T>`

In the previous section, we wanted to get the inner `T` value out of the `Some`
case when using `Option<T>`; we can also handle `Option<T>` using `match` as we
did with the `Coin` enum! Instead of comparing coins, we’ll compare the
variants of `Option<T>`, but the way that the `match` expression works remains
the same.

Let’s say we want to write a function that takes an `Option<i32>` and, if
there’s a value inside, adds 1 to that value. If there isn’t a value inside,
the function should return the `None` value and not attempt to perform any
operations.

This function is very easy to write, thanks to `match`, and will look like
Listing 6-5.

```
fn plus_one(x: Option<i32>) -> Option<i32> {
    match x {
        None => None,
        Some(i) => Some(i + 1),
    }
}

let five = Some(5);
let six = plus_one(five);
let none = plus_one(None);
```

Listing 6-5: A function that uses a `match` expression on an `Option<i32>`

Let’s examine the first execution of `plus_one` in more detail. When we call
`plus_one(five)`, the variable `x` in the body of `plus_one` will have the
value `Some(5)`. We then compare that against each match arm.

```
None => None,
```

The `Some(5)` value doesn’t match the pattern `None`, so we continue to the
next arm.

```
Some(i) => Some(i + 1),
```

Does `Some(5)` match `Some(i)`? Why yes it does! We have the same variant. The
`i` binds to the value contained in `Some`, so `i` takes the value `5`. The
code in the match arm is then executed, so we add 1 to the value of `i` and
create a new `Some` value with our total `6` inside.

Now let’s consider the second call of `plus_one` in Listing 6-5, where `x` is
`None`. We enter the `match` and compare to the first arm.

```
None => None,
```

It matches! There’s no value to add to, so the program stops and returns the
`None` value on the right side of `=>`. Because the first arm matched, no other
arms are compared.

Combining `match` and enums is useful in many situations. You’ll see this
pattern a lot in Rust code: `match` against an enum, bind a variable to the
data inside, and then execute code based on it. It’s a bit tricky at first, but
once you get used to it, you’ll wish you had it in all languages. It’s
consistently a user favorite.

### Matches Are Exhaustive

There’s one other aspect of `match` we need to discuss: the arms’ patterns must
cover all possibilities. Consider this version of our `plus_one` function,
which has a bug and won’t compile:

```
fn plus_one(x: Option<i32>) -> Option<i32> {
    match x {
        Some(i) => Some(i + 1),
    }
}
```

We didn’t handle the `None` case, so this code will cause a bug. Luckily, it’s
a bug Rust knows how to catch. If we try to compile this code, we’ll get this
error:

```
$ cargo run
   Compiling enums v0.1.0 (file:///projects/enums)
error[E0004]: non-exhaustive patterns: `None` not covered
   --> src/main.rs:3:15
    |
3   |         match x {
    |               ^ pattern `None` not covered
    |
    = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
    = note: the matched value is of type `Option<i32>`
```

Rust knows that we didn’t cover every possible case and even knows which
pattern we forgot! Matches in Rust are *exhaustive*: we must exhaust every last
possibility in order for the code to be valid. Especially in the case of
`Option<T>`, when Rust prevents us from forgetting to explicitly handle the
`None` case, it protects us from assuming that we have a value when we might
have null, thus making the billion-dollar mistake discussed earlier impossible.

### Catch-all Patterns and the `_` Placeholder

Using enums, we can also take special actions for a few particular values, but
for all other values take one default action. Imagine we’re implementing a game
where, if you roll a 3 on a dice roll, your player doesn’t move, but instead
gets a new fancy hat. If you roll a 7, your player loses a fancy hat. For all
other values, your player moves that number of spaces on the game board. Here’s
a `match` that implements that logic, with the result of the dice roll
hardcoded rather than a random value, and all other logic represented by
functions without bodies because actually implementing them is out of scope for
this example:

```
let dice_roll = 9;
match dice_roll {
    3 => add_fancy_hat(),
    7 => remove_fancy_hat(),
    other => move_player(other),
}

fn add_fancy_hat() {}
fn remove_fancy_hat() {}
fn move_player(num_spaces: u8) {}
```

For the first two arms, the patterns are the literal values 3 and 7. For the
last arm that covers every other possible value, the pattern is the variable
we’ve chosen to name `other`. The code that runs for the `other` arm uses the
variable by passing it to the `move_player` function.

This code compiles, even though we haven’t listed all the possible values a
`u8` can have, because the last pattern will match all values not specifically
listed. This catch-all pattern meets the requirement that `match` must be
exhaustive. Note that we have to put the catch-all arm last because the
patterns are evaluated in order. If we put the catch-all arm earlier, the other
arms would never run, so Rust will warn us if we add arms after a catch-all!

Rust also has a pattern we can use when we want a catch-all but don’t want to
*use* the value in the catch-all pattern: `_` is a special pattern that matches
any value and does not bind to that value. This tells Rust we aren’t going to
use the value, so Rust won’t warn us about an unused variable.

Let’s change the rules of the game: now, if you roll anything other than a 3 or
a 7, you must roll again. We no longer need to use the catch-all value, so we
can change our code to use `_` instead of the variable named `other`:

```
let dice_roll = 9;
match dice_roll {
    3 => add_fancy_hat(),
    7 => remove_fancy_hat(),
    _ => reroll(),
}

fn add_fancy_hat() {}
fn remove_fancy_hat() {}
fn reroll() {}
```

This example also meets the exhaustiveness requirement because we’re explicitly
ignoring all other values in the last arm; we haven’t forgotten anything.

Finally, we’ll change the rules of the game one more time, so that nothing else
happens on your turn if you roll anything other than a 3 or a 7. We can express
that by using the unit value (the empty tuple type we mentioned in “The Tuple
Type” section) as the code that goes with the `_` arm:

```
let dice_roll = 9;
match dice_roll {
    3 => add_fancy_hat(),
    7 => remove_fancy_hat(),
    _ => (),
}

fn add_fancy_hat() {}
fn remove_fancy_hat() {}
```

Here, we’re telling Rust explicitly that we aren’t going to use any other value
that doesn’t match a pattern in an earlier arm, and we don’t want to run any
code in this case.

There’s more about patterns and matching that we’ll cover in Chapter 18. For
now, we’re going to move on to the `if let` syntax, which can be useful in
situations where the `match` expression is a bit wordy.

## Concise Control Flow with `if let`

The `if let` syntax lets you combine `if` and `let` into a less verbose way to
handle values that match one pattern while ignoring the rest. Consider the
program in Listing 6-6 that matches on an `Option<u8>` value in the `config_max`
variable but only wants to execute code if the value is the `Some` variant.

```
let config_max = Some(3u8);
match config_max {
    Some(max) => println!("The maximum is configured to be {}", max),
    _ => (),
}
```

Listing 6-6: A `match` that only cares about executing code when the value is
`Some`

If the value is `Some`, we print out the value in the `Some` variant by binding
the value to the variable `max` in the pattern. We don’t want to do anything
with the `None` value. To satisfy the `match` expression, we have to add `_ =>
()` after processing just one variant, which is annoying boilerplate code to
add.

Instead, we could write this in a shorter way using `if let`. The following
code behaves the same as the `match` in Listing 6-6:

```
let config_max = Some(3u8);
if let Some(max) = config_max {
    println!("The maximum is configured to be {}", max);
}
```

The syntax `if let` takes a pattern and an expression separated by an equal
sign. It works the same way as a `match`, where the expression is given to the
`match` and the pattern is its first arm. In this case, the pattern is
`Some(max)`, and the `max` binds to the value inside the `Some`. We can then
use `max` in the body of the `if let` block in the same way as we used `max` in
the corresponding `match` arm. The code in the `if let` block isn’t run if the
value doesn’t match the pattern.

Using `if let` means less typing, less indentation, and less boilerplate code.
However, you lose the exhaustive checking that `match` enforces. Choosing
between `match` and `if let` depends on what you’re doing in your particular
situation and whether gaining conciseness is an appropriate trade-off for
losing exhaustive checking.

In other words, you can think of `if let` as syntax sugar for a `match` that
runs code when the value matches one pattern and then ignores all other values.

We can include an `else` with an `if let`. The block of code that goes with the
`else` is the same as the block of code that would go with the `_` case in the
`match` expression that is equivalent to the `if let` and `else`. Recall the
`Coin` enum definition in Listing 6-4, where the `Quarter` variant also held a
`UsState` value. If we wanted to count all non-quarter coins we see while also
announcing the state of the quarters, we could do that with a `match`
expression like this:

```
let mut count = 0;
match coin {
    Coin::Quarter(state) => println!("State quarter from {:?}!", state),
    _ => count += 1,
}
```

Or we could use an `if let` and `else` expression like this:

```
let mut count = 0;
if let Coin::Quarter(state) = coin {
    println!("State quarter from {:?}!", state);
} else {
    count += 1;
}
```

If you have a situation in which your program has logic that is too verbose to
express using a `match`, remember that `if let` is in your Rust toolbox as well.

## Summary

We’ve now covered how to use enums to create custom types that can be one of a
set of enumerated values. We’ve shown how the standard library’s `Option<T>`
type helps you use the type system to prevent errors. When enum values have
data inside them, you can use `match` or `if let` to extract and use those
values, depending on how many cases you need to handle.

Your Rust programs can now express concepts in your domain using structs and
enums. Creating custom types to use in your API ensures type safety: the
compiler will make certain your functions get only values of the type each
function expects.

In order to provide a well-organized API to your users that is straightforward
to use and only exposes exactly what your users will need, let’s now turn to
Rust’s modules.

<!--- I'm of two minds whether `?` should squeeze in here? We talk about `if
let` but then switch topics next chapter and talk about modules. In the wild,
I'd bet `?` would be as common, perhaps more common, than `if let`.

But I'll defer to your pedagogy plan. Just wanted to share the thought.
/JT --->
<!-- Yeah introducing `?` here would be a big change; I don't want to talk
about that until we've talked about `Result` even though you can use `?` on
`Option` now, because `?` is still way more common with `Result`. /Carol -->