path: root/src/doc/book/nostarch/chapter18.md

<!-- DO NOT EDIT THIS FILE.

This file is periodically generated from the content in the `/src/`
directory, so all fixes need to be made in `/src/`.
-->

[TOC]

# Patterns and Matching

*Patterns* are a special syntax in Rust for matching against the structure of
types, both complex and simple. Using patterns in conjunction with `match`
expressions and other constructs gives you more control over a program’s
control flow. A pattern consists of some combination of the following:

* Literals
* Destructured arrays, enums, structs, or tuples
* Variables
* Wildcards
* Placeholders

Some example patterns include `x`, `(a, 3)`, and `Some(Color::Red)`. In the
contexts in which patterns are valid, these components describe the shape of
data. Our program then matches values against the patterns to determine whether
it has the correct shape of data to continue running a particular piece of code.
<!-- is there some generic pattern we can show as an example, early on, or
is it too dependent on where the pattern is used? /LC -->
<!-- Yeah, if a pattern is out of context, it doesn't look special. Like `3`
can be a pattern. /Carol -->
<!-- We could mention something like, "If you've written a little Rust, you've
already used patterns without knowing it. For example `let x = 3`, the `x` is
a pattern." Though looks like we use this later.

Or, we could say, "Some example patterns include: `x`, `(a, b)`, and `Color::Red`
/JT -->
<!-- Ok, I've tried rewording this paragraph to include some examples, I do
think it's important to emphasize that these are only patterns in the contexts
patterns may exist, because without the context, there's no way to distinguish
patterns from regular values /Carol -->

To use a pattern, we compare it to some value. If the pattern matches the
value, we use the value parts in our code. Recall the `match` expressions in
Chapter 6 that used patterns, such as the coin-sorting machine example. If the
value fits the shape of the pattern, we can use the named pieces. If it
doesn’t, the code associated with the pattern won’t run.

This chapter is a reference on all things related to patterns. We’ll cover the
valid places to use patterns, the difference between refutable and irrefutable
patterns, and the different kinds of pattern syntax that you might see. By the
end of the chapter, you’ll know how to use patterns to express many concepts in
a clear way.

## All the Places Patterns Can Be Used

Patterns pop up in a number of places in Rust, and you’ve been using them a lot
without realizing it! This section discusses all the places where patterns are
valid.

### `match` Arms

As discussed in Chapter 6, we use patterns in the arms of `match` expressions.
Formally, `match` expressions are defined as the keyword `match`, a value to
match on, and one or more match arms that consist of a pattern and an
expression to run if the value matches that arm’s pattern, like this:

```
match VALUE {
    PATTERN => EXPRESSION,
    PATTERN => EXPRESSION,
    PATTERN => EXPRESSION,
}
```

For example, here's the `match` expression from Listing 6-5 that matches on an
`Option<i32>` value in the variable `x`:

```
match x {
    None => None,
    Some(i) => Some(i + 1),
}
```

The patterns in this `match` expression are the `None` and `Some(i)` on the
left of each arrow.

One requirement for `match` expressions is that they need to be *exhaustive* in
the sense that all possibilities for the value in the `match` expression must
be accounted for. One way to ensure you’ve covered every possibility is to have
a catchall pattern for the last arm: for example, a variable name matching any
value can never fail and thus covers every remaining case.

The particular pattern `_` will match anything, but it never binds to a
variable, so it’s often used in the last match arm. The `_` pattern can be
useful when you want to ignore any value not specified, for example. We’ll
cover the `_` pattern in more detail in the “Ignoring Values in a Pattern”
section later in this chapter.

### Conditional `if let` Expressions

In Chapter 6 we discussed how to use `if let` expressions mainly as a shorter
way to write the equivalent of a `match` that only matches one case.
Optionally, `if let` can have a corresponding `else` containing code to run if
the pattern in the `if let` doesn’t match.

Listing 18-1 shows that it’s also possible to mix and match `if let`, `else
if`, and `else if let` expressions. Doing so gives us more flexibility than a
`match` expression in which we can express only one value to compare with the
patterns. Also, Rust doesn't require that the conditions in a series of `if
let`, `else if`, `else if let` arms relate to each other.

The code in Listing 18-1 determines what color to make your background based on
a series of checks for several conditions. For this example, we’ve created
variables with hardcoded values that a real program might receive from user
input.

Filename: src/main.rs

```
fn main() {
    let favorite_color: Option<&str> = None;
    let is_tuesday = false;
    let age: Result<u8, _> = "34".parse();

    [1] if let Some(color) = favorite_color {
        [2] println!("Using your favorite color, {color}, as the background");
    [3] } else if is_tuesday {
        [4] println!("Tuesday is green day!");
    [5]} else if let Ok(age) = age {
        [6] if age > 30 {
            [7] println!("Using purple as the background color");
        } else {
            [8] println!("Using orange as the background color");
        }
    [9] } else {
        [10] println!("Using blue as the background color");
    }
}
```

Listing 18-1: Mixing `if let`, `else if`, `else if let`, and `else`

If the user specifies a favorite color [1], that color is used as the
background [2]. If no favorite color is specified and today is Tuesday [3], the
background color is green [4]. Otherwise, if the user specifies their age as a
string and we can parse it as a number successfully [5], the color is either
purple [7] or orange [8] depending on the value of the number [6]. If none of
these conditions apply [9], the background color is blue.

This conditional structure lets us support complex requirements. With the
hardcoded values we have here, this example will print `Using purple as the
background color`.

You can see that `if let` can also introduce shadowed variables in the same way
that `match` arms can: the line `if let Ok(age) = age` [5] introduces a new
shadowed `age` variable that contains the value inside the `Ok` variant. This
means we need to place the `if age > 30` condition [6] within that block: we
can’t combine these two conditions into `if let Ok(age) = age && age > 30`. The
shadowed `age` we want to compare to 30 isn’t valid until the new scope starts
with the curly bracket.

<!-- Have we given them an intuition yet for why this is? I may be forgetting
something from the earlier chapters, but I wonder if we should reiterate that
when a pattern matches, the variable that gets bound is only valid for the
expression or block that follows the match. /JT -->
<!-- I don't really see a difference between saying "The shadowed `age` we want
to compare to 30 isn't valid until the new scope starts with the curly bracket"
and "when a pattern matches, the variable that gets bound is only valid for the
expression or block that follows the match"? To me, it sounds like this would
be saying the same thing twice, so I'm not going to change anything here.
/Carol -->

The downside of using `if let` expressions is that the compiler doesn’t check
for exhaustiveness, whereas with `match` expressions it does. If we omitted the
last `else` block [9] and therefore missed handling some cases, the compiler
would not alert us to the possible logic bug.

### `while let` Conditional Loops

Similar in construction to `if let`, the `while let` conditional loop allows a
`while` loop to run for as long as a pattern continues to match. In Listing
18-2 we code a `while let` loop that uses a vector as a stack and prints the
values in the vector in the opposite order in which they were pushed.

```
let mut stack = Vec::new();

stack.push(1);
stack.push(2);
stack.push(3);

while let Some(top) = stack.pop() {
    println!("{}", top);
}
```

Listing 18-2: Using a `while let` loop to print values for as long as
`stack.pop()` returns `Some`

This example prints 3, 2, and then 1. The `pop` method takes the last element
out of the vector and returns `Some(value)`. If the vector is empty, `pop`
returns `None`. The `while` loop continues running the code in its block as
long as `pop` returns `Some`. When `pop` returns `None`, the loop stops. We can
use `while let` to pop every element off our stack.

### `for` Loops

In a `for` loop, the value that directly follows the keyword `for` is a
pattern. For example, in `for x in y` the `x` is the pattern. Listing 18-3
demonstrates how to use a pattern in a `for` loop to destructure, or break
apart, a tuple as part of the `for` loop.

```
let v = vec!['a', 'b', 'c'];

for (index, value) in v.iter().enumerate() {
    println!("{} is at index {}", value, index);
}
```

Listing 18-3: Using a pattern in a `for` loop to destructure a tuple

The code in Listing 18-3 will print the following:

```
a is at index 0
b is at index 1
c is at index 2
```

We adapt an iterator using the `enumerate` method so it produces a value and
the index for that value, placed into a tuple. The first value produced is the
tuple `(0, 'a')`. When this value is matched to the pattern `(index, value)`,
`index` will be `0` and `value` will be `'a'`, printing the first line of the
output.

### `let` Statements

Prior to this chapter, we had only explicitly discussed using patterns with
`match` and `if let`, but in fact, we’ve used patterns in other places as well,
including in `let` statements. For example, consider this straightforward
variable assignment with `let`:

```
let x = 5;
```

Every time you've used a `let` statement like this you've been using patterns,
although you might not have realized it! More formally, a `let` statement looks
like this:

```
let PATTERN = EXPRESSION;
```

In statements like `let x = 5;` with a variable name in the `PATTERN` slot, the
variable name is just a particularly simple form of a pattern. Rust compares
the expression against the pattern and assigns any names it finds. So in the
`let x = 5;` example, `x` is a pattern that means “bind what matches here to
the variable `x`.” Because the name `x` is the whole pattern, this pattern
effectively means “bind everything to the variable `x`, whatever the value is.”

To see the pattern matching aspect of `let` more clearly, consider Listing
18-4, which uses a pattern with `let` to destructure a tuple.

```
let (x, y, z) = (1, 2, 3);
```

Listing 18-4: Using a pattern to destructure a tuple and create three variables
at once

Here, we match a tuple against a pattern. Rust compares the value `(1, 2, 3)`
to the pattern `(x, y, z)` and sees that the value matches the pattern, in that
it sees that the number of elements is the same in both, so Rust
binds `1` to `x`, `2` to `y`, and `3` to `z`. You can think of this tuple
pattern as nesting three individual variable patterns inside it.

If the number of elements in the pattern doesn’t match the number of elements
in the tuple, the overall type won’t match and we’ll get a compiler error. For
example, Listing 18-5 shows an attempt to destructure a tuple with three
elements into two variables, which won’t work.

```
let (x, y) = (1, 2, 3);
```

Listing 18-5: Incorrectly constructing a pattern whose variables don’t match
the number of elements in the tuple

Attempting to compile this code results in this type error:

```
error[E0308]: mismatched types
 --> src/main.rs:2:9
  |
2 |     let (x, y) = (1, 2, 3);
  |         ^^^^^^   --------- this expression has type `({integer}, {integer}, {integer})`
  |         |
  |         expected a tuple with 3 elements, found one with 2 elements
  |
  = note: expected tuple `({integer}, {integer}, {integer})`
             found tuple `(_, _)`
```

To fix the error, we could ignore one or more of the values in the tuple using
`_` or `..`, as you’ll see in the “Ignoring Values in a Pattern” section. If
the problem is that we have too many variables in the pattern, the solution is
to make the types match by removing variables so the number of variables equals
the number of elements in the tuple.

### Function Parameters

Function parameters can also be patterns. The code in Listing 18-6, which
declares a function named `foo` that takes one parameter named `x` of type
`i32`, should by now look familiar.

```
fn foo(x: i32) {
    // code goes here
}
```

Listing 18-6: A function signature uses patterns in the parameters

The `x` part is a pattern! As we did with `let`, we could match a tuple in a
function’s arguments to the pattern. Listing 18-7 splits the values in a tuple
as we pass it to a function.

Filename: src/main.rs

```
fn print_coordinates(&(x, y): &(i32, i32)) {
    println!("Current location: ({}, {})", x, y);
}

fn main() {
    let point = (3, 5);
    print_coordinates(&point);
}
```

Listing 18-7: A function with parameters that destructure a tuple

This code prints `Current location: (3, 5)`. The values `&(3, 5)` match the
pattern `&(x, y)`, so `x` is the value `3` and `y` is the value `5`.

We can also use patterns in closure parameter lists in the same way as in
function parameter lists, because closures are similar to functions, as
discussed in Chapter 13.

At this point, you’ve seen several ways of using patterns, but patterns don’t
work the same in every place we can use them. In some places, the patterns must
be irrefutable; in other circumstances, they can be refutable. We’ll discuss
these two concepts next.

## Refutability: Whether a Pattern Might Fail to Match

Patterns come in two forms: refutable and irrefutable. Patterns that will match
for any possible value passed are *irrefutable*. An example would be `x` in the
statement `let x = 5;` because `x` matches anything and therefore cannot fail
to match. Patterns that can fail to match for some possible value are
*refutable*. An example would be `Some(x)` in the expression `if let Some(x) =
a_value` because if the value in the `a_value` variable is `None` rather than
`Some`, the `Some(x)` pattern will not match.

Function parameters, `let` statements, and `for` loops can only accept
irrefutable patterns, because the program cannot do anything meaningful when
values don’t match. The `if let` and `while let` expressions accept
refutable and irrefutable patterns, but the compiler warns against
irrefutable patterns because by definition they’re intended to handle possible
failure: the functionality of a conditional is in its ability to perform
differently depending on success or failure.

In general, you shouldn’t have to worry about the distinction between refutable
and irrefutable patterns; however, you do need to be familiar with the concept
of refutability so you can respond when you see it in an error message. In
those cases, you’ll need to change either the pattern or the construct you’re
using the pattern with, depending on the intended behavior of the code.

Let’s look at an example of what happens when we try to use a refutable pattern
where Rust requires an irrefutable pattern and vice versa. Listing 18-8 shows a
`let` statement, but for the pattern we’ve specified `Some(x)`, a refutable
pattern. As you might expect, this code will not compile.

```
let Some(x) = some_option_value;
```

Listing 18-8: Attempting to use a refutable pattern with `let`

If `some_option_value` was a `None` value, it would fail to match the pattern
`Some(x)`, meaning the pattern is refutable. However, the `let` statement can
only accept an irrefutable pattern because there is nothing valid the code can
do with a `None` value. At compile time, Rust will complain that we’ve tried to
use a refutable pattern where an irrefutable pattern is required:

```
error[E0005]: refutable pattern in local binding: `None` not covered
   --> src/main.rs:3:9
    |
3   |     let Some(x) = some_option_value;
    |         ^^^^^^^ pattern `None` not covered
    |
    = note: `let` bindings require an "irrefutable pattern", like a `struct` or an `enum` with only one variant
    = note: for more information, visit https://doc.rust-lang.org/book/ch18-02-refutability.html
    = note: the matched value is of type `Option<i32>`
help: you might want to use `if let` to ignore the variant that isn't matched
    |
3   |     if let Some(x) = some_option_value { /* */ }
    |
```

Because we didn’t cover (and couldn’t cover!) every valid value with the
pattern `Some(x)`, Rust rightfully produces a compiler error.

If we have a refutable pattern where an irrefutable pattern is needed, we can
fix it by changing the code that uses the pattern: instead of using `let`, we
can use `if let`. Then if the pattern doesn’t match, the code will just skip
the code in the curly brackets, giving it a way to continue validly. Listing
18-9 shows how to fix the code in Listing 18-8.

```
if let Some(x) = some_option_value {
    println!("{}", x);
}
```

Listing 18-9: Using `if let` and a block with refutable patterns instead of
`let`

We’ve given the code an out! This code is perfectly valid, although it means we
cannot use an irrefutable pattern without receiving an error. If we give `if
let` a pattern that will always match, such as `x`, as shown in Listing 18-10,
the compiler will give a warning.

```
if let x = 5 {
    println!("{}", x);
};
```

Listing 18-10: Attempting to use an irrefutable pattern with `if let`

Rust complains that it doesn’t make sense to use `if let` with an irrefutable
pattern:

```
warning: irrefutable `if let` pattern
 --> src/main.rs:2:8
  |
2 |     if let x = 5 {
  |        ^^^^^^^^^
  |
  = note: `#[warn(irrefutable_let_patterns)]` on by default
  = note: this pattern will always match, so the `if let` is useless
  = help: consider replacing the `if let` with a `let`
```

For this reason, match arms must use refutable patterns, except for the last
arm, which should match any remaining values with an irrefutable pattern. Rust
allows us to use an irrefutable pattern in a `match` with only one arm, but
this syntax isn’t particularly useful and could be replaced with a simpler
`let` statement.

Now that you know where to use patterns and the difference between refutable
and irrefutable patterns, let’s cover all the syntax we can use to create
patterns.

## Pattern Syntax

In this section, we gather all the syntax valid in patterns and discuss why and
when you might want to use each one.

### Matching Literals

As you saw in Chapter 6, you can match patterns against literals directly. The
following code gives some examples:

```
let x = 1;

match x {
    1 => println!("one"),
    2 => println!("two"),
    3 => println!("three"),
    _ => println!("anything"),
}
```

This code prints `one` because the value in `x` is 1. This syntax is useful
when you want your code to take an action if it gets a particular concrete
value.

### Matching Named Variables

Named variables are irrefutable patterns that match any value, and we’ve used
them many times in the book. However, there is a complication when you use
named variables in `match` expressions. Because `match` starts a new scope,
variables declared as part of a pattern inside the `match` expression will
shadow those with the same name outside the `match` construct, as is the case
with all variables. In Listing 18-11, we declare a variable named `x` with the
value `Some(5)` and a variable `y` with the value `10`. We then create a
`match` expression on the value `x`. Look at the patterns in the match arms and
`println!` at the end, and try to figure out what the code will print before
running this code or reading further.

Filename: src/main.rs

```
fn main() {
    [1] let x = Some(5);
    [2] let y = 10;

    match x {
        [3] Some(50) => println!("Got 50"),
        [4] Some(y) => println!("Matched, y = {y}"),
        [5] _ => println!("Default case, x = {:?}", x),
    }

    [6] println!("at the end: x = {:?}, y = {y}", x);
}
```

Listing 18-11: A `match` expression with an arm that introduces a shadowed
variable `y`

Let’s walk through what happens when the `match` expression runs. The pattern
in the first match arm [3] doesn’t match the defined value of `x` [1], so the
code continues.

The pattern in the second match arm [4] introduces a new variable named `y`
that will match any value inside a `Some` value. Because we’re in a new scope
inside the `match` expression, this is a new `y` variable, not the `y` we
declared at the beginning with the value 10 [2]. This new `y` binding will
match any value inside a `Some`, which is what we have in `x`. Therefore, this
new `y` binds to the inner value of the `Some` in `x`. That value is `5`, so
the expression for that arm executes and prints `Matched, y = 5`.

If `x` had been a `None` value instead of `Some(5)`, the patterns in the first
two arms wouldn’t have matched, so the value would have matched to the
underscore [5]. We didn’t introduce the `x` variable in the pattern of the
underscore arm, so the `x` in the expression is still the outer `x` that hasn’t
been shadowed. In this hypothetical case, the `match` would print `Default
case, x = None`.

When the `match` expression is done, its scope ends, and so does the scope of
the inner `y`. The last `println!` [6] produces `at the end: x = Some(5), y =
10`.

To create a `match` expression that compares the values of the outer `x` and
`y`, rather than introducing a shadowed variable, we would need to use a match
guard conditional instead. We’ll talk about match guards later in the “Extra
Conditionals with Match Guards” section.

### Multiple Patterns

In `match` expressions, you can match multiple patterns using the `|` syntax,
which is the pattern *or* operator. For example, in the following code we match
the value of `x` against the match arms, the first of which has an *or* option,
meaning if the value of `x` matches either of the values in that arm, that
arm’s code will run:

```
let x = 1;

match x {
    1 | 2 => println!("one or two"),
    3 => println!("three"),
    _ => println!("anything"),
}
```

This code prints `one or two`.

### Matching Ranges of Values with `..=`

The `..=` syntax allows us to match to an inclusive range of values. In the
following code, when a pattern matches any of the values within the given
range, that arm will execute:

```
let x = 5;

match x {
    1..=5 => println!("one through five"),
    _ => println!("something else"),
}
```

If `x` is 1, 2, 3, 4, or 5, the first arm will match. This syntax is more
convenient for multiple match values than using the `|` operator to express the
same idea; if we were to use `|` we would have to specify `1 | 2 | 3 | 4 | 5`.
Specifying a range is much shorter, especially if we want to match, say, any
number between 1 and 1,000!

The compiler checks that the range isn’t empty at compile time, and because the
only types for which Rust can tell if a range is empty or not are `char` and
numeric values, ranges are only allowed with numeric or `char` values.

Here is an example using ranges of `char` values:

```
let x = 'c';

match x {
    'a'..='j' => println!("early ASCII letter"),
    'k'..='z' => println!("late ASCII letter"),
    _ => println!("something else"),
}
```

Rust can tell that `'c'` is within the first pattern’s range and prints `early
ASCII letter`.

### Destructuring to Break Apart Values

We can also use patterns to destructure structs, enums, and tuples to use
different parts of these values. Let’s walk through each value.

#### Destructuring Structs

Listing 18-12 shows a `Point` struct with two fields, `x` and `y`, that we can
break apart using a pattern with a `let` statement.

Filename: src/main.rs

```
struct Point {
    x: i32,
    y: i32,
}

fn main() {
    let p = Point { x: 0, y: 7 };

    let Point { x: a, y: b } = p;
    assert_eq!(0, a);
    assert_eq!(7, b);
}
```

Listing 18-12: Destructuring a struct’s fields into separate variables

This code creates the variables `a` and `b` that match the values of the `x`
and `y` fields of the `p` struct. This example shows that the names of the
variables in the pattern don’t have to match the field names of the struct.
However, it’s common to match the variable names to the field names to make it
easier to remember which variables came from which fields. Because of this
common usage, and because writing `let Point { x: x, y: y } = p;` contains a
lot of duplication, Rust has a shorthand for patterns that match struct fields:
you only need to list the name of the struct field, and the variables created
from the pattern will have the same names. Listing 18-13 behaves in the same
way as the code in Listing 18-12, but the variables created in the `let`
pattern are `x` and `y` instead of `a` and `b`.

Filename: src/main.rs

```
struct Point {
    x: i32,
    y: i32,
}

fn main() {
    let p = Point { x: 0, y: 7 };

    let Point { x, y } = p;
    assert_eq!(0, x);
    assert_eq!(7, y);
}
```

Listing 18-13: Destructuring struct fields using struct field shorthand

This code creates the variables `x` and `y` that match the `x` and `y` fields
of the `p` variable. The outcome is that the variables `x` and `y` contain the
values from the `p` struct.

We can also destructure with literal values as part of the struct pattern
rather than creating variables for all the fields. Doing so allows us to test
some of the fields for particular values while creating variables to
destructure the other fields.

In Listing 18-14, we have a `match` expression that separates `Point` values
into three cases: points that lie directly on the `x` axis (which is true when
`y = 0`), on the `y` axis (`x = 0`), or neither.

Filename: src/main.rs

```
fn main() {
    let p = Point { x: 0, y: 7 };

    match p {
        Point { x, y: 0 } => println!("On the x axis at {}", x),
        Point { x: 0, y } => println!("On the y axis at {}", y),
        Point { x, y } => println!("On neither axis: ({}, {})", x, y),
    }
}
```

Listing 18-14: Destructuring and matching literal values in one pattern

The first arm will match any point that lies on the `x` axis by specifying that
the `y` field matches if its value matches the literal `0`. The pattern still
creates an `x` variable that we can use in the code for this arm.

Similarly, the second arm matches any point on the `y` axis by specifying that
the `x` field matches if its value is `0` and creates a variable `y` for the
value of the `y` field. The third arm doesn’t specify any literals, so it
matches any other `Point` and creates variables for both the `x` and `y` fields.

In this example, the value `p` matches the second arm by virtue of `x`
containing a 0, so this code will print `On the y axis at 7`.

Remember that a `match` expression stops checking arms once it has found the
first matching pattern, so even though `Point { x: 0, y: 0}` is on the `x` axis
and the `y` axis, this code would only print `On the x axis at 0`.

<!-- We should remind them that we stop at the first pattern, so even though
0,0 is on both x- and y-axis in a sense, you'll only ever see the "on x-axis
message" /JT -->
<!-- Done! /Carol -->

#### Destructuring Enums

We've destructured enums in this book (for example, Listing 6-5 in Chapter 6),
but haven’t yet explicitly discussed that the pattern to destructure an enum
corresponds to the way the data stored within the enum is defined. As an
example, in Listing 18-15 we use the `Message` enum from Listing 6-2 and write
a `match` with patterns that will destructure each inner value.

Filename: src/main.rs

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

fn main() {
    [1] let msg = Message::ChangeColor(0, 160, 255);

    match msg {
        [2] Message::Quit => {
            println!("The Quit variant has no data to destructure.")
        }
        [3] Message::Move { x, y } => {
            println!(
                "Move in the x direction {} and in the y direction {}",
                x, y
            );
        }
        [4] Message::Write(text) => println!("Text message: {}", text),
        [5] Message::ChangeColor(r, g, b) => println!(
            "Change the color to red {}, green {}, and blue {}",
            r, g, b
        ),
    }
}
```

Listing 18-15: Destructuring enum variants that hold different kinds of values

This code will print `Change the color to red 0, green 160, and blue 255`. Try
changing the value of `msg` [1] to see the code from the other arms run.

For enum variants without any data, like `Message::Quit` [2], we can’t
destructure the value any further. We can only match on the literal
`Message::Quit` value, and no variables are in that pattern.

For struct-like enum variants, such as `Message::Move` [3], we can use a
pattern similar to the pattern we specify to match structs. After the variant
name, we place curly brackets and then list the fields with variables so we
break apart the pieces to use in the code for this arm. Here we use the
shorthand form as we did in Listing 18-13.

For tuple-like enum variants, like `Message::Write` that holds a tuple with one
element [4] and `Message::ChangeColor` that holds a tuple with three elements
[5], the pattern is similar to the pattern we specify to match tuples. The
number of variables in the pattern must match the number of elements in the
variant we’re matching.

#### Destructuring Nested Structs and Enums

So far, our examples have all been matching structs or enums one level deep,
but matching can work on nested items too! For example, we can refactor the
code in Listing 18-15 to support RGB and HSV colors in the `ChangeColor`
message, as shown in Listing 18-16.

```
enum Color {
    Rgb(i32, i32, i32),
    Hsv(i32, i32, i32),
}

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

fn main() {
    let msg = Message::ChangeColor(Color::Hsv(0, 160, 255));

    match msg {
        Message::ChangeColor(Color::Rgb(r, g, b)) => println!(
            "Change the color to red {}, green {}, and blue {}",
            r, g, b
        ),
        Message::ChangeColor(Color::Hsv(h, s, v)) => println!(
            "Change the color to hue {}, saturation {}, and value {}",
            h, s, v
        ),
        _ => (),
    }
}
```

Listing 18-16: Matching on nested enums

The pattern of the first arm in the `match` expression matches a
`Message::ChangeColor` enum variant that contains a `Color::Rgb` variant; then
the pattern binds to the three inner `i32` values. The pattern of the second
arm also matches a `Message::ChangeColor` enum variant, but the inner enum
matches `Color::Hsv` instead. We can specify these complex conditions in one
`match` expression, even though two enums are involved.

#### Destructuring Structs and Tuples

We can mix, match, and nest destructuring patterns in even more complex ways.
The following example shows a complicated destructure where we nest structs and
tuples inside a tuple and destructure all the primitive values out:

```
let ((feet, inches), Point { x, y }) = ((3, 10), Point { x: 3, y: -10 });
```

This code lets us break complex types into their component parts so we can use
the values we’re interested in separately.

Destructuring with patterns is a convenient way to use pieces of values, such
as the value from each field in a struct, separately from each other.

### Ignoring Values in a Pattern

You’ve seen that it’s sometimes useful to ignore values in a pattern, such as
in the last arm of a `match`, to get a catchall that doesn’t actually do
anything but does account for all remaining possible values. There are a few
ways to ignore entire values or parts of values in a pattern: using the `_`
pattern (which you’ve seen), using the `_` pattern within another pattern,
using a name that starts with an underscore, or using `..` to ignore remaining
parts of a value. Let’s explore how and why to use each of these patterns.

#### Ignoring an Entire Value with `_`

We’ve used the underscore as a wildcard pattern that will match any value but
not bind to the value. This is especially useful as the last arm in a `match`
expression, but we can also use it in any pattern, including function
parameters, as shown in Listing 18-17.

Filename: src/main.rs

```
fn foo(_: i32, y: i32) {
    println!("This code only uses the y parameter: {}", y);
}

fn main() {
    foo(3, 4);
}
```

Listing 18-17: Using `_` in a function signature

This code will completely ignore the value `3` passed as the first argument,
and will print `This code only uses the y parameter: 4`.

In most cases when you no longer need a particular function parameter, you
would change the signature so it doesn’t include the unused parameter. Ignoring
a function parameter can be especially useful in cases when, for example,
you're implementing a trait when you need a certain type signature but the
function body in your implementation doesn’t need one of the parameters. You
then avoid getting a compiler warning about unused function parameters, as you
would if you used a name instead.

#### Ignoring Parts of a Value with a Nested `_`

We can also use `_` inside another pattern to ignore just part of a value, for
example, when we want to test for only part of a value but have no use for the
other parts in the corresponding code we want to run. Listing 18-18 shows code
responsible for managing a setting’s value. The business requirements are that
the user should not be allowed to overwrite an existing customization of a
setting but can unset the setting and give it a value if it is currently unset.

```
let mut setting_value = Some(5);
let new_setting_value = Some(10);

match (setting_value, new_setting_value) {
    (Some(_), Some(_)) => {
        println!("Can't overwrite an existing customized value");
    }
    _ => {
        setting_value = new_setting_value;
    }
}

println!("setting is {:?}", setting_value);
```

Listing 18-18: Using an underscore within patterns that match `Some` variants
when we don’t need to use the value inside the `Some`

This code will print `Can't overwrite an existing customized value` and then
`setting is Some(5)`. In the first match arm, we don’t need to match on or use
the values inside either `Some` variant, but we do need to test for the case
when `setting_value` and `new_setting_value` are the `Some` variant. In that
case, we print the reason for not changing `setting_value`, and it doesn’t get
changed.

In all other cases (if either `setting_value` or `new_setting_value` are
`None`) expressed by the `_` pattern in the second arm, we want to allow
`new_setting_value` to become `setting_value`.

We can also use underscores in multiple places within one pattern to ignore
particular values. Listing 18-19 shows an example of ignoring the second and
fourth values in a tuple of five items.

```
let numbers = (2, 4, 8, 16, 32);

match numbers {
    (first, _, third, _, fifth) => {
        println!("Some numbers: {first}, {third}, {fifth}")
    }
}
```

Listing 18-19: Ignoring multiple parts of a tuple

This code will print `Some numbers: 2, 8, 32`, and the values 4 and 16 will be
ignored.

#### Ignoring an Unused Variable by Starting Its Name with `_`

If you create a variable but don’t use it anywhere, Rust will usually issue a
warning because an unused variable could be a bug. However, sometimes it’s
useful to be able to create a variable you won’t use yet, such as when you’re
prototyping or just starting a project. In this situation, you can tell Rust
not to warn you about the unused variable by starting the name of the variable
with an underscore. In Listing 18-20, we create two unused variables, but when
we compile this code, we should only get a warning about one of them.

Filename: src/main.rs

```
fn main() {
    let _x = 5;
    let y = 10;
}
```

Listing 18-20: Starting a variable name with an
underscore to avoid getting unused variable warnings

Here we get a warning about not using the variable `y`, but we don’t get a
warning about not using `_x`.

Note that there is a subtle difference between using only `_` and using a name
that starts with an underscore. The syntax `_x` still binds the value to the
variable, whereas `_` doesn’t bind at all. To show a case where this
distinction matters, Listing 18-21 will provide us with an error.

```
let s = Some(String::from("Hello!"));

if let Some(_s) = s {
    println!("found a string");
}

println!("{:?}", s);
```

Listing 18-21: An unused variable starting with an underscore still binds the
value, which might take ownership of the value

We’ll receive an error because the `s` value will still be moved into `_s`,
which prevents us from using `s` again. However, using the underscore by itself
doesn’t ever bind to the value. Listing 18-22 will compile without any errors
because `s` doesn’t get moved into `_`.

```
let s = Some(String::from("Hello!"));

if let Some(_) = s {
    println!("found a string");
}

println!("{:?}", s);
```

Listing 18-22: Using an underscore does not bind the value

This code works just fine because we never bind `s` to anything; it isn’t moved.

#### Ignoring Remaining Parts of a Value with `..`

With values that have many parts, we can use the `..` syntax to use specific
parts and ignore the rest, avoiding the need to list underscores for each
ignored value. The `..` pattern ignores any parts of a value that we haven’t
explicitly matched in the rest of the pattern. In Listing 18-23, we have a
`Point` struct that holds a coordinate in three-dimensional space. In the
`match` expression, we want to operate only on the `x` coordinate and ignore
the values in the `y` and `z` fields.

```
struct Point {
    x: i32,
    y: i32,
    z: i32,
}

let origin = Point { x: 0, y: 0, z: 0 };

match origin {
    Point { x, .. } => println!("x is {}", x),
}
```

Listing 18-23: Ignoring all fields of a `Point` except for `x` by using `..`

We list the `x` value and then just include the `..` pattern. This is quicker
than having to list `y: _` and `z: _`, particularly when we’re working with
structs that have lots of fields in situations where only one or two fields are
relevant.

The syntax `..` will expand to as many values as it needs to be. Listing 18-24
shows how to use `..` with a tuple.

Filename: src/main.rs

```
fn main() {
    let numbers = (2, 4, 8, 16, 32);

    match numbers {
        (first, .., last) => {
            println!("Some numbers: {first}, {last}");
        }
    }
}
```

Listing 18-24: Matching only the first and last values in a tuple and ignoring
all other values

In this code, the first and last value are matched with `first` and `last`. The
`..` will match and ignore everything in the middle.

However, using `..` must be unambiguous. If it is unclear which values are
intended for matching and which should be ignored, Rust will give us an error.
Listing 18-25 shows an example of using `..` ambiguously, so it will not
compile.

Filename: src/main.rs

```
fn main() {
    let numbers = (2, 4, 8, 16, 32);

    match numbers {
        (.., second, ..) => {
            println!("Some numbers: {}", second)
        },
    }
}
```

Listing 18-25: An attempt to use `..` in an ambiguous way

When we compile this example, we get this error:

```
error: `..` can only be used once per tuple pattern
 --> src/main.rs:5:22
  |
5 |         (.., second, ..) => {
  |          --          ^^ can only be used once per tuple pattern
  |          |
  |          previously used here
```

It’s impossible for Rust to determine how many values in the tuple to ignore
before matching a value with `second` and then how many further values to
ignore thereafter. This code could mean that we want to ignore `2`, bind
`second` to `4`, and then ignore `8`, `16`, and `32`; or that we want to ignore
`2` and `4`, bind `second` to `8`, and then ignore `16` and `32`; and so forth.
The variable name `second` doesn’t mean anything special to Rust, so we get a
compiler error because using `..` in two places like this is ambiguous.

### Extra Conditionals with Match Guards

A *match guard* is an additional `if` condition, specified after the pattern in
a `match` arm, that must also match for that arm to be chosen. Match guards are
useful for expressing more complex ideas than a pattern alone allows.

The condition can use variables created in the pattern. Listing 18-26 shows a
`match` where the first arm has the pattern `Some(x)` and also has a match
guard of `if x % 2 == 0` (which will be true if the number is even).

```
let num = Some(4);

match num {
    Some(x) if x % 2 == 0 => println!("The number {} is even", x),
    Some(x) => println!("The number {} is odd", x),
    None => (),
}
```

Listing 18-26: Adding a match guard to a pattern

This example will print `The number 4 is even`. When `num` is compared to the
pattern in the first arm, it matches, because `Some(4)` matches `Some(x)`. Then
the match guard checks whether the remainder of dividing `x` by 2 is equal to
0, and because it is, the first arm is selected.

If `num` had been `Some(5)` instead, the match guard in the first arm would
have been false because the remainder of 5 divided by 2 is 1, which is not
equal to 0. Rust would then go to the second arm, which would match because the
second arm doesn’t have a match guard and therefore matches any `Some` variant.

There is no way to express the `if x % 2 == 0` condition within a pattern, so
the match guard gives us the ability to express this logic. The downside of
this additional expressiveness is that the compiler doesn't try to check for
exhaustiveness when match guard expressions are involved.

In Listing 18-11, we mentioned that we could use match guards to solve our
pattern-shadowing problem. Recall that we created a new variable inside the
pattern in the `match` expression instead of using the variable outside the
`match`. That new variable meant we couldn’t test against the value of the
outer variable. Listing 18-27 shows how we can use a match guard to fix this
problem.

Filename: src/main.rs

```
fn main() {
    let x = Some(5);
    let y = 10;

    match x {
        Some(50) => println!("Got 50"),
        Some(n) if n == y => println!("Matched, n = {n}"),
        _ => println!("Default case, x = {:?}", x),
    }

    println!("at the end: x = {:?}, y = {y}", x);
}
```

Listing 18-27: Using a match guard to test for equality with an outer variable

This code will now print `Default case, x = Some(5)`. The pattern in the second
match arm doesn’t introduce a new variable `y` that would shadow the outer `y`,
meaning we can use the outer `y` in the match guard. Instead of specifying the
pattern as `Some(y)`, which would have shadowed the outer `y`, we specify
`Some(n)`. This creates a new variable `n` that doesn’t shadow anything because
there is no `n` variable outside the `match`.

The match guard `if n == y` is not a pattern and therefore doesn’t introduce
new variables. This `y` *is* the outer `y` rather than a new shadowed `y`, and
we can look for a value that has the same value as the outer `y` by comparing
`n` to `y`.

You can also use the *or* operator `|` in a match guard to specify multiple
patterns; the match guard condition will apply to all the patterns. Listing
18-28 shows the precedence when combining a pattern that uses `|` with a match
guard. The important part of this example is that the `if y` match guard
applies to `4`, `5`, *and* `6`, even though it might look like `if y` only
applies to `6`.

```
let x = 4;
let y = false;

match x {
    4 | 5 | 6 if y => println!("yes"),
    _ => println!("no"),
}
```

Listing 18-28: Combining multiple patterns with a match guard

The match condition states that the arm only matches if the value of `x` is
equal to `4`, `5`, or `6` *and* if `y` is `true`. When this code runs, the
pattern of the first arm matches because `x` is `4`, but the match guard `if y`
is false, so the first arm is not chosen. The code moves on to the second arm,
which does match, and this program prints `no`. The reason is that the `if`
condition applies to the whole pattern `4 | 5 | 6`, not only to the last value
`6`. In other words, the precedence of a match guard in relation to a pattern
behaves like this:

```
(4 | 5 | 6) if y => ...
```

rather than this:

```
4 | 5 | (6 if y) => ...
```

After running the code, the precedence behavior is evident: if the match guard
were applied only to the final value in the list of values specified using the
`|` operator, the arm would have matched and the program would have printed
`yes`.

### `@` Bindings

The *at* operator `@` lets us create a variable that holds a value at the same
time as we’re testing that value for a pattern match. In Listing 18-29, we want
to test that a `Message::Hello` `id` field is within the range `3..=7`. We also
want to bind the value to the variable `id_variable` so we can use it in the
code associated with the arm. We could name this variable `id`, the same as the
field, but for this example we’ll use a different name.

```
enum Message {
    Hello { id: i32 },
}

let msg = Message::Hello { id: 5 };

match msg {
    Message::Hello {
        id: id_variable @ 3..=7,
    } => println!("Found an id in range: {}", id_variable),
    Message::Hello { id: 10..=12 } => {
        println!("Found an id in another range")
    }
    Message::Hello { id } => println!("Found some other id: {}", id),
}
```

Listing 18-29: Using `@` to bind to a value in a pattern while also testing it

This example will print `Found an id in range: 5`. By specifying `id_variable
@` before the range `3..=7`, we’re capturing whatever value matched the range
while also testing that the value matched the range pattern.

In the second arm, where we only have a range specified in the pattern, the code
associated with the arm doesn’t have a variable that contains the actual value
of the `id` field. The `id` field’s value could have been 10, 11, or 12, but
the code that goes with that pattern doesn’t know which it is. The pattern code
isn’t able to use the value from the `id` field, because we haven’t saved the
`id` value in a variable.

In the last arm, where we’ve specified a variable without a range, we do have
the value available to use in the arm’s code in a variable named `id`. The
reason is that we’ve used the struct field shorthand syntax. But we haven’t
applied any test to the value in the `id` field in this arm, as we did with the
first two arms: any value would match this pattern.

Using `@` lets us test a value and save it in a variable within one pattern.

## Summary

Rust’s patterns are very useful in distinguishing between different kinds of
data. When used in `match` expressions, Rust ensures your patterns cover every
possible value, or your program won’t compile. Patterns in `let` statements and
function parameters make those constructs more useful, enabling the
destructuring of values into smaller parts at the same time as assigning to
variables. We can create simple or complex patterns to suit our needs.

Next, for the penultimate chapter of the book, we’ll look at some advanced
aspects of a variety of Rust’s features.