All the Pattern Syntax - All the Pattern Syntax - The Rust Programming Language

## All the Pattern Syntax

We’ve seen examples of many different kinds of patterns throughout the book, so we’ll gather all the syntax valid in patterns in one place here, and why you might want to use each of them.

### Matching Literals

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

    # #![allow(unused_variables)]
    #fn main() {
    let x = 1;

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

This prints `one` since the value in `x` is 1. This is useful when you want to take some action if you get a concrete value in particular.

### Matching Named Variables

Named variables are irrefutable patterns that match any value, which we have used many times before. There is a complication, however, when used 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`. Take a look at the patterns in the match arms and `println!` at the end, and try to figure out what will be printed before running this code or reading further:

Filename: src/main.rs

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

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

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

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

Let’s walk through what happens when the `match` statement runs. The pattern in the first match arm does not match the defined value of `x`, so we continue.

The pattern in the second match arm introduces a new variable name `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 variable, and not the `y` we declared at the beginning with the value 10. This new `y` binding will match any value inside a `Some`, which is what we have in `x`. Therefore this `y` binds to the inner value of the `Some` in `x`. That value is 5, and 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 would not have matched, so we would have matched to the underscore. We did not introduce the `x` variable in the pattern of that arm, so the `x` in the expression is still the outer `x` that has not been shadowed. In this hypothetical case, the `match` would print `Default case, x = None`.

Once the `match` expression is over, its scope ends, and so does the scope of the inner `y`. The last `println!` 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 be talking about match guards later in this section.

### Multiple Patterns

In `match` expressions you can match multiple patterns using the `|` syntax, which means or. For example, the following code matches 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, it will run:

    # #![allow(unused_variables)]
    #fn main() {
    let x = 1;

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

This code will print `one or two`.

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

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

    # #![allow(unused_variables)]
    #fn main() {
    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 is more convenient than using the `|` operator to express the same idea; instead of `1 ... 5` we would have to specify `1 | 2 | 3 | 4 | 5` using `|`. Specifying a range instead is much shorter, especially if we wanted to match, say, any number between 1 and 1,000!

Ranges are only allowed with numeric values or `char` values, because the compiler checks that the range isn’t empty at compile time. `char` and numeric values are the only types that Rust knows how to tell if a range is empty or not.

Here’s an example using ranges of `char` values:

    # #![allow(unused_variables)]
    #fn main() {
    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 this will print `early ASCII letter`.

### Destructuring to Break Apart Values

We can also use patterns to destructure structs, enums, tuples, and references in order to use different parts of these values. Let’s go through each of those!

#### 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` variable.

This example shows that the names of the variable names in the pattern don’t have to match the field names of the struct, but it’s common to want the variable names to match the field names to make it easier to remember which variables came from which fields. Because having variable names match the fields is common, and because writing `let Point { x: x, y: y } = p;` contains a lot of duplication, there’s 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 shows code that 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` 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 of the fields. This allows us to test some of the fields for particular values while creating variables to destructure the other fields.

Listing 18-14 shows a `match` statement 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

    # struct Point {
    #     x: i32,
    #     y: i32,
    # }
    #
    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 will print `On the y axis at 7`.

#### Destructuring Enums

We’ve destructured enums before in this book, like in Listing 6-5 in Chapter 6 when we destructured an `Option<i32>`. One detail we haven’t mentioned explicitly is that the pattern to destructure an enum should correspond to the way the data stored within the enum is defined. For example, let’s take the `Message` enum from Listing 6-2 and write a `match` with patterns that will destructure each inner value in Listing 18-15:

Filename: src/main.rs

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

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

        match msg {
            Message::Quit => {
                println!("The Quit variant has no data to destructure.")
            },
            Message::Move { x: x, y: y } => {
                println!(
                    "Move in the x direction {} and in the y direction {}",
                    x,
                    y
                );
            }
            Message::Write(text) => println!("Text message: {}", text),
            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` to see the code from the other arms run.

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

For struct-like enum variants such as `Message::Move`, 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 that we break apart the pieces to use in the code for this arm.

For tuple-like enum variants like `Message::Write`, that holds a tuple with one element, and `Message::ChangeColor` that holds a tuple with three elements, 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 References

When the value we’re matching to our pattern contains a reference, we need to destructure the reference from the value, which we can do can by specifying a `&` in the pattern. This lets us get a variable holding the value that the reference points to rather than getting a variable that holds the reference.

This is especially useful in closures where we have iterators that iterate over references, but we want to use the values in the closure rather than the references.

The example in Listing 18-16 iterates over references to `Point` instances in a vector, and destructures both the reference and the struct so we can perform calculations on the `x` and `y` values easily:

    # #![allow(unused_variables)]
    #fn main() {
    # struct Point {
    #     x: i32,
    #     y: i32,
    # }
    #
    let points = vec![
        Point { x: 0, y: 0 },
        Point { x: 1, y: 5 },
        Point { x: 10, y: -3 },
    ];

    let sum_of_squares: i32 = points
        .iter()
        .map(|&Point { x, y }| x * x + y * y)
        .sum();
    #}

Listing 18-16: Destructuring a reference to a struct into the struct field values

This code results in the value 135 in the variable `sum_of_squares`, which is the result from squaring the `x` value and the `y` value, adding those together, and then adding the result for each `Point` in the `points` vector to get one number.

If we had not included the `&` in `&Point { x, y }` we’d get a type mismatch error, because `iter` would then iterate over references to the items in the vector rather than the values themselves. The error would look like this:

    error[E0308]: mismatched types
      -->
       |
    14 |         .map(|Point { x, y }| x * x + y * y)
       |               ^^^^^^^^^^^^ expected &Point, found struct `Point`
       |
       = note: expected type `&Point`
                  found type `Point`

This tells us that Rust was expecting our closure to match `&Point`, but we tried to match directly to a `Point` value, and not a reference to a `Point`.

#### Destructuring Structs and Tuples

We can mix, match, and nest destructuring patterns in even more complex way. Here’s an example of a complicated destructure, where we nest structs and tuples inside a tuple, and destructure all the primitive values out:

    # #![allow(unused_variables)]
    #fn main() {
    # struct Point {
    #     x: i32,
    #     y: i32,
    # }
    #
    let ((feet, inches), Point {x, y}) = ((3, 10), Point { x: 3, y: -10 });
    #}

This lets us break complex types into their component parts so that 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

We’ve seen that it’s sometimes useful to ignore values in a pattern, such as in the last arm of a `match` to give us a catch-all 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 we’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 do each of these.

#### 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. While the underscore pattern is especially useful as the last arm in a `match` expression, we can 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 passed as the first argument, 3, and will print out `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 some cases, such as when implementing a trait, when you need a certain type signature but the function body in your implementation doesn’t need one of the parameters. The compiler will then not warn about unused function parameters, as it would if we used a name instead.

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

We can also use `_` inside of another pattern to ignore just part of a value, when we only want to test for 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 giving a setting a 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 can give the setting a value if it is currently unset.

    # #![allow(unused_variables)]
    #fn main() {
    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; the important part we need to test for is the case when both `setting_value` and `new_setting_value` are the `Some` variant. In that case, we want to print out why we’re not changing `setting_value`, and we don’t change it.

In all other cases (if either `setting_value` or `new_setting_value` are `None`), which is expressed by the `_` pattern in the second arm, we do 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, as shown in Listing 18-19 where we’re ignoring the second and fourth values in a tuple of five items:

    # #![allow(unused_variables)]
    #fn main() {
    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 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 an Underscore

If you create a variable but don’t use it anywhere, Rust will usually issue a warning, since that could be a bug. Sometimes, though, it’s useful to create a variable you won’t use yet, like if you’re prototyping or just starting a project. In this situation you’ll want to tell Rust not to warn you about the unused variable, which you can do by starting the name of the variable with an underscore. In Listing 18-20 we create two unused variables, but when we run 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 in order to not get unused variable warnings

Here we get a warning about not using the variable `y`, but not about not using the variable preceded by the underscore.

Note that there is a subtle difference between using only `_` and using a name that starts with an underscore. Something like `_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 may 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. Using the underscore by itself, however, doesn’t ever bind to the value. Listing 18-22 will compile without any errors since `s` does not get moved into `_`:

    # #![allow(unused_variables)]
    #fn main() {
    let s = Some(String::from("Hello!"));

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

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

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

This 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 only a few parts and ignore the rest, while avoiding having to list underscores for each ignored value. The `..` pattern will ignore 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:

    # #![allow(unused_variables)]
    #fn main() {
    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 out `y: _` and `z: _`, particularly when working with structs that have lots of fields, in situations where only one or two fields are relevant.

`..` will expand to as many values as it needs to be. Listing 18-24 shows a use of `..` 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

Here, we have the first and last value matched with `first` and `last`. The `..` will match and ignore everything in the middle.

Using `..` must be unambiguous, however. If it is not clear which values are intended for matching, and which to be ignored, Rust will error. Listing 18-25 shows an example of using `..` ambiguously that will not compile due to this ambiguity:

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 a way that is ambiguous

If we compile this example, we get this error:

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

It’s not possible 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 after that. This code could mean that we intend to ignore 2, bind `second` to 4, then ignore 8, 16, and 32; or we could mean that we want to ignore 2 and 4, bind `second` to 8, 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 since using `..` in two places like this is ambiguous.

### `ref` and `ref mut` to Create References in Patterns

Here we’ll look at using `ref` to make references so ownership of the values isn’t moved to variables in the pattern. Usually, when you match against a pattern, the variables introduced by the pattern are bound to a value. Rust’s ownership rules mean the value will be moved into the `match`, or wherever you’re using the pattern. Listing 18-26 shows an example of a `match` that has a pattern with a variable, and then another usage of the entire value after the `match`. This will fail to compile because ownership of part of the `robot_name` value is transferred to the `name` variable in the pattern of the first `match` arm:

    let robot_name = Some(String::from("Bors"));

    match robot_name {
        Some(name) => println!("Found a name: {}", name),
        None => (),
    }

    println!("robot_name is: {:?}", robot_name);

Listing 18-26: Creating a variable in a match arm pattern takes ownership of the value

This example will fail because the value inside `Some` in `robot_name` is moved to within the `match` when `name` binds to that value. Because ownership of part of `robot_name` has been moved to `name`, we can no longer use `robot_name` in the `println!` after the `match` because `robot_name` no longer has ownership.

In order to fix this code, we want to have the `Some(name)` pattern borrow that part of `robot_name` rather than taking ownership. Outside of patterns, we’ve seen that the way to borrow a value is to create a reference using `&`, so you may think the solution is changing `Some(name)` to `Some(&name)`.

However, we saw in the “Destructuring to Break Apart Values” section that `&` in patterns does not create a reference, it matches an existing reference in the value. Because `&` already has that meaning in patterns, we can’t use `&` to create a reference in a pattern.

Instead, to create a reference in a pattern, we do this by using the `ref` keyword before the new variable, as shown in Listing 18-27:

    # #![allow(unused_variables)]
    #fn main() {
    let robot_name = Some(String::from("Bors"));

    match robot_name {
        Some(ref name) => println!("Found a name: {}", name),
        None => (),
    }

    println!("robot_name is: {:?}", robot_name);
    #}

Listing 18-27: Creating a reference so that a pattern variable does not take ownership of a value

This example will compile because the value in the `Some` variant in `robot_name` is not moved into the `match`; the `match` only took a reference to the data in `robot_name` rather than moving it.

To create a mutable reference in order to be able to mutate a value matched in a pattern, use `ref mut` instead of `&mut` for the same reason that we use `ref` instead of `&`: `&mut` in patterns is for matching existing mutable references, not creating new ones. Listing 18-28 shows an example of a pattern creating a mutable reference:

    # #![allow(unused_variables)]
    #fn main() {
    let mut robot_name = Some(String::from("Bors"));

    match robot_name {
        Some(ref mut name) => *name = String::from("Another name"),
        None => (),
    }

    println!("robot_name is: {:?}", robot_name);
    #}

Listing 18-28: Creating a mutable reference to a value as part of a pattern using `ref mut`

This example will compile and print `robot_name is: Some("Another name")`. Because `name` is a mutable reference, we need to dereference within the match arm code using the `*` operator in order to be able to mutate the value.

### Extra Conditionals with Match Guards

A match guard is an additional `if` condition specified after the pattern in a `match` arm that also must match if the pattern matches in order 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 18-29 shows a `match` where the first arm has the pattern `Some(x)` and then also has a match guard of `if x < 5`:

    # #![allow(unused_variables)]
    #fn main() {
    let num = Some(4);

    match num {
        Some(x) if x < 5 => println!("less than five: {}", x),
        Some(x) => println!("{}", x),
        None => (),
    }
    #}

Listing 18-29: Adding a match guard to a pattern

This example will print `less than five: 4`. When `num` is compared to the pattern in the first arm, it matches since `Some(4)` matches `Some(x)`. Then the match guard checks to see if the value in `x` is less than 5, and because 4 is less than 5, the first arm is selected.

If `num` had been `Some(10)` instead, the match guard in the first arm would have been false since 10 is not less than 5. Rust would then go to the second arm, which would match because the second arm does not have a match guard and therefore matches any `Some` variant.

There’s no way to express the `if x < 5` condition within a pattern, so the match guard has given us the ability to express this logic.

In Listing 18-11, we mentioned that we could use match guards to solve our pattern shadowing problem, where a new variable was created 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 that the outer variable had. Listing 18-30 shows how we can use a match guard to fix this:

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 = {:?}", x, y);
    }

Listing 18-30: Using a match guard to test for equality with an outer variable

This will now print `Default case, x = Some(5)`. The pattern in the second match arm is now not introducing 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 does not shadow anything because there is no `n` variable outside the `match`.

In the match guard, `if n == y`, this is not a pattern and therefore does not introduce new variables. This `y` is the outer `y` rather than a new shadowed `y`, and we can express the idea that we’re looking 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, and the match guard condition will apply to all of the patterns. Listing 18-31 shows the precedence of combining a match guard with a pattern that uses `|`. 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:

    # #![allow(unused_variables)]
    #fn main() {
    let x = 4;
    let y = false;

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

Listing 18-31: 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`. What happens when this code runs is that 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`.

This is because the `if` condition applies to the whole pattern `4 | 5 | 6`, and 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) => ...

We can tell this from what happened when we ran the code: if the match guard was only applied 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 we’re testing that value to see if it matches a pattern. Listing 18-32 shows an example where we want to test that a `Message::Hello` `id` field is within the range `3...7` but also be able to bind the value to the variable `id_variable` so that we can use it in the code associated with the arm. We could have named `id_variable` `id`, the same as the field, but for the purposes of this example we’ve chosen to give it a different name:

    # #![allow(unused_variables)]
    #fn main() {
    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 18-32: 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 one and 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` because we’ve used the struct field shorthand syntax. We haven’t applied any test to the value in the `id` field in this arm, though, like the first two arms did: any value would match this pattern.

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

## Summary

Patterns are a useful feature of Rust that help distinguish between different kinds of data. When used in `match` statements, Rust makes sure your patterns cover every possible value, or your program will not compile. Patterns in `let` statements and function parameters make those constructs more powerful, 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.

Now, for the penultimate chapter of the book, let’s take a look at some advanced parts of a variety of Rust’s features.