Pattern Syntax

Throughout the book, you’ve seen examples of many kinds of patterns. In this section, we gather all the syntax valid in patterns and discuss why 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:

fn main() {
    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() {
    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 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 doesn’t match the defined value of x, so the code continues.

The pattern in the second match arm 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. 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. 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! 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 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, that arm’s code will run:

fn main() {
    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 range, that arm will execute:

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 syntax 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 if we used |. Specifying a range is much shorter, especially if we want 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. The only types for which Rust can tell if a range is empty or not are char and numeric values.

Here is an example using ranges of char values:

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 prints early ASCII letter.

Destructuring to Break Apart Values

We can also use patterns to destructure structs, enums, tuples, and references 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. 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 is 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 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.

Listing 18-14 shows 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

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 code will print On the y axis at 7.

Destructuring Enums

We’ve destructured enums earlier in this book, for example, when we destructured Option<i32> in Listing 6-5 in Chapter 6. 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. 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() {
    let msg = Message::ChangeColor(0, 160, 255);
    match msg {
        Message::Quit => {
            println!("The Quit variant has no data to destructure.")
        }
        Message::Move { x, 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 no variables are 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 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 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 Nested Structs and Enums

Until now, all our examples have been matching structs or enums that were one level deep. 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 the Color::Hsv variant 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:

fn main() {
    struct Point {
        x: i32,
        y: i32,
    }
    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. Although 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 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, for example, 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 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.

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, 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 why we’re 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.

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 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 that could be a bug. But sometimes it’s useful 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 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. 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.

fn main() { 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 _.

fn main() {
    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 only a few 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.

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 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:

$ cargo run Compiling patterns v0.1.0 (file:///projects/patterns) 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 error: aborting due to previous error error: could not compile `patterns` To learn more, run the command again with --verbose.

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, along with the pattern matching, 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 < 5.

fn main() {
    let num = Some(4);
    match num {
        Some(x) if x < 5 => println!("less than five: {}", x),
        Some(x) => println!("{}", x),
        None => (),
    }
}

Listing 18-26: 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, because Some(4) matches Some(x). Then the match guard checks whether the value in x is less than 5, and because it is, the first arm is selected.

If num had been Some(10) instead, the match guard in the first arm would have been false because 10 is not less than 5. 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 < 5 condition within a pattern, so the match guard gives 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. Recall that 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 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 = {}", x, y);
}

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 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.

fn main() {
    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 we’re testing that value to see whether it matches a pattern. Listing 18-29 shows an example where we want to test that a Message::Hello id field is within the range 3..=7. But 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.

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 18-29: 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 that they help distinguish 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.