# Patterns

Patterns are quite common in Rust. We use them in variable bindings, match expressions, and other places, too. Let’s go on a whirlwind tour of all of the things patterns can do!

A quick refresher: you can match against literals directly, and `_` acts as an ‘any’ case:

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

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

    #}

This prints `one`.

It's possible to create a binding for the value in the any case:

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

    match x {
        y => println!("x: {} y: {}", x, y),
    }

    #}

This prints:

    x: 1 y: 1

Note it is an error to have both a catch-all `_` and a catch-all binding in the same match block:

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

    match x {
        y => println!("x: {} y: {}", x, y),
        _ => println!("anything"), // this causes an error as it is unreachable
    }

    #}

There’s one pitfall with patterns: like anything that introduces a new binding, they introduce shadowing. For example:

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

    match c {
        x => println!("x: {} c: {}", x, c),
    }

    println!("x: {}", x)

    #}

This prints:

    x: c c: c
    x: 1

In other words, `x =>` matches the pattern and introduces a new binding named `x`. This new binding is in scope for the match arm and takes on the value of `c`. Notice that the value of `x` outside the scope of the match has no bearing on the value of `x` within it. Because we already have a binding named `x`, this new `x` shadows it.

# Multiple patterns

You can match multiple patterns with `|`:

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

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

    #}

This prints `one or two`.

# Destructuring

If you have a compound data type, like a `struct`, you can destructure it inside of a pattern:

    # #![allow(unused_variables)]
    #fn main() {
    struct Point {
        x: i32,
        y: i32,
    }

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

    match origin {
        Point { x, y } => println!("({},{})", x, y),
    }

    #}

We can use `:` to give a value a different name.

    # #![allow(unused_variables)]
    #fn main() {
    struct Point {
        x: i32,
        y: i32,
    }

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

    match origin {
        Point { x: x1, y: y1 } => println!("({},{})", x1, y1),
    }

    #}

If we only care about some of the values, we don’t have to give them all names:

    # #![allow(unused_variables)]
    #fn main() {
    struct Point {
        x: i32,
        y: i32,
    }

    let point = Point { x: 2, y: 3 };

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

    #}

This prints `x is 2`.

You can do this kind of match on any member, not only the first:

    # #![allow(unused_variables)]
    #fn main() {
    struct Point {
        x: i32,
        y: i32,
    }

    let point = Point { x: 2, y: 3 };

    match point {
        Point { y, .. } => println!("y is {}", y),
    }

    #}

This prints `y is 3`.

This ‘destructuring’ behavior works on any compound data type, like tuples or enums.

# Ignoring bindings

You can use `_` in a pattern to disregard the type and value. For example, here’s a `match` against a `Result<T, E>`:

    # #![allow(unused_variables)]
    #fn main() {
    # let some_value: Result<i32, &'static str> = Err("There was an error");
    match some_value {
        Ok(value) => println!("got a value: {}", value),
        Err(_) => println!("an error occurred"),
    }

    #}

In the first arm, we bind the value inside the `Ok` variant to `value`. But in the `Err` arm, we use `_` to disregard the specific error, and print a general error message.

`_` is valid in any pattern that creates a binding. This can be useful to ignore parts of a larger structure:

    # #![allow(unused_variables)]
    #fn main() {
    fn coordinate() -> (i32, i32, i32) {
        // Generate and return some sort of triple tuple.
    # (1, 2, 3)
    }

    let (x, _, z) = coordinate();

    #}

Here, we bind the first and last element of the tuple to `x` and `z`, but ignore the middle element.

It’s worth noting that using `_` never binds the value in the first place, which means that the value does not move:

    # #![allow(unused_variables)]
    #fn main() {
    let tuple: (u32, String) = (5, String::from("five"));

    // Here, tuple is moved, because the String moved:
    let (x, _s) = tuple;

    // The next line would give "error: use of partially moved value: `tuple`".
    // println!("Tuple is: {:?}", tuple);

    // However,

    let tuple = (5, String::from("five"));

    // Here, tuple is _not_ moved, as the String was never moved, and u32 is Copy:
    let (x, _) = tuple;

    // That means this works:
    println!("Tuple is: {:?}", tuple);

    #}

This also means that any temporary variables will be dropped at the end of the statement:

    # #![allow(unused_variables)]
    #fn main() {
    // Here, the String created will be dropped immediately, as it’s not bound:

    let _ = String::from("  hello  ").trim();

    #}

You can also use `..` in a pattern to disregard multiple values:

    # #![allow(unused_variables)]
    #fn main() {
    enum OptionalTuple {
        Value(i32, i32, i32),
        Missing,
    }

    let x = OptionalTuple::Value(5, -2, 3);

    match x {
        OptionalTuple::Value(..) => println!("Got a tuple!"),
        OptionalTuple::Missing => println!("No such luck."),
    }

    #}

This prints `Got a tuple!`.

# ref and ref mut

If you want to get a reference, use the `ref` keyword:

    # #![allow(unused_variables)]
    #fn main() {
    let x = 5;

    match x {
        ref r => println!("Got a reference to {}", r),
    }

    #}

This prints `Got a reference to 5`.

Here, the `r` inside the `match` has the type `&i32`. In other words, the `ref` keyword creates a reference, for use in the pattern. If you need a mutable reference, `ref mut` will work in the same way:

    # #![allow(unused_variables)]
    #fn main() {
    let mut x = 5;

    match x {
        ref mut mr => println!("Got a mutable reference to {}", mr),
    }

    #}

# Ranges

You can match a range of values with `...`:

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

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

    #}

This prints `one through five`.

Ranges are mostly used with integers and `char`s:

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

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

    #}

This prints `something else`.

# Bindings

You can bind values to names with `@`:

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

    match x {
        e @ 1 ... 5 => println!("got a range element {}", e),
        _ => println!("anything"),
    }

    #}

This prints `got a range element 1`. This is useful when you want to do a complicated match of part of a data structure:

    # #![allow(unused_variables)]
    #fn main() {
    #[derive(Debug)]
    struct Person {
        name: Option<String>,
    }

    let name = "Steve".to_string();
    let x: Option<Person> = Some(Person { name: Some(name) });
    match x {
        Some(Person { name: ref a @ Some(_), .. }) => println!("{:?}", a),
        _ => {}
    }

    #}

This prints `Some("Steve")`: we’ve bound the inner `name` to `a`.

If you use `@` with `|`, you need to make sure the name is bound in each part of the pattern:

    # #![allow(unused_variables)]
    #fn main() {
    let x = 5;

    match x {
        e @ 1 ... 5 | e @ 8 ... 10 => println!("got a range element {}", e),
        _ => println!("anything"),
    }

    #}

# Guards

You can introduce ‘match guards’ with `if`:

    # #![allow(unused_variables)]
    #fn main() {
    enum OptionalInt {
        Value(i32),
        Missing,
    }

    let x = OptionalInt::Value(5);

    match x {
        OptionalInt::Value(i) if i > 5 => println!("Got an int bigger than five!"),
        OptionalInt::Value(..) => println!("Got an int!"),
        OptionalInt::Missing => println!("No such luck."),
    }

    #}

This prints `Got an int!`.

If you’re using `if` with multiple patterns, the `if` applies to both sides:

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

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

    #}

This prints `no`, because the `if` applies to the whole of `4 | 5`, and not to only the `5`. In other words, the precedence of `if` behaves like this:

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

not this:

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

# Mix and Match

Whew! That’s a lot of different ways to match things, and they can all be mixed and matched, depending on what you’re doing:

    match x {
        Foo { x: Some(ref name), y: None } => ...
    }

Patterns are very powerful. Make good use of them.