patterns.md

% 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:

let x = 1;

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

This prints one.

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

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 |:

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:

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.

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:

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:

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>:

# 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:

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:

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:

// 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:

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:

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:

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 ...:

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 chars:

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 @:

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:

#[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:

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:

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:

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.