Imported Upstream version 1.5.0+dfsg1

[rustc.git] / src / doc / trpl / match.md

% Match

Often, a simple [`if`][if]/`else` isn’t enough, because you have more than two
possible options. Also, conditions can get quite complex. Rust
has a keyword, `match`, that allows you to replace complicated `if`/`else`
groupings with something more powerful. Check it out:

```rust
let x = 5;

match x {
    1 => println!("one"),
    2 => println!("two"),
    3 => println!("three"),
    4 => println!("four"),
    5 => println!("five"),
    _ => println!("something else"),
}
```

[if]: if.html

`match` takes an expression and then branches based on its value. Each ‘arm’ of
the branch is of the form `val => expression`. When the value matches, that arm’s
expression will be evaluated. It’s called `match` because of the term ‘pattern
matching’, which `match` is an implementation of. There’s an [entire section on
patterns][patterns] that covers all the patterns that are possible here.

[patterns]: patterns.html

So what’s the big advantage? Well, there are a few. First of all, `match`
enforces ‘exhaustiveness checking’. Do you see that last arm, the one with the
underscore (`_`)? If we remove that arm, Rust will give us an error:

```text
error: non-exhaustive patterns: `_` not covered
```

In other words, Rust is trying to tell us we forgot a value. Because `x` is an
integer, Rust knows that it can have a number of different values – for
example, `6`. Without the `_`, however, there is no arm that could match, and
so Rust refuses to compile the code. `_` acts like a ‘catch-all arm’. If none
of the other arms match, the arm with `_` will, and since we have this
catch-all arm, we now have an arm for every possible value of `x`, and so our
program will compile successfully.

`match` is also an expression, which means we can use it on the right-hand
side of a `let` binding or directly where an expression is used:

```rust
let x = 5;

let number = match x {
    1 => "one",
    2 => "two",
    3 => "three",
    4 => "four",
    5 => "five",
    _ => "something else",
};
```

Sometimes it’s a nice way of converting something from one type to another.

# Matching on enums

Another important use of the `match` keyword is to process the possible
variants of an enum:

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

fn quit() { /* ... */ }
fn change_color(r: i32, g: i32, b: i32) { /* ... */ }
fn move_cursor(x: i32, y: i32) { /* ... */ }

fn process_message(msg: Message) {
    match msg {
        Message::Quit => quit(),
        Message::ChangeColor(r, g, b) => change_color(r, g, b),
        Message::Move { x: x, y: y } => move_cursor(x, y),
        Message::Write(s) => println!("{}", s),
    };
}
```

Again, the Rust compiler checks exhaustiveness, so it demands that you
have a match arm for every variant of the enum. If you leave one off, it
will give you a compile-time error unless you use `_`.

Unlike the previous uses of `match`, you can’t use the normal `if`
statement to do this. You can use the [`if let`][if-let] statement,
which can be seen as an abbreviated form of `match`.

[if-let]: if-let.html