3 Often, a simple [`if`][if]/`else` isn’t enough, because you have more than two
4 possible options. Also, conditions can get quite complex. Rust
5 has a keyword, `match`, that allows you to replace complicated `if`/`else`
6 groupings with something more powerful. Check it out:
14 3 => println!("three"),
15 4 => println!("four"),
16 5 => println!("five"),
17 _ => println!("something else"),
23 `match` takes an expression and then branches based on its value. Each ‘arm’ of
24 the branch is of the form `val => expression`. When the value matches, that arm’s
25 expression will be evaluated. It’s called `match` because of the term ‘pattern
26 matching’, which `match` is an implementation of. There’s an [entire section on
27 patterns][patterns] that covers all the patterns that are possible here.
29 [patterns]: patterns.html
31 So what’s the big advantage? Well, there are a few. First of all, `match`
32 enforces ‘exhaustiveness checking’. Do you see that last arm, the one with the
33 underscore (`_`)? If we remove that arm, Rust will give us an error:
36 error: non-exhaustive patterns: `_` not covered
39 In other words, Rust is trying to tell us we forgot a value. Because `x` is an
40 integer, Rust knows that it can have a number of different values – for
41 example, `6`. Without the `_`, however, there is no arm that could match, and
42 so Rust refuses to compile the code. `_` acts like a ‘catch-all arm’. If none
43 of the other arms match, the arm with `_` will, and since we have this
44 catch-all arm, we now have an arm for every possible value of `x`, and so our
45 program will compile successfully.
47 `match` is also an expression, which means we can use it on the right-hand
48 side of a `let` binding or directly where an expression is used:
53 let number = match x {
59 _ => "something else",
63 Sometimes it’s a nice way of converting something from one type to another.
67 Another important use of the `match` keyword is to process the possible
73 ChangeColor(i32, i32, i32),
74 Move { x: i32, y: i32 },
78 fn quit() { /* ... */ }
79 fn change_color(r: i32, g: i32, b: i32) { /* ... */ }
80 fn move_cursor(x: i32, y: i32) { /* ... */ }
82 fn process_message(msg: Message) {
84 Message::Quit => quit(),
85 Message::ChangeColor(r, g, b) => change_color(r, g, b),
86 Message::Move { x: x, y: y } => move_cursor(x, y),
87 Message::Write(s) => println!("{}", s),
92 Again, the Rust compiler checks exhaustiveness, so it demands that you
93 have a match arm for every variant of the enum. If you leave one off, it
94 will give you a compile-time error unless you use `_`.
96 Unlike the previous uses of `match`, you can’t use the normal `if`
97 statement to do this. You can use the [`if let`][if-let] statement,
98 which can be seen as an abbreviated form of `match`.
100 [if-let]: if-let.html