101 lines
3.1 KiB
Markdown
101 lines
3.1 KiB
Markdown
% Match
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Often, a simple [`if`][if]/`else` isn’t enough, because you have more than two
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possible options. Also, conditions can get quite complex. Rust
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has a keyword, `match`, that allows you to replace complicated `if`/`else`
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groupings with something more powerful. Check it out:
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```rust
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let x = 5;
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match x {
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1 => println!("one"),
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2 => println!("two"),
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3 => println!("three"),
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4 => println!("four"),
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5 => println!("five"),
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_ => println!("something else"),
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}
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```
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[if]: if.html
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`match` takes an expression and then branches based on its value. Each ‘arm’ of
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the branch is of the form `val => expression`. When the value matches, that arm’s
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expression will be evaluated. It’s called `match` because of the term ‘pattern
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matching’, which `match` is an implementation of. There’s an [entire section on
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patterns][patterns] that covers all the patterns that are possible here.
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[patterns]: patterns.html
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So what’s the big advantage? Well, there are a few. First of all, `match`
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enforces ‘exhaustiveness checking’. Do you see that last arm, the one with the
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underscore (`_`)? If we remove that arm, Rust will give us an error:
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```text
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error: non-exhaustive patterns: `_` not covered
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```
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In other words, Rust is trying to tell us we forgot a value. Because `x` is an
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integer, Rust knows that it can have a number of different values – for
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example, `6`. Without the `_`, however, there is no arm that could match, and
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so Rust refuses to compile the code. `_` acts like a ‘catch-all arm’. If none
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of the other arms match, the arm with `_` will, and since we have this
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catch-all arm, we now have an arm for every possible value of `x`, and so our
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program will compile successfully.
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`match` is also an expression, which means we can use it on the right-hand
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side of a `let` binding or directly where an expression is used:
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```rust
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let x = 5;
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let number = match x {
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1 => "one",
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2 => "two",
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3 => "three",
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4 => "four",
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5 => "five",
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_ => "something else",
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};
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```
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Sometimes it’s a nice way of converting something from one type to another.
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# Matching on enums
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Another important use of the `match` keyword is to process the possible
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variants of an enum:
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```rust
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enum Message {
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Quit,
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ChangeColor(i32, i32, i32),
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Move { x: i32, y: i32 },
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Write(String),
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}
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fn quit() { /* ... */ }
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fn change_color(r: i32, g: i32, b: i32) { /* ... */ }
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fn move_cursor(x: i32, y: i32) { /* ... */ }
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fn process_message(msg: Message) {
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match msg {
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Message::Quit => quit(),
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Message::ChangeColor(r, g, b) => change_color(r, g, b),
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Message::Move { x: x, y: y } => move_cursor(x, y),
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Message::Write(s) => println!("{}", s),
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};
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}
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```
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Again, the Rust compiler checks exhaustiveness, so it demands that you
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have a match arm for every variant of the enum. If you leave one off, it
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will give you a compile-time error unless you use `_`.
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Unlike the previous uses of `match`, you can’t use the normal `if`
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statement to do this. You can use the [`if let`][if-let] statement,
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which can be seen as an abbreviated form of `match`.
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[if-let]: if-let.html
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