src/doc/trpl/patterns.md

   1 % Patterns
   2
   3 Patterns are quite common in Rust. We use them in [variable
   4 bindings][bindings], [match statements][match], and other places, too. Let’s go
   5 on a whirlwind tour of all of the things patterns can do!
   6
   7 [bindings]: variable-bindings.html
   8 [match]: match.html
   9
  10 A quick refresher: you can match against literals directly, and `_` acts as an
  11 ‘any’ case:
  12
  13 ```rust
  14 let x = 1;
  15
  16 match x {
  17     1 => println!("one"),
  18     2 => println!("two"),
  19     3 => println!("three"),
  20     _ => println!("anything"),
  21 }
  22 ```
  23
  24 This prints `one`.
  25
  26 # Multiple patterns
  27
  28 You can match multiple patterns with `|`:
  29
  30 ```rust
  31 let x = 1;
  32
  33 match x {
  34     1 | 2 => println!("one or two"),
  35     3 => println!("three"),
  36     _ => println!("anything"),
  37 }
  38 ```
  39
  40 This prints `one or two`.
  41
  42 # Ranges
  43
  44 You can match a range of values with `...`:
  45
  46 ```rust
  47 let x = 1;
  48
  49 match x {
  50     1 ... 5 => println!("one through five"),
  51     _ => println!("anything"),
  52 }
  53 ```
  54
  55 This prints `one through five`.
  56
  57 Ranges are mostly used with integers and `char`s:
  58
  59 ```rust
  60 let x = '💅';
  61
  62 match x {
  63     'a' ... 'j' => println!("early letter"),
  64     'k' ... 'z' => println!("late letter"),
  65     _ => println!("something else"),
  66 }
  67 ```
  68
  69 This prints `something else`.
  70
  71 # Bindings
  72
  73 You can bind values to names with `@`:
  74
  75 ```rust
  76 let x = 1;
  77
  78 match x {
  79     e @ 1 ... 5 => println!("got a range element {}", e),
  80     _ => println!("anything"),
  81 }
  82 ```
  83
  84 This prints `got a range element 1`. This is useful when you want to
  85 do a complicated match of part of a data structure:
  86
  87 ```rust
  88 #[derive(Debug)]
  89 struct Person {
  90     name: Option<String>,
  91 }
  92
  93 let name = "Steve".to_string();
  94 let mut x: Option<Person> = Some(Person { name: Some(name) });
  95 match x {
  96     Some(Person { name: ref a @ Some(_), .. }) => println!("{:?}", a),
  97     _ => {}
  98 }
  99 ```
 100
 101 This prints `Some("Steve")`: We’ve bound the inner `name` to `a`.
 102
 103 If you use `@` with `|`, you need to make sure the name is bound in each part
 104 of the pattern:
 105
 106 ```rust
 107 let x = 5;
 108
 109 match x {
 110     e @ 1 ... 5 | e @ 8 ... 10 => println!("got a range element {}", e),
 111     _ => println!("anything"),
 112 }
 113 ```
 114
 115 # Ignoring variants
 116
 117 If you’re matching on an enum which has variants, you can use `..` to
 118 ignore the value and type in the variant:
 119
 120 ```rust
 121 enum OptionalInt {
 122     Value(i32),
 123     Missing,
 124 }
 125
 126 let x = OptionalInt::Value(5);
 127
 128 match x {
 129     OptionalInt::Value(..) => println!("Got an int!"),
 130     OptionalInt::Missing => println!("No such luck."),
 131 }
 132 ```
 133
 134 This prints `Got an int!`.
 135
 136 # Guards
 137
 138 You can introduce ‘match guards’ with `if`:
 139
 140 ```rust
 141 enum OptionalInt {
 142     Value(i32),
 143     Missing,
 144 }
 145
 146 let x = OptionalInt::Value(5);
 147
 148 match x {
 149     OptionalInt::Value(i) if i > 5 => println!("Got an int bigger than five!"),
 150     OptionalInt::Value(..) => println!("Got an int!"),
 151     OptionalInt::Missing => println!("No such luck."),
 152 }
 153 ```
 154
 155 This prints `Got an int!`.
 156
 157 If you’re using `if` with multiple patterns, the `if` applies to both sides:
 158
 159 ```rust
 160 let x = 4;
 161 let y = false;
 162
 163 match x {
 164     4 | 5 if y => println!("yes"),
 165     _ => println!("no"),
 166 }
 167 ```
 168
 169 This prints `no`, because the `if` applies to the whole of `4 | 5`, and not to
 170 just the `5`, In other words, the the precedence of `if` behaves like this:
 171
 172 ```text
 173 (4 | 5) if y => ...
 174 ```
 175
 176 not this:
 177
 178 ```text
 179 4 | (5 if y) => ...
 180 ```
 181
 182 # ref and ref mut
 183
 184 If you want to get a [reference][ref], use the `ref` keyword:
 185
 186 ```rust
 187 let x = 5;
 188
 189 match x {
 190     ref r => println!("Got a reference to {}", r),
 191 }
 192 ```
 193
 194 This prints `Got a reference to 5`.
 195
 196 [ref]: references-and-borrowing.html
 197
 198 Here, the `r` inside the `match` has the type `&i32`. In other words, the `ref`
 199 keyword _creates_ a reference, for use in the pattern. If you need a mutable
 200 reference, `ref mut` will work in the same way:
 201
 202 ```rust
 203 let mut x = 5;
 204
 205 match x {
 206     ref mut mr => println!("Got a mutable reference to {}", mr),
 207 }
 208 ```
 209
 210 # Destructuring
 211
 212 If you have a compound data type, like a [`struct`][struct], you can destructure it
 213 inside of a pattern:
 214
 215 ```rust
 216 struct Point {
 217     x: i32,
 218     y: i32,
 219 }
 220
 221 let origin = Point { x: 0, y: 0 };
 222
 223 match origin {
 224     Point { x, y } => println!("({},{})", x, y),
 225 }
 226 ```
 227
 228 [struct]: structs.html
 229
 230 We can use `:` to give a value a different name.
 231
 232 ```rust
 233 struct Point {
 234     x: i32,
 235     y: i32,
 236 }
 237
 238 let origin = Point { x: 0, y: 0 };
 239
 240 match origin {
 241     Point { x: x1, y: y1 } => println!("({},{})", x1, y1),
 242 }
 243 ```
 244
 245 If we only care about some of the values, we don’t have to give them all names:
 246
 247 ```rust
 248 struct Point {
 249     x: i32,
 250     y: i32,
 251 }
 252
 253 let origin = Point { x: 0, y: 0 };
 254
 255 match origin {
 256     Point { x, .. } => println!("x is {}", x),
 257 }
 258 ```
 259
 260 This prints `x is 0`.
 261
 262 You can do this kind of match on any member, not just the first:
 263
 264 ```rust
 265 struct Point {
 266     x: i32,
 267     y: i32,
 268 }
 269
 270 let origin = Point { x: 0, y: 0 };
 271
 272 match origin {
 273     Point { y, .. } => println!("y is {}", y),
 274 }
 275 ```
 276
 277 This prints `y is 0`.
 278
 279 This ‘destructuring’ behavior works on any compound data type, like
 280 [tuples][tuples] or [enums][enums].
 281
 282 [tuples]: primitive-types.html#tuples
 283 [enums]: enums.html
 284
 285 # Mix and Match
 286
 287 Whew! That’s a lot of different ways to match things, and they can all be
 288 mixed and matched, depending on what you’re doing:
 289
 290 ```rust,ignore
 291 match x {
 292     Foo { x: Some(ref name), y: None } => ...
 293 }
 294 ```
 295
 296 Patterns are very powerful. Make good use of them.