Binding
Indirectly accessing a variable makes it impossible to branch and use that
variable without re-binding. match provides the @ sigil for binding values to
names:
// A function `age` which returns a `u32`.
fn age() -> u32 {
15
}
fn main() {
println!("Tell me what type of person you are");
match age() {
0 => println!("I haven't celebrated my first birthday yet"),
// Could `match` 1 ..= 12 directly but then what age
// would the child be?
// Could `match` n and use an `if` guard, but would
// not contribute to exhaustiveness checks.
// (Although in this case that would not matter since
// a "catch-all" pattern is present at the bottom)
// Instead, bind to `n` for the sequence of 1 ..= 12.
// Now the age can be reported.
n @ 1 ..= 12 => println!("I'm a child of age {:?}", n),
n @ 13 ..= 19 => println!("I'm a teen of age {:?}", n),
// A similar binding can be done when matching several values.
n @ (1 | 7 | 15 | 13) => println!("I'm a teen of age {:?}", n),
// Nothing bound. Return the result.
n => println!("I'm an old person of age {:?}", n),
}
}You can also use binding to “destructure” enum variants, such as Option:
fn some_number() -> Option<u32> {
Some(42)
}
fn main() {
match some_number() {
// Got `Some` variant, match if its value, bound to `n`,
// is equal to 42.
// Could also use `Some(42)` and print `"The Answer: 42!"`
// but that would require changing `42` in 2 spots should
// you ever wish to change it.
// Could also use `Some(n) if n == 42` and print `"The Answer: {n}!"`
// but that would not contribute to exhaustiveness checks.
// (Although in this case that would not matter since
// the next arm is a "catch-all" pattern)
Some(n @ 42) => println!("The Answer: {}!", n),
// Match any other number.
Some(n) => println!("Not interesting... {}", n),
// Match anything else (`None` variant).
_ => (),
}
}