match
match compares a value against a series of patterns and runs the first arm that matches. It is exhaustive: the compiler requires every possible value to be covered, so a match can never silently fall through.
match status {
200 => Print("OK"),
404 => Print("Not Found"),
500 => Print("Server Error"),
_ => Print("Unknown"),
}
Each arm is written pattern => body,. Patterns are checked top to bottom, and the first match wins.
Patterns
A pattern can be a literal, a range, or a destructuring of an enum variant or a struct:
// Literal and range patterns
match code {
0 => Print("zero"),
1..9 => Print("single digit"),
_ => Print("other"),
}
// Enum destructuring — bind the variant's data
match event {
Event::Click(x, y) => HandleClick(x, y),
Event::KeyPress(key) => HandleKey(key),
Event::Quit => Exit(),
}
// Struct destructuring — match and bind fields
match point {
Point { x: 0, y: 0 } => Print("origin"),
Point { x, y } => Print(x, y),
}
Guards
Add if <condition> to an arm to match only when an extra condition also holds. Guarded arms are tested in order, so list the most specific first:
match temperature {
t if t < 0.0 => Print("freezing"),
t if t < 20.0 => Print("cold"),
t if t < 30.0 => Print("comfortable"),
_ => Print("hot"),
}
The Wildcard
The wildcard _ matches any value and discards it. It is the usual way to satisfy exhaustiveness for all remaining cases:
match code {
0 => Print("ok"),
_ => Print("error"),
}
match as an Expression
A match can produce a value. Used this way, every arm must yield a value of the same type, and the result can be bound directly:
let label = match status {
200 => "OK",
404 => "Not Found",
_ => "Unknown",
};
See Also
if/else— simpler branching for one or two cases- Type Tests —
isfor a single type check without exhaustiveness - Enumerations — the tagged values
matchmost often destructures
if / else
if runs its block when the condition is true. Add else if to test further conditions, and a final else to handle everything that remains. The first matching block runs; the rest are skipped.
while
while repeats its body as long as the condition holds. The condition is tested before each iteration, so the body may run zero times.