match
expressions
Syntax
MatchExpression :
match
Expressionexcept struct expression{
InnerAttribute*
MatchArms?
}
MatchArms :
( MatchArm=>
( BlockExpression,
? | Expression,
) )*
MatchArm=>
( BlockExpression | Expression ),
?MatchArm :
OuterAttribute* MatchArmPatterns MatchArmGuard?MatchArmPatterns :
|
? Pattern (|
Pattern )*MatchArmGuard :
if
Expression
A match
expression branches on a pattern. The exact form of matching that
occurs depends on the pattern. A match
expression has a scrutinee expression, which is the value to compare to the
patterns. The scrutinee expression and the patterns must have the same type.
A match
behaves differently depending on whether or not the scrutinee
expression is a place expression or value expression.
If the scrutinee expression is a value expression, it is first evaluated into
a temporary location, and the resulting value is sequentially compared to the
patterns in the arms until a match is found. The first arm with a matching
pattern is chosen as the branch target of the match
, any variables bound by
the pattern are assigned to local variables in the arm's block, and control
enters the block.
When the scrutinee expression is a place expression, the match does not allocate a temporary location; however, a by-value binding may copy or move from the memory location. When possible, it is preferable to match on place expressions, as the lifetime of these matches inherits the lifetime of the place expression rather than being restricted to the inside of the match.
An example of a match
expression:
# #![allow(unused_variables)] #fn main() { let x = 1; match x { 1 => println!("one"), 2 => println!("two"), 3 => println!("three"), 4 => println!("four"), 5 => println!("five"), _ => println!("something else"), } #}
Variables bound within the pattern are scoped to the match guard and the arm's expression. The binding mode (move, copy, or reference) depends on the pattern.
Multiple match patterns may be joined with the |
operator. Each pattern will be
tested in left-to-right sequence until a successful match is found.
# #![allow(unused_variables)] #fn main() { # let x = 9; let message = match x { 0 | 1 => "not many", 2 ..= 9 => "a few", _ => "lots" }; assert_eq!(message, "a few"); // Demonstration of pattern match order. struct S(i32, i32); match S(1, 2) { S(z @ 1, _) | S(_, z @ 2) => assert_eq!(z, 1), _ => panic!(), } #}
Note: The
2..=9
is a Range Pattern, not a Range Expression. Thus, only those types of ranges supported by range patterns can be used in match arms.
Every binding in each |
separated pattern must appear in all of the patterns
in the arm. Every binding of the same name must have the same type, and have
the same binding mode.
Match arms can accept match guards to further refine the
criteria for matching a case. Pattern guards appear after the pattern and
consist of a bool-typed expression following the if
keyword. A pattern guard
may refer to the variables bound within the pattern they follow.
When the pattern matches successfully, the pattern guard expression is executed.
If the expression evaluates to true, the pattern is successfully matched against.
Otherwise, the next pattern, including other matches with the |
operator in
the same arm, is tested.
# #![allow(unused_variables)] #fn main() { # let maybe_digit = Some(0); # fn process_digit(i: i32) { } # fn process_other(i: i32) { } let message = match maybe_digit { Some(x) if x < 10 => process_digit(x), Some(x) => process_other(x), None => panic!(), }; #}
Note: Multiple matches using the
|
operator can cause the pattern guard and the side effects it has to execute multiple times. For example:# #![allow(unused_variables)] #fn main() { # use std::cell::Cell; let i : Cell<i32> = Cell::new(0); match 1 { 1 | _ if { i.set(i.get() + 1); false } => {} _ => {} } assert_eq!(i.get(), 2); #}
Attributes on match arms
Outer attributes are allowed on match arms. The only attributes that have
meaning on match arms are cfg
, cold
, and the lint check attributes.
Inner attributes are allowed directly after the opening brace of the match expression in the same expression contexts as attributes on block expressions.