# `match` expressions

A `match` expression branches on a *pattern*. The exact form of matching that
occurs depends on the pattern. Patterns consist of some combination of
literals, destructured arrays or enum constructors, structs and tuples,
variable binding specifications, wildcards (`..`), and placeholders (`_`). A
`match` expression has a *head expression*, which is the value to compare to
the patterns. The type of the patterns must equal the type of the head
expression.

A `match` behaves differently depending on whether or not the head expression
is a [place expression or value expression][place expression].
If the head 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 head 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:

```rust
let x = 1;

match x {
    1 => println!("one"),
    2 => println!("two"),
    3 => println!("three"),
    4 => println!("four"),
    5 => println!("five"),
    _ => println!("something else"),
}
```

Patterns that bind variables default to binding to a copy or move of the
matched value (depending on the matched value's type). This can be changed to
bind to a reference by using the `ref` keyword, or to a mutable reference using
`ref mut`.

Patterns can be used to *destructure* structs, enums, and tuples. Destructuring
breaks a value up into its component pieces. The syntax used is the same as
when creating such values. When destructing a data structure with named (but
not numbered) fields, it is allowed to write `fieldname` as a shorthand for
`fieldname: fieldname`. In a pattern whose head expression has a `struct`,
`enum` or `tupl` type, a placeholder (`_`) stands for a *single* data field,
whereas a wildcard `..` stands for *all* the fields of a particular variant.

```rust
# enum Message {
#     Quit,
#     WriteString(String),
#     Move { x: i32, y: i32 },
#     ChangeColor(u8, u8, u8),
# }
# let message = Message::Quit;
match message {
    Message::Quit => println!("Quit"),
    Message::WriteString(write) => println!("{}", &write),
    Message::Move{ x, y: 0 } => println!("move {} horizontally", x),
    Message::Move{ .. } => println!("other move"),
    Message::ChangeColor { 0: red, 1: green, 2: _ } => {
        println!("color change, red: {}, green: {}", red, green);
    }
};
```

Patterns can also dereference pointers by using the `&`, `&mut` and `box`
symbols, as appropriate. For example, these two matches on `x: &i32` are
equivalent:

```rust
# let x = &3;
let y = match *x { 0 => "zero", _ => "some" };
let z = match x { &0 => "zero", _ => "some" };

assert_eq!(y, z);
```

Subpatterns can also be bound to variables by the use of the syntax `variable @
subpattern`. For example:

```rust
let x = 1;

match x {
    e @ 1 ... 5 => println!("got a range element {}", e),
    _ => println!("anything"),
}
```

Multiple match patterns may be joined with the `|` operator. A range of values
may be specified with `...`. For example:

```rust
# let x = 2;
let message = match x {
    0 | 1  => "not many",
    2 ... 9 => "a few",
    _      => "lots"
};
```

Range patterns only work on scalar types (like integers and characters; not
like arrays and structs, which have sub-components). A range pattern may not be
a sub-range of another range pattern inside the same `match`.

Finally, match patterns can accept *pattern 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.

```rust
# 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!(),
};
```

[place expression]: expressions.html#place-expressions-and-value-expressions
[value expression]: expressions.html#place-expressions-and-value-expressions