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Pattern syntax.
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ch18-03-pattern-syntax.md
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## All the Pattern Syntax
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We've seen some examples of different kinds of patterns throughout the book.
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This section lists all the syntax valid in patterns and why you might want to
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use each of them.
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### Literals
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As we saw in Chapter 6, you can match against literals directly:
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```rust
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let x = 1;
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match x {
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1 => println!("one"),
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2 => println!("two"),
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3 => println!("three"),
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_ => println!("anything"),
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}
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```
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This prints `one` since the value in `x` is 1.
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### Named Variables
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Named variables are irrefutable patterns that match any value.
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As with all variables, variables declared as part of a pattern will shadow
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variables with the same name outside of the `match` construct since a `match`
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starts a new scope. In Listing 18-10, we declare a variable named `x` with the
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value `Some(5)` and a variable `y` with the value `10`. Then we have a `match`
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expression on the value `x`. Take a look at the patterns in the match arms and
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the `println!` at the end, and make a guess about what will be printed before
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running this code or reading further:
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<span class="filename">Filename: src/main.rs</span>
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```rust
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fn main() {
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let x = Some(5);
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let y = 10;
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match x {
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Some(50) => println!("Got 50"),
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Some(y) => println!("Matched, y = {:?}", y),
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_ => println!("Default case, x = {:?}", x),
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}
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println!("at the end: x = {:?}, y = {:?}", x, y);
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}
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```
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<span class="caption">Listing 18-10: A `match` statement with an arm that
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introduces a shadowed variable `y`</span>
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<!-- NEXT PARAGRAPH WRAPPED WEIRD INTENTIONALLY SEE #199 -->
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Let's walk through what happens when the `match` statement runs. The first
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match arm has the pattern `Some(50)`, and the value in `x` (`Some(5)`) does not
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match `Some(50)`, so we continue. In the second match arm, the pattern
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`Some(y)` introduces a new variable name `y` that will match any value inside a
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`Some` value. Because we're in a new scope inside the `match` expression, this
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is a new variable, not the `y` we declared at the beginning that has the
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value 10. The new `y` binding will match any value inside a `Some`, which is
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what we have in `x`, so we execute the expression for that arm and print
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`Matched, y = 5` since this `y` binds to the inner value of the `Some` in `x`,
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which is 5.
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If `x` had been a `None` value instead of `Some(5)`, we would have matched the
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underscore since the other two arms' patterns would not have matched. In the
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expression for that match arm, since we did not introduce an `x` variable in
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the pattern of the arm, this `x` is still the outer `x` that has not been
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shadowed. In this hypothetical case, the `match` would print `Default case, x =
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None`.
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Once the `match` expression is over, its scope ends, and so does the scope of
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the inner `y`. The last `println!` produces `at the end: x = Some(5), y = 10`.
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In order to make a `match` expression that compares the values of the outer `x`
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and `y` rather than introducing a shadowed variable, we would need to use a
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match guard conditional instead. We'll be talking about match guards later in
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this section.
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### Multiple patterns
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In `match` expressions only, you can match multiple patterns with `|`, which
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means *or*:
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```rust
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let x = 1;
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match x {
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1 | 2 => println!("one or two"),
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3 => println!("three"),
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_ => println!("anything"),
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}
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```
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This prints `one or two`.
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### Matching Ranges of Values with `...`
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You can match an inclusive range of values with `...`:
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```rust
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let x = 5;
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match x {
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1 ... 5 => println!("one through five"),
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_ => println!("something else"),
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}
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```
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If `x` is 1, 2, 3, 4, or 5, the first arm will match.
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Ranges are only allowed with numeric values or `char` values. Here's an example
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using ranges of `char` values:
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```rust
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let x = 'c';
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match x {
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'a' ... 'j' => println!("early ASCII letter"),
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'k' ... 'z' => println!("late ASCII letter"),
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_ => println!("something else"),
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}
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```
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This will print `early ASCII letter`.
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### Destructuring to Break Apart Values
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Patterns can be used to *destructure* structs, enums, tuples, and references.
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Destructuring means to break a value up into its component pieces. Listing
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18-11 shows a `Point` struct with two fields, `x` and `y`, that we can break
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apart by using a pattern with a `let` statement:
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<span class="filename">Filename: src/main.rs</span>
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```rust
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struct Point {
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x: i32,
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y: i32,
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}
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fn main() {
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let p = Point { x: 0, y: 7 };
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let Point { x, y } = p;
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assert_eq!(0, x);
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assert_eq!(7, y);
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}
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```
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<span class="caption">Listing 18-11: Destructuring using struct field
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shorthand</span>
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This creates the variables `x` and `y` that match the `x` and `y` of `p`. The
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names of the variables must match the names of the fields to use this
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shorthand. If we wanted to use names different than the variable names, we can
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specify `field_name: variable_name` in the pattern. In Listing 18-12, `a` will
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have the value in the `Point` instance's `x` field and `b` will have the value
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in the `y` field:
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<span class="filename">Filename: src/main.rs</span>
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```rust
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struct Point {
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x: i32,
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y: i32,
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}
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fn main() {
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let p = Point { x: 0, y: 7 };
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let Point { x: a, y: b } = p;
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assert_eq!(0, a);
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assert_eq!(7, b);
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}
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```
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<span class="caption">Listing 18-12: Destructuring struct fields into variables
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with different names than the fields</span>
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We can also use destructuring with literal values in order to test and use
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inner parts of a value. Listing 18-13 shows a `match` statement that determines
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whether a point lies directly on the `x` axis (which is true when `y = 0`), on
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the `y` axis (`x = 0`), or neither:
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```rust
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# struct Point {
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# x: i32,
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# y: i32,
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# }
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#
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fn main() {
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let p = Point { x: 0, y: 7 };
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match p {
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Point { x, y: 0 } => println!("On the x axis at {}", x),
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Point { x: 0, y } => println!("On the y axis at {}", y),
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Point { x, y } => println!("On neither axis: ({}, {})", x, y),
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}
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}
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```
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<span class="caption">Listing 18-13: Destructuring and matching literal values
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in one pattern</span>
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This will print `On the y axis at 7` since the value `p` matches the second arm
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by virtue of `x` having the value 0.
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We used destructuring on enums in Chapter 6, such as in Listing 6-5 where we
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destructured an `Option<i32>` using a `match` expression and added one to the
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inner value of the `Some` variant.
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When the value we're matching against a pattern contains a reference, we can
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specify a `&` in the pattern in order to separate the reference and the value.
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This is especially useful in closures used with iterators that iterate over
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references to values when we want to use the values in the closure rather than
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the references. Listing 18-14 shows how to iterate over references to `Point`
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instances in a vector, and destructure both the reference and the struct in
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order to be able to perform calculations on the `x` and `y` values easily:
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```rust
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# struct Point {
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# x: i32,
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# y: i32,
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# }
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#
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let points = vec![
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Point { x: 0, y: 0 },
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Point { x: 1, y: 5 },
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Point { x: 10, y: -3 },
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];
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let sum_of_squares: i32 = points
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.iter()
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.map(|&Point {x, y}| x * x + y * y)
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.sum();
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```
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<span class="caption">Listing 18-14: Destructuring a reference to a struct into
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the struct field values</span>
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Because `iter` iterates over references to the items in the vector, if we
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forgot the `&` in the closure arguments in the `map`, we'd get a type mismatch
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error like this:
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```text
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error[E0308]: mismatched types
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-->
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14 | .map(|Point {x, y}| x * x + y * y)
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| ^^^^^^^^^^^^ expected &Point, found struct `Point`
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|
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= note: expected type `&Point`
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found type `Point`
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```
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This says Rust was expecting our closure to match `&Point`, but we tried to
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match the value with a pattern that was a `Point` value, not a reference to a
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`Point`.
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We can mix, match, and nest destructuring patterns in even more complex ways:
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we can do something complicated like this example where we nest structs and
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tuples inside of a tuple and destructure all the primitive values out:
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```rust
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# struct Point {
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# x: i32,
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# y: i32,
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# }
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#
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let ((feet, inches), Point {x, y}) = ((3, 10), Point { x: 3, y: -10 });
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```
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This lets us break complex types into their component parts.
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### Ignoring Values in a Pattern
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There are a few ways to ignore entire values or parts of values: using the `_`
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pattern, using the `_` pattern within another pattern, using a name that starts
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with an underscore, or using `..` to ignore all remaining parts of a value.
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Let's explore how and why to do each of these.
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#### Ignoring an Entire Value with `_`
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We've seen the use of underscore as a wildcard pattern that will match any value
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but not bind to the value. While the underscore pattern is especially useful as
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the last arm in a `match` expression, we can use it in any pattern, such as
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function arguments as shown in Listing 18-15:
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```rust
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fn foo(_: i32) {
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// code goes here
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}
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```
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<span class="caption">Listing 18-15: Using `_` in a function signature</span>
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Normally, you would change the signature to not have the unused parameter. In
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cases such as implementing a trait, where you need a certain type signature,
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using an underscore lets you ignore a parameter, and the compiler won't warn
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about unused function parameters like it would if we had used a name instead.
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#### Ignoring Parts of a Value with a Nested `_`
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We can also use `_` inside of another pattern to ignore just part of a value.
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In Listing 18-16, the first `match` arm's pattern matches a `Some` value but
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ignores the value inside of the `Some` variant as specified by the underscore:
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```rust
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let x = Some(5);
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match x {
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Some(_) => println!("got a Some and I don't care what's inside"),
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None => (),
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}
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```
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<span class="caption">Listing 18-16: Ignoring the value inside of the `Some`
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variant by using a nested underscore</span>
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This is useful when the code associated with the `match` arm doesn't use the
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nested part of the variable at all.
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We can also use underscores in multiple places within one pattern, as shown in
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Listing 18-17 where we're ignoring the second and fourth values in a tuple of
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five items:
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```rust
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let numbers = (2, 4, 8, 16, 32);
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match numbers {
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(first, _, third, _, fifth) => {
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println!("Some numbers: {}, {}, {}", first, third, fifth)
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},
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}
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```
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<span class="caption">Listing 18-17: Ignoring multiple parts of a tuple</span>
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This will print `Some numbers: 2, 8, 32`, and the values 4 and 16 will be
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ignored.
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#### Ignoring an Unused Variable by Starting its Name with an Underscore
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Usually, Rust will warn you if you create a variable but don't use it anywhere,
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since that could be a bug. If you're prototyping or just starting a project,
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though, you might create a variable that you'll use eventually, but temporarily
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it will be unused. If you're in this situation and would like to tell Rust not
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to warn you about the unused variable, you can start the name of the variable
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with an underscore. This works just like a variable name in any pattern, only
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Rust won't warn you if the variable goes unused. In Listing 18-18, we
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do get a warning about not using the variable `y`, but we don't get a warning
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about not using the variable `_x`:
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```rust
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fn main() {
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let _x = 5;
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let y = 10;
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}
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```
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<span class="caption">Listing 18-18: Starting a variable name with an underscore
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in order to not get unused variable warnings</span>
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Note that there is a subtle difference between using only `_` and using a name
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that starts with an underscore like `_x`: `_x` still binds the value to the
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variable, but `_` doesn't bind at all.
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Listing 18-19 shows a case where this distinction matters: `s` will still be
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moved into `_s`, which prevents us from using `s` again:
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```rust,ignore
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let s = Some(String::from("Hello!"));
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if let Some(_s) = s {
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println!("found a string");
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}
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println!("{:?}", s);
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```
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<span class="caption">Listing 18-19: An unused variable starting with an
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underscore still binds the value, which may take ownership of the value</span>
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Using underscore by itself, however, doesn't ever bind to the value. Listing
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18-20 will compile without any errors since `s` does not get moved into `_`:
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```rust
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let s = Some(String::from("Hello!"));
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if let Some(_) = s {
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println!("found a string");
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}
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println!("{:?}", s);
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```
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<span class="caption">Listing 18-20: Using underscore does not bind the
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value</span>
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This works just fine. Because we never bind `s` to anything, it's not moved.
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#### Ignoring Remaining Parts of a Value with `..`
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With values that have many parts, we can extract only a few parts and avoid
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having to list underscores for each remaining part by instead using `..`. The
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`..` pattern will ignore any parts of a value that we haven't explicitly
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matched in the rest of the pattern. In Listing 18-21, we have a `Point` struct
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that holds a coordinate in three dimensional space. In the `match` expression,
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we only want to operate on the `x` coordinate and ignore the values in the `y`
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and `z` fields:
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|
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```rust
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struct Point {
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x: i32,
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y: i32,
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z: i32,
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}
|
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let origin = Point { x: 0, y: 0, z: 0 };
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|
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match origin {
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Point { x, .. } => println!("x is {}", x),
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}
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```
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<span class="caption">Listing 18-21: Ignoring all fields of a `Point` except
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for `x` by using `..`</span>
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|
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Using `..` is shorter to type than having to list out `y: _` and `z: _`. The
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`..` pattern is especially useful when working with structs that have lots of
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fields in situations where only one or two fields are relevant.
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`..` will expand to as many values as it needs to be. Listing 18-22 shows a use
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of `..` with a tuple:
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```rust
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fn main() {
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let numbers = (2, 4, 8, 16, 32);
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match numbers {
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(first, .., last) => {
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println!("Some numbers: {}, {}", first, last);
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},
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}
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}
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```
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<span class="caption">Listing 18-22: Matching only the first and last values in
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a tuple and ignoring all other values with `..`</span>
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Here, we have the first and last value matched, with `first` and `last`. The
|
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`..` will match and ignore all of the things in the middle.
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|
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Using `..` must be unambiguous, however. Listing 18-23 shows an example where
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it's not clear to Rust which values we want to match and which values we want
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to ignore:
|
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|
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```rust,ignore
|
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fn main() {
|
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let numbers = (2, 4, 8, 16, 32);
|
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|
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match numbers {
|
||||
(.., second, ..) => {
|
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println!("Some numbers: {}", second)
|
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},
|
||||
}
|
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}
|
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```
|
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|
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<span class="caption">Listing 18-23: An attempt to use `..` in a way that is
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ambiguous</span>
|
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|
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If we compile this example, we get this error:
|
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|
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```text
|
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error: `..` can only be used once per tuple or tuple struct pattern
|
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--> src/main.rs:5:22
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|
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5 | (.., second, ..) => {
|
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| ^^
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```
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|
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It's not possible to determine how many values in the tuple should be ignored
|
||||
before one value is matched with `second`, and then how many further values are
|
||||
ignored after that. We could mean that we want to ignore 2, bind `second` to 4,
|
||||
then ignore 8, 16, and 32, or we could mean that we want to ignore 2 and 4,
|
||||
bind `second` to 8, then ignore 16 and 32, and so forth. The variable name
|
||||
`second` doesn't mean anything special to Rust, so we get a compiler error
|
||||
since using `..` in two places like this is ambiguous.
|
||||
|
||||
### `ref` and `ref mut` to Create References in Patterns
|
||||
|
||||
Usually, when you match against a pattern, the variables that the pattern
|
||||
introduces are bound to a value. This means you'll end up moving the value into
|
||||
the `match` (or wherever you're using the pattern) since the ownership rules
|
||||
apply. Listing 18-24 shows an example:
|
||||
|
||||
```rust,ignore
|
||||
let robot_name = Some(String::from("Bors"));
|
||||
|
||||
match robot_name {
|
||||
Some(name) => println!("Found a name: {}", name),
|
||||
None => (),
|
||||
}
|
||||
|
||||
println!("robot_name is: {:?}", robot_name);
|
||||
```
|
||||
|
||||
<span class="caption">Listing 18-24: Creating a variable in a match arm pattern
|
||||
takes ownership of the value</span>
|
||||
|
||||
This example will fail to compile since the value inside the `Some` value in
|
||||
`robot_name` is moved within the `match` when `name` binds to that value.
|
||||
|
||||
Using `&` in a pattern matches an existing reference in the value, as we saw in
|
||||
the "Destructuring to Break Apart Values" section. If you want to create a
|
||||
reference instead in order to borrow the value in a pattern variable, use the
|
||||
`ref` keyword before the new variable, as shown in Listing 18-25:
|
||||
|
||||
```rust
|
||||
let robot_name = Some(String::from("Bors"));
|
||||
|
||||
match robot_name {
|
||||
Some(ref name) => println!("Found a name: {}", name),
|
||||
None => (),
|
||||
}
|
||||
|
||||
println!("robot_name is: {:?}", robot_name);
|
||||
```
|
||||
|
||||
<span class="caption">Listing 18-25: Creating a reference so that a pattern
|
||||
variable does not take ownership of a value</span>
|
||||
|
||||
This example will compile because the value in the `Some` variant in
|
||||
`robot_name` is not moved into the `Some(ref name)` arm of the match; the match
|
||||
only took a reference to the data in `robot_name` rather than moving it.
|
||||
|
||||
To create a mutable reference, use `ref mut` for the same reason as shown in
|
||||
Listing 18-26:
|
||||
|
||||
```rust
|
||||
let mut robot_name = Some(String::from("Bors"));
|
||||
|
||||
match robot_name {
|
||||
Some(ref mut name) => *name = String::from("Another name"),
|
||||
None => (),
|
||||
}
|
||||
|
||||
println!("robot_name is: {:?}", robot_name);
|
||||
```
|
||||
|
||||
<span class="caption">Listing 18-26: Creating a mutable reference to a value as
|
||||
part of a pattern using `ref mut`</span>
|
||||
|
||||
This example will compile and print `robot_name is: Some("Another name")`.
|
||||
Since `name` is a mutable reference, within the match arm code, we need to
|
||||
dereference using the `*` operator in order to be able to mutate the value.
|
||||
|
||||
### Extra Conditionals with Match Guards
|
||||
|
||||
You can introduce *match guards* as part of a match arm by specifying an
|
||||
additional `if` conditional after the pattern. The conditional can use
|
||||
variables created in the pattern. Listing 18-27 has a `match` expression with a
|
||||
match guard in the first arm:
|
||||
|
||||
```rust
|
||||
let num = Some(4);
|
||||
|
||||
match num {
|
||||
Some(x) if x < 5 => println!("less than five: {}", x),
|
||||
Some(x) => println!("{}", x),
|
||||
None => (),
|
||||
}
|
||||
```
|
||||
|
||||
<span class="caption">Listing 18-27: Adding a match guard to a pattern</span>
|
||||
|
||||
This example will print `less than five: 4`. If `num` was instead `Some(7)`,
|
||||
this example would print `7`. Match guards allow you to express more complexity
|
||||
than patterns alone give you.
|
||||
|
||||
In Listing 18-10, we saw that since patterns shadow variables, we weren't able
|
||||
to specify a pattern to express the case when a value was equal to a variable
|
||||
outside the `match`. Listing 18-28 shows how we can use a match guard to
|
||||
accomplish this:
|
||||
|
||||
```rust
|
||||
fn main() {
|
||||
let x = Some(5);
|
||||
let y = 10;
|
||||
|
||||
match x {
|
||||
Some(50) => println!("Got 50"),
|
||||
Some(n) if n == y => println!("Matched, n = {:?}", n),
|
||||
_ => println!("Default case, x = {:?}", x),
|
||||
}
|
||||
|
||||
println!("at the end: x = {:?}, y = {:?}", x, y);
|
||||
}
|
||||
```
|
||||
|
||||
<span class="caption">Listing 18-28: Using a match guard to test for equality
|
||||
with an outer variable</span>
|
||||
|
||||
This will now print `Default case, x = Some(5)`. Because the second match arm
|
||||
is not introducing a new variable `y` that shadows the outer `y` in the
|
||||
pattern, we can use `y` in the match guard. We're still destructuring `x` to
|
||||
get the inner value `n`, and then we can compare `n` and `y` in the match guard.
|
||||
|
||||
If you're using a match guard with multiple patterns specified by `|`, the
|
||||
match guard condition applies to all of the patterns. Listing 18-29 shows a
|
||||
match guard that applies to the value matched by all three patterns in the
|
||||
first arm:
|
||||
|
||||
```rust
|
||||
let x = 4;
|
||||
let y = false;
|
||||
|
||||
match x {
|
||||
4 | 5 | 6 if y => println!("yes"),
|
||||
_ => println!("no"),
|
||||
}
|
||||
```
|
||||
|
||||
<span class="caption">Listing 18-29: Combining multiple patterns with a match
|
||||
guard</span>
|
||||
|
||||
This prints `no` since the `if` condition applies to the whole pattern `4 | 5 |
|
||||
6`, not only to the last value `6`. In other words, the precedence of a match
|
||||
guard in relation to a pattern behaves like this:
|
||||
|
||||
```text
|
||||
(4 | 5 | 6) if y => ...
|
||||
```
|
||||
|
||||
rather than this:
|
||||
|
||||
```text
|
||||
4 | 5 | (6 if y) => ...
|
||||
```
|
||||
|
||||
### `@` Bindings
|
||||
|
||||
In order to test a value in a pattern but also be able to create a variable
|
||||
bound to the value, we can use `@`. Listing 18-30 shows an example where we
|
||||
want to test that a `Message::Hello` `id` field is within the range `3...7` but
|
||||
also be able to bind to the value so that we can use it in the code associated
|
||||
with the arm:
|
||||
|
||||
```rust
|
||||
enum Message {
|
||||
Hello { id: i32 },
|
||||
}
|
||||
|
||||
let msg = Message::Hello { id: 5 };
|
||||
|
||||
match msg {
|
||||
Message::Hello { id: id @ 3...7 } => {
|
||||
println!("Found an id in range: {}", id)
|
||||
},
|
||||
Message::Hello { id: 10...12 } => {
|
||||
println!("Found an id in another range")
|
||||
},
|
||||
Message::Hello { id } => {
|
||||
println!("Found some other id: {}", id)
|
||||
},
|
||||
}
|
||||
```
|
||||
|
||||
<span class="caption">Listing 18-30: Using `@` to bind to a value in a pattern
|
||||
while also testing it</span>
|
||||
|
||||
This example will print `Found an id in range: 5`. By specifying `id @` before
|
||||
the range, we're capturing whatever value matched the range while also testing
|
||||
it. In the second arm where we only have a range specified in the pattern, the
|
||||
code associated with the arm doesn't know if `id` is 10, 11, or 12, since we
|
||||
haven't saved the `id` value in a variable: we only know that the value matched
|
||||
something in that range if that arm's code is executed. In the last arm where
|
||||
we've specified a variable without a range, we do have the value available to
|
||||
use in the arm's code, but we haven't applied any other test to the value.
|
||||
Using `@` lets us test a value and save it in a variable within one pattern.
|
||||
|
||||
## Summary
|
||||
|
||||
Patterns are a useful feature of Rust that help to distinguish between
|
||||
different kinds of data. When used in `match` statements, Rust makes sure that
|
||||
your patterns cover every possible value. Patterns in `let` statements and
|
||||
function parameters make those constructs more powerful, enabling the
|
||||
destructuring of values into smaller parts at the same time as assigning to
|
||||
variables.
|
||||
|
||||
Now, for the penultimate chapter of the book, let's take a look at some
|
||||
advanced parts of a variety of Rust's features.
|
||||
Loading…
Reference in New Issue
Block a user