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[rustc.git] / src / doc / book / src / ch18-03-pattern-syntax.md

## Pattern Syntax

Throughout the book, you’ve seen examples of many kinds of patterns. In this
section, we gather all the syntax valid in patterns and discuss why you might
want to use each one.

### Matching Literals

As you saw in Chapter 6, you can match patterns against literals directly. The
following code gives some examples:

```rust
{{#rustdoc_include ../listings/ch18-patterns-and-matching/no-listing-01-literals/src/main.rs:here}}
```

This code prints `one` because the value in `x` is 1. This syntax is useful
when you want your code to take an action if it gets a particular concrete
value.

### Matching Named Variables

Named variables are irrefutable patterns that match any value, and we’ve used
them many times in the book. However, there is a complication when you use
named variables in `match` expressions. Because `match` starts a new scope,
variables declared as part of a pattern inside the `match` expression will
shadow those with the same name outside the `match` construct, as is the case
with all variables. In Listing 18-11, we declare a variable named `x` with the
value `Some(5)` and a variable `y` with the value `10`. We then create a
`match` expression on the value `x`. Look at the patterns in the match arms and
`println!` at the end, and try to figure out what the code will print before
running this code or reading further.

<span class="filename">Filename: src/main.rs</span>

```rust
{{#rustdoc_include ../listings/ch18-patterns-and-matching/listing-18-11/src/main.rs:here}}
```

<span class="caption">Listing 18-11: A `match` expression with an arm that
introduces a shadowed variable `y`</span>

Let’s walk through what happens when the `match` expression runs. The pattern
in the first match arm doesn’t match the defined value of `x`, so the code
continues.

The pattern in the second match arm introduces a new variable named `y` that
will match any value inside a `Some` value. Because we’re in a new scope inside
the `match` expression, this is a new `y` variable, not the `y` we declared at
the beginning with the value 10. This new `y` binding will match any value
inside a `Some`, which is what we have in `x`. Therefore, this new `y` binds to
the inner value of the `Some` in `x`. That value is `5`, so the expression for
that arm executes and prints `Matched, y = 5`.

If `x` had been a `None` value instead of `Some(5)`, the patterns in the first
two arms wouldn’t have matched, so the value would have matched to the
underscore. We didn’t introduce the `x` variable in the pattern of the
underscore arm, so the `x` in the expression is still the outer `x` that hasn’t
been shadowed. In this hypothetical case, the `match` would print `Default
case, x = None`.

When the `match` expression is done, its scope ends, and so does the scope of
the inner `y`. The last `println!` produces `at the end: x = Some(5), y = 10`.

To create a `match` expression that compares the values of the outer `x` and
`y`, rather than introducing a shadowed variable, we would need to use a match
guard conditional instead. We’ll talk about match guards later in the [“Extra
Conditionals with Match Guards”](#extra-conditionals-with-match-guards)<!--
ignore --> section.

### Multiple Patterns

In `match` expressions, you can match multiple patterns using the `|` syntax,
which means *or*. For example, the following code matches the value of `x`
against the match arms, the first of which has an *or* option, meaning if the
value of `x` matches either of the values in that arm, that arm’s code will
run:

```rust
{{#rustdoc_include ../listings/ch18-patterns-and-matching/no-listing-02-multiple-patterns/src/main.rs:here}}
```

This code prints `one or two`.

### Matching Ranges of Values with `..=`

The `..=` syntax allows us to match to an inclusive range of values. In the
following code, when a pattern matches any of the values within the range, that
arm will execute:

```rust
{{#rustdoc_include ../listings/ch18-patterns-and-matching/no-listing-03-ranges/src/main.rs:here}}
```

If `x` is 1, 2, 3, 4, or 5, the first arm will match. This syntax is more
convenient than using the `|` operator to express the same idea; instead of
`1..=5`, we would have to specify `1 | 2 | 3 | 4 | 5` if we used `|`.
Specifying a range is much shorter, especially if we want to match, say, any
number between 1 and 1,000!

Ranges are only allowed with numeric values or `char` values, because the
compiler checks that the range isn’t empty at compile time. The only types for
which Rust can tell if a range is empty or not are `char` and numeric values.

Here is an example using ranges of `char` values:

```rust
{{#rustdoc_include ../listings/ch18-patterns-and-matching/no-listing-04-ranges-of-char/src/main.rs:here}}
```

Rust can tell that `c` is within the first pattern’s range and prints `early
ASCII letter`.

### Destructuring to Break Apart Values

We can also use patterns to destructure structs, enums, tuples, and references
to use different parts of these values. Let’s walk through each value.

#### Destructuring Structs

Listing 18-12 shows a `Point` struct with two fields, `x` and `y`, that we can
break apart using a pattern with a `let` statement.

<span class="filename">Filename: src/main.rs</span>

```rust
{{#rustdoc_include ../listings/ch18-patterns-and-matching/listing-18-12/src/main.rs}}
```

<span class="caption">Listing 18-12: Destructuring a struct’s fields into
separate variables</span>

This code creates the variables `a` and `b` that match the values of the `x`
and `y` fields of the `p` struct. This example shows that the names of the
variables in the pattern don’t have to match the field names of the struct. But
it’s common to want the variable names to match the field names to make it
easier to remember which variables came from which fields.

Because having variable names match the fields is common and because writing
`let Point { x: x, y: y } = p;` contains a lot of duplication, there is a
shorthand for patterns that match struct fields: you only need to list the name
of the struct field, and the variables created from the pattern will have the
same names. Listing 18-13 shows code that behaves in the same way as the code
in Listing 18-12, but the variables created in the `let` pattern are `x` and
`y` instead of `a` and `b`.

<span class="filename">Filename: src/main.rs</span>

```rust
{{#rustdoc_include ../listings/ch18-patterns-and-matching/listing-18-13/src/main.rs}}
```

<span class="caption">Listing 18-13: Destructuring struct fields using struct
field shorthand</span>

This code creates the variables `x` and `y` that match the `x` and `y` fields
of the `p` variable. The outcome is that the variables `x` and `y` contain the
values from the `p` struct.

We can also destructure with literal values as part of the struct pattern
rather than creating variables for all the fields. Doing so allows us to test
some of the fields for particular values while creating variables to
destructure the other fields.

Listing 18-14 shows a `match` expression that separates `Point` values into
three cases: points that lie directly on the `x` axis (which is true when `y =
0`), on the `y` axis (`x = 0`), or neither.

<span class="filename">Filename: src/main.rs</span>

```rust
{{#rustdoc_include ../listings/ch18-patterns-and-matching/listing-18-14/src/main.rs:here}}
```

<span class="caption">Listing 18-14: Destructuring and matching literal values
in one pattern</span>

The first arm will match any point that lies on the `x` axis by specifying that
the `y` field matches if its value matches the literal `0`. The pattern still
creates an `x` variable that we can use in the code for this arm.

Similarly, the second arm matches any point on the `y` axis by specifying that
the `x` field matches if its value is `0` and creates a variable `y` for the
value of the `y` field. The third arm doesn’t specify any literals, so it
matches any other `Point` and creates variables for both the `x` and `y` fields.

In this example, the value `p` matches the second arm by virtue of `x`
containing a 0, so this code will print `On the y axis at 7`.

#### Destructuring Enums

We’ve destructured enums earlier in this book, for example, when we
destructured `Option<i32>` in Listing 6-5 in Chapter 6. One detail we haven’t
mentioned explicitly is that the pattern to destructure an enum should
correspond to the way the data stored within the enum is defined. As an
example, in Listing 18-15 we use the `Message` enum from Listing 6-2 and write
a `match` with patterns that will destructure each inner value.

<span class="filename">Filename: src/main.rs</span>

```rust
{{#rustdoc_include ../listings/ch18-patterns-and-matching/listing-18-15/src/main.rs}}
```

<span class="caption">Listing 18-15: Destructuring enum variants that hold
different kinds of values</span>

This code will print `Change the color to red 0, green 160, and blue 255`. Try
changing the value of `msg` to see the code from the other arms run.

For enum variants without any data, like `Message::Quit`, we can’t destructure
the value any further. We can only match on the literal `Message::Quit` value,
and no variables are in that pattern.

For struct-like enum variants, such as `Message::Move`, we can use a pattern
similar to the pattern we specify to match structs. After the variant name, we
place curly brackets and then list the fields with variables so we break apart
the pieces to use in the code for this arm. Here we use the shorthand form as
we did in Listing 18-13.

For tuple-like enum variants, like `Message::Write` that holds a tuple with one
element and `Message::ChangeColor` that holds a tuple with three elements, the
pattern is similar to the pattern we specify to match tuples. The number of
variables in the pattern must match the number of elements in the variant we’re
matching.

#### Destructuring Nested Structs and Enums

Until now, all our examples have been matching structs or enums that were one
level deep. Matching can work on nested items too!

For example, we can refactor the code in Listing 18-15 to support RGB and HSV
colors in the `ChangeColor` message, as shown in Listing 18-16.

```rust
{{#rustdoc_include ../listings/ch18-patterns-and-matching/listing-18-16/src/main.rs}}
```

<span class="caption">Listing 18-16: Matching on nested enums</span>

The pattern of the first arm in the `match` expression matches a
`Message::ChangeColor` enum variant that contains a `Color::Rgb` variant; then
the pattern binds to the three inner `i32` values. The pattern of the second
arm also matches a `Message::ChangeColor` enum variant, but the inner enum
matches the `Color::Hsv` variant instead. We can specify these complex
conditions in one `match` expression, even though two enums are involved.

#### Destructuring Structs and Tuples

We can mix, match, and nest destructuring patterns in even more complex ways.
The following example shows a complicated destructure where we nest structs and
tuples inside a tuple and destructure all the primitive values out:

```rust
{{#rustdoc_include ../listings/ch18-patterns-and-matching/no-listing-05-destructuring-structs-and-tuples/src/main.rs:here}}
```

This code lets us break complex types into their component parts so we can use
the values we’re interested in separately.

Destructuring with patterns is a convenient way to use pieces of values, such
as the value from each field in a struct, separately from each other.

### Ignoring Values in a Pattern

You’ve seen that it’s sometimes useful to ignore values in a pattern, such as
in the last arm of a `match`, to get a catchall that doesn’t actually do
anything but does account for all remaining possible values. There are a few
ways to ignore entire values or parts of values in a pattern: using the `_`
pattern (which you’ve seen), using the `_` pattern within another pattern,
using a name that starts with an underscore, or using `..` to ignore remaining
parts of a value. Let’s explore how and why to use each of these patterns.

#### Ignoring an Entire Value with `_`

We’ve used the underscore (`_`) as a wildcard pattern that will match any value
but not bind to the value. Although the underscore `_` pattern is especially
useful as the last arm in a `match` expression, we can use it in any pattern,
including function parameters, as shown in Listing 18-17.

<span class="filename">Filename: src/main.rs</span>

```rust
{{#rustdoc_include ../listings/ch18-patterns-and-matching/listing-18-17/src/main.rs}}
```

<span class="caption">Listing 18-17: Using `_` in a function signature</span>

This code will completely ignore the value passed as the first argument, `3`,
and will print `This code only uses the y parameter: 4`.

In most cases when you no longer need a particular function parameter, you
would change the signature so it doesn’t include the unused parameter. Ignoring
a function parameter can be especially useful in some cases, for example, when
implementing a trait when you need a certain type signature but the function
body in your implementation doesn’t need one of the parameters. The compiler
will then not warn about unused function parameters, as it would if you used a
name instead.

#### Ignoring Parts of a Value with a Nested `_`

We can also use `_` inside another pattern to ignore just part of a value, for
example, when we want to test for only part of a value but have no use for the
other parts in the corresponding code we want to run. Listing 18-18 shows code
responsible for managing a setting’s value. The business requirements are that
the user should not be allowed to overwrite an existing customization of a
setting but can unset the setting and give it a value if it is currently unset.

```rust
{{#rustdoc_include ../listings/ch18-patterns-and-matching/listing-18-18/src/main.rs:here}}
```

<span class="caption">Listing 18-18: Using an underscore within patterns that
match `Some` variants when we don’t need to use the value inside the
`Some`</span>

This code will print `Can't overwrite an existing customized value` and then
`setting is Some(5)`. In the first match arm, we don’t need to match on or use
the values inside either `Some` variant, but we do need to test for the case
when `setting_value` and `new_setting_value` are the `Some` variant. In that
case, we print why we’re not changing `setting_value`, and it doesn’t get
changed.

In all other cases (if either `setting_value` or `new_setting_value` are
`None`) expressed by the `_` pattern in the second arm, we want to allow
`new_setting_value` to become `setting_value`.

We can also use underscores in multiple places within one pattern to ignore
particular values. Listing 18-19 shows an example of ignoring the second and
fourth values in a tuple of five items.

```rust
{{#rustdoc_include ../listings/ch18-patterns-and-matching/listing-18-19/src/main.rs:here}}
```

<span class="caption">Listing 18-19: Ignoring multiple parts of a tuple</span>

This code will print `Some numbers: 2, 8, 32`, and the values 4 and 16 will be
ignored.

#### Ignoring an Unused Variable by Starting Its Name with `_`

If you create a variable but don’t use it anywhere, Rust will usually issue a
warning because that could be a bug. But sometimes it’s useful to create a
variable you won’t use yet, such as when you’re prototyping or just starting a
project. In this situation, you can tell Rust not to warn you about the unused
variable by starting the name of the variable with an underscore. In Listing
18-20, we create two unused variables, but when we compile this code, we should
only get a warning about one of them.

<span class="filename">Filename: src/main.rs</span>

```rust
{{#rustdoc_include ../listings/ch18-patterns-and-matching/listing-18-20/src/main.rs}}
```

<span class="caption">Listing 18-20: Starting a variable name with an
underscore to avoid getting unused variable warnings</span>

Here we get a warning about not using the variable `y`, but we don’t get a
warning about not using the variable preceded by the underscore.

Note that there is a subtle difference between using only `_` and using a name
that starts with an underscore. The syntax `_x` still binds the value to the
variable, whereas `_` doesn’t bind at all. To show a case where this
distinction matters, Listing 18-21 will provide us with an error.

```rust,ignore,does_not_compile
{{#rustdoc_include ../listings/ch18-patterns-and-matching/listing-18-21/src/main.rs:here}}
```

<span class="caption">Listing 18-21: An unused variable starting with an
underscore still binds the value, which might take ownership of the value</span>

We’ll receive an error because the `s` value will still be moved into `_s`,
which prevents us from using `s` again. However, using the underscore by itself
doesn’t ever bind to the value. Listing 18-22 will compile without any errors
because `s` doesn’t get moved into `_`.

```rust
{{#rustdoc_include ../listings/ch18-patterns-and-matching/listing-18-22/src/main.rs:here}}
```

<span class="caption">Listing 18-22: Using an underscore does not bind the
value</span>

This code works just fine because we never bind `s` to anything; it isn’t moved.

#### Ignoring Remaining Parts of a Value with `..`

With values that have many parts, we can use the `..` syntax to use only a few
parts and ignore the rest, avoiding the need to list underscores for each
ignored value. The `..` pattern ignores any parts of a value that we haven’t
explicitly matched in the rest of the pattern. In Listing 18-23, we have a
`Point` struct that holds a coordinate in three-dimensional space. In the
`match` expression, we want to operate only on the `x` coordinate and ignore
the values in the `y` and `z` fields.

```rust
{{#rustdoc_include ../listings/ch18-patterns-and-matching/listing-18-23/src/main.rs:here}}
```

<span class="caption">Listing 18-23: Ignoring all fields of a `Point` except
for `x` by using `..`</span>

We list the `x` value and then just include the `..` pattern. This is quicker
than having to list `y: _` and `z: _`, particularly when we’re working with
structs that have lots of fields in situations where only one or two fields are
relevant.

The syntax `..` will expand to as many values as it needs to be. Listing 18-24
shows how to use `..` with a tuple.

<span class="filename">Filename: src/main.rs</span>

```rust
{{#rustdoc_include ../listings/ch18-patterns-and-matching/listing-18-24/src/main.rs}}
```

<span class="caption">Listing 18-24: Matching only the first and last values in
a tuple and ignoring all other values</span>

In this code, the first and last value are matched with `first` and `last`. The
`..` will match and ignore everything in the middle.

However, using `..` must be unambiguous. If it is unclear which values are
intended for matching and which should be ignored, Rust will give us an error.
Listing 18-25 shows an example of using `..` ambiguously, so it will not
compile.

<span class="filename">Filename: src/main.rs</span>

```rust,ignore,does_not_compile
{{#rustdoc_include ../listings/ch18-patterns-and-matching/listing-18-25/src/main.rs}}
```

<span class="caption">Listing 18-25: An attempt to use `..` in an ambiguous
way</span>

When we compile this example, we get this error:

```console
{{#include ../listings/ch18-patterns-and-matching/listing-18-25/output.txt}}
```

It’s impossible for Rust to determine how many values in the tuple to ignore
before matching a value with `second` and then how many further values to
ignore thereafter. This code could mean that we want to ignore `2`, bind
`second` to `4`, and then ignore `8`, `16`, and `32`; or that we want to ignore
`2` and `4`, bind `second` to `8`, and 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 because using `..` in two places like this is ambiguous.

### Extra Conditionals with Match Guards

A *match guard* is an additional `if` condition specified after the pattern in
a `match` arm that must also match, along with the pattern matching, for that
arm to be chosen. Match guards are useful for expressing more complex ideas
than a pattern alone allows.

The condition can use variables created in the pattern. Listing 18-26 shows a
`match` where the first arm has the pattern `Some(x)` and also has a match
guard of `if x < 5`.

```rust
{{#rustdoc_include ../listings/ch18-patterns-and-matching/listing-18-26/src/main.rs:here}}
```

<span class="caption">Listing 18-26: Adding a match guard to a pattern</span>

This example will print `less than five: 4`. When `num` is compared to the
pattern in the first arm, it matches, because `Some(4)` matches `Some(x)`. Then
the match guard checks whether the value in `x` is less than `5`, and because
it is, the first arm is selected.

If `num` had been `Some(10)` instead, the match guard in the first arm would
have been false because 10 is not less than 5. Rust would then go to the second
arm, which would match because the second arm doesn’t have a match guard and
therefore matches any `Some` variant.

There is no way to express the `if x < 5` condition within a pattern, so the
match guard gives us the ability to express this logic.

In Listing 18-11, we mentioned that we could use match guards to solve our
pattern-shadowing problem. Recall that a new variable was created inside the
pattern in the `match` expression instead of using the variable outside the
`match`. That new variable meant we couldn’t test against the value of the
outer variable. Listing 18-27 shows how we can use a match guard to fix this
problem.

<span class="filename">Filename: src/main.rs</span>

```rust
{{#rustdoc_include ../listings/ch18-patterns-and-matching/listing-18-27/src/main.rs}}
```

<span class="caption">Listing 18-27: Using a match guard to test for equality
with an outer variable</span>

This code will now print `Default case, x = Some(5)`. The pattern in the second
match arm doesn’t introduce a new variable `y` that would shadow the outer `y`,
meaning we can use the outer `y` in the match guard. Instead of specifying the
pattern as `Some(y)`, which would have shadowed the outer `y`, we specify
`Some(n)`. This creates a new variable `n` that doesn’t shadow anything because
there is no `n` variable outside the `match`.

The match guard `if n == y` is not a pattern and therefore doesn’t introduce
new variables. This `y` *is* the outer `y` rather than a new shadowed `y`, and
we can look for a value that has the same value as the outer `y` by comparing
`n` to `y`.

You can also use the *or* operator `|` in a match guard to specify multiple
patterns; the match guard condition will apply to all the patterns. Listing
18-28 shows the precedence of combining a match guard with a pattern that uses
`|`. The important part of this example is that the `if y` match guard applies
to `4`, `5`, *and* `6`, even though it might look like `if y` only applies to
`6`.

```rust
{{#rustdoc_include ../listings/ch18-patterns-and-matching/listing-18-28/src/main.rs:here}}
```

<span class="caption">Listing 18-28: Combining multiple patterns with a match
guard</span>

The match condition states that the arm only matches if the value of `x` is
equal to `4`, `5`, or `6` *and* if `y` is `true`. When this code runs, the
pattern of the first arm matches because `x` is `4`, but the match guard `if y`
is false, so the first arm is not chosen. The code moves on to the second arm,
which does match, and this program prints `no`. The reason is that 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) => ...
```

After running the code, the precedence behavior is evident: if the match guard
were applied only to the final value in the list of values specified using the
`|` operator, the arm would have matched and the program would have printed
`yes`.

### `@` Bindings

The *at* operator (`@`) lets us create a variable that holds a value at the
same time we’re testing that value to see whether it matches a pattern. Listing
18-29 shows an example where we want to test that a `Message::Hello` `id` field
is within the range `3..=7`. But we also want to bind the value to the variable
`id_variable` so we can use it in the code associated with the arm. We could
name this variable `id`, the same as the field, but for this example we’ll use
a different name.

```rust
{{#rustdoc_include ../listings/ch18-patterns-and-matching/listing-18-29/src/main.rs:here}}
```

<span class="caption">Listing 18-29: 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_variable
@` before the range `3..=7`, we’re capturing whatever value matched the range
while also testing that the value matched the range pattern.

In the second arm, where we only have a range specified in the pattern, the code
associated with the arm doesn’t have a variable that contains the actual value
of the `id` field. The `id` field’s value could have been 10, 11, or 12, but
the code that goes with that pattern doesn’t know which it is. The pattern code
isn’t able to use the value from the `id` field, because we haven’t saved the
`id` value in a variable.

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 in a variable named `id`. The
reason is that we’ve used the struct field shorthand syntax. But we haven’t
applied any test to the value in the `id` field in this arm, as we did with the
first two arms: any value would match this pattern.

Using `@` lets us test a value and save it in a variable within one pattern.

## Summary

Rust’s patterns are very useful in that they help distinguish between different
kinds of data. When used in `match` expressions, Rust ensures your patterns
cover every possible value, or your program won’t compile. Patterns in `let`
statements and function parameters make those constructs more useful, enabling
the destructuring of values into smaller parts at the same time as assigning to
variables. We can create simple or complex patterns to suit our needs.

Next, for the penultimate chapter of the book, we’ll look at some advanced
aspects of a variety of Rust’s features.