Rust has an extremely powerful control flow construct called match that
allows you to compare a value against a series of patterns and then execute
code based on which pattern matches. Patterns can be made up of literal values,
variable names, wildcards, and many other things; Chapter
18 covers all the different kinds of patterns
and what they do. The power of match comes from the expressiveness of the
patterns and the fact that the compiler confirms that all possible cases are
handled.
Think of a match expression as being like a coin-sorting machine: coins slide
down a track with variously sized holes along it, and each coin falls through
the first hole it encounters that it fits into. In the same way, values go
through each pattern in a match, and at the first pattern the value “fits,”
the value falls into the associated code block to be used during execution.
Speaking of coins, let’s use them as an example using match! We can write a
function that takes an unknown US coin and, in a similar way as the counting
machine, determines which coin it is and returns its value in cents, as shown
in Listing 6-3.
{{#rustdoc_include ../listings/ch06-enums-and-pattern-matching/listing-06-03/src/main.rs:here}}Let’s break down the match in the value_in_cents function. First we list
the match keyword followed by an expression, which in this case is the value
coin. This seems very similar to a conditional expression used with if, but
there’s a big difference: with if, the condition needs to evaluate to a
Boolean value, but here it can be any type. The type of coin in this example
is the Coin enum that we defined on the first line.
Next are the match arms. An arm has two parts: a pattern and some code. The
first arm here has a pattern that is the value Coin::Penny and then the =>
operator that separates the pattern and the code to run. The code in this case
is just the value 1. Each arm is separated from the next with a comma.
When the match expression executes, it compares the resultant value against
the pattern of each arm, in order. If a pattern matches the value, the code
associated with that pattern is executed. If that pattern doesn’t match the
value, execution continues to the next arm, much as in a coin-sorting machine.
We can have as many arms as we need: in Listing 6-3, our match has four arms.
The code associated with each arm is an expression, and the resultant value of
the expression in the matching arm is the value that gets returned for the
entire match expression.
We don’t typically use curly brackets if the match arm code is short, as it is
in Listing 6-3 where each arm just returns a value. If you want to run multiple
lines of code in a match arm, you must use curly brackets, and the comma
following the arm is then optional. For example, the following code prints
“Lucky penny!” every time the method is called with a Coin::Penny, but still
returns the last value of the block, 1:
{{#rustdoc_include ../listings/ch06-enums-and-pattern-matching/no-listing-08-match-arm-multiple-lines/src/main.rs:here}}Another useful feature of match arms is that they can bind to the parts of the values that match the pattern. This is how we can extract values out of enum variants.
As an example, let’s change one of our enum variants to hold data inside it.
From 1999 through 2008, the United States minted quarters with different
designs for each of the 50 states on one side. No other coins got state
designs, so only quarters have this extra value. We can add this information to
our enum by changing the Quarter variant to include a UsState value
stored inside it, which we’ve done in Listing 6-4.
{{#rustdoc_include ../listings/ch06-enums-and-pattern-matching/listing-06-04/src/main.rs:here}}Let’s imagine that a friend is trying to collect all 50 state quarters. While we sort our loose change by coin type, we’ll also call out the name of the state associated with each quarter so that if it’s one our friend doesn’t have, they can add it to their collection.
In the match expression for this code, we add a variable called state to the
pattern that matches values of the variant Coin::Quarter. When a
Coin::Quarter matches, the state variable will bind to the value of that
quarter’s state. Then we can use state in the code for that arm, like so:
{{#rustdoc_include ../listings/ch06-enums-and-pattern-matching/no-listing-09-variable-in-pattern/src/main.rs:here}}If we were to call value_in_cents(Coin::Quarter(UsState::Alaska)), coin
would be Coin::Quarter(UsState::Alaska). When we compare that value with each
of the match arms, none of them match until we reach Coin::Quarter(state). At
that point, the binding for state will be the value UsState::Alaska. We can
then use that binding in the println! expression, thus getting the inner
state value out of the Coin enum variant for Quarter.
In the previous section, we wanted to get the inner T value out of the Some
case when using Option<T>; we can also handle Option<T> using match, as
we did with the Coin enum! Instead of comparing coins, we’ll compare the
variants of Option<T>, but the way the match expression works remains the
same.
Let’s say we want to write a function that takes an Option<i32> and, if
there’s a value inside, adds 1 to that value. If there isn’t a value inside,
the function should return the None value and not attempt to perform any
operations.
This function is very easy to write, thanks to match, and will look like
Listing 6-5.
{{#rustdoc_include ../listings/ch06-enums-and-pattern-matching/listing-06-05/src/main.rs:here}}Let’s examine the first execution of plus_one in more detail. When we call
plus_one(five), the variable x in the body of plus_one will have the
value Some(5). We then compare that against each match arm:
{{#rustdoc_include ../listings/ch06-enums-and-pattern-matching/listing-06-05/src/main.rs:first_arm}}The Some(5) value doesn’t match the pattern None, so we continue to the
next arm:
{{#rustdoc_include ../listings/ch06-enums-and-pattern-matching/listing-06-05/src/main.rs:second_arm}}Does Some(5) match Some(i)? It does! We have the same variant. The i
binds to the value contained in Some, so i takes the value 5. The code in
the match arm is then executed, so we add 1 to the value of i and create a
new Some value with our total 6 inside.
Now let’s consider the second call of plus_one in Listing 6-5, where x is
None. We enter the match and compare to the first arm:
{{#rustdoc_include ../listings/ch06-enums-and-pattern-matching/listing-06-05/src/main.rs:first_arm}}It matches! There’s no value to add to, so the program stops and returns the
None value on the right side of =>. Because the first arm matched, no other
arms are compared.
Combining match and enums is useful in many situations. You’ll see this
pattern a lot in Rust code: match against an enum, bind a variable to the
data inside, and then execute code based on it. It’s a bit tricky at first, but
once you get used to it, you’ll wish you had it in all languages. It’s
consistently a user favorite.
There’s one other aspect of match we need to discuss: the arms’ patterns must
cover all possibilities. Consider this version of our plus_one function,
which has a bug and won’t compile:
{{#rustdoc_include ../listings/ch06-enums-and-pattern-matching/no-listing-10-non-exhaustive-match/src/main.rs:here}}We didn’t handle the None case, so this code will cause a bug. Luckily, it’s
a bug Rust knows how to catch. If we try to compile this code, we’ll get this
error:
{{#include ../listings/ch06-enums-and-pattern-matching/no-listing-10-non-exhaustive-match/output.txt}}Rust knows that we didn’t cover every possible case, and even knows which
pattern we forgot! Matches in Rust are exhaustive: we must exhaust every last
possibility in order for the code to be valid. Especially in the case of
Option<T>, when Rust prevents us from forgetting to explicitly handle the
None case, it protects us from assuming that we have a value when we might
have null, thus making the billion-dollar mistake discussed earlier impossible.
Using enums, we can also take special actions for a few particular values, but
for all other values take one default action. Imagine we’re implementing a game
where, if you roll a 3 on a dice roll, your player doesn’t move, but instead
gets a new fancy hat. If you roll a 7, your player loses a fancy hat. For all
other values, your player moves that number of spaces on the game board. Here’s
a match that implements that logic, with the result of the dice roll
hardcoded rather than a random value, and all other logic represented by
functions without bodies because actually implementing them is out of scope for
this example:
{{#rustdoc_include ../listings/ch06-enums-and-pattern-matching/no-listing-15-binding-catchall/src/main.rs:here}}For the first two arms, the patterns are the literal values 3 and 7. For
the last arm that covers every other possible value, the pattern is the
variable we’ve chosen to name other. The code that runs for the other arm
uses the variable by passing it to the move_player function.
This code compiles, even though we haven’t listed all the possible values a
u8 can have, because the last pattern will match all values not specifically
listed. This catch-all pattern meets the requirement that match must be
exhaustive. Note that we have to put the catch-all arm last because the
patterns are evaluated in order. If we put the catch-all arm earlier, the other
arms would never run, so Rust will warn us if we add arms after a catch-all!
Rust also has a pattern we can use when we want a catch-all but don’t want to
use the value in the catch-all pattern: _ is a special pattern that matches
any value and does not bind to that value. This tells Rust we aren’t going to
use the value, so Rust won’t warn us about an unused variable.
Let’s change the rules of the game: now, if you roll anything other than a 3 or
a 7, you must roll again. We no longer need to use the catch-all value, so we
can change our code to use _ instead of the variable named other:
{{#rustdoc_include ../listings/ch06-enums-and-pattern-matching/no-listing-16-underscore-catchall/src/main.rs:here}}This example also meets the exhaustiveness requirement because we’re explicitly ignoring all other values in the last arm; we haven’t forgotten anything.
Finally, we’ll change the rules of the game one more time so that nothing else
happens on your turn if you roll anything other than a 3 or a 7. We can express
that by using the unit value (the empty tuple type we mentioned in “The Tuple
Type” section) as the code that goes with the _ arm:
{{#rustdoc_include ../listings/ch06-enums-and-pattern-matching/no-listing-17-underscore-unit/src/main.rs:here}}Here, we’re telling Rust explicitly that we aren’t going to use any other value that doesn’t match a pattern in an earlier arm, and we don’t want to run any code in this case.
There’s more about patterns and matching that we’ll cover in Chapter
19. For now, we’re going to move on to the
if let syntax, which can be useful in situations where the match expression
is a bit wordy.