Patterns come in two forms: refutable and irrefutable. Patterns that will match
for any possible value passed are said to be irrefutable. An example would be
x in the statement let x = 5; because x matches anything and so cannot
fail to match. Patterns that may fail to match for some possible value are said
to be refutable. An example of this would be Some(x) in the expression if let Some(x) = a_value; if the value in the a_value variable is None rather
than Some, then the Some(x) pattern would not match.
let statements, function parameters, and for loops can only accept
irrefutable patterns, because the program cannot do anything
meaningful with values that don’t match. The if let and while let
expressions are restricted to only accept refutable patterns, because by
definition they’re intended to handle possible failure---the functionality of a
conditional is in its ability to perform differently upon success and failure.
In general, you shouldn’t have to worry about the distinction between refutable and irrefutable patterns, but you do need to be familiar with the concept of refutability so you can respond when you see it in an error message. In those cases, you’ll need to change either the pattern or the construct you’re using the pattern with, depending on your intentions for the behavior of the code.
Let’s look at an example of what happens if we try to use a refutable pattern
where Rust requires an irrefutable pattern and vice versa. In Listing 18-8, we
have a let statement, but for the pattern we’ve specified Some(x), a
refutable pattern. As you might expect, this will error:
let Some(x) = some_option_value;Listing 18-8: Attempting to use a refutable pattern with
let
If some_option_value was a None value, it would fail to match the pattern
Some(x), meaning the pattern is refutable. The let statement, however, can
only accept an irrefutable patterns because there’s nothing valid the code
could do with a None value. At compile time, Rust will complain that we’ve
tried to use a refutable pattern where an irrefutable pattern is required:
error[E0005]: refutable pattern in local binding: `None` not covered
--> <anon>:3:5
|
3 | let Some(x) = some_option_value;
| ^^^^^^^ pattern `None` not covered
We didn’t cover (and couldn’t cover!) every valid value with the pattern
Some(x), so Rust will rightfully complain.
To fix the case where we have a refutable pattern in a place where an
irrefutable pattern is needed, we can change the code that uses the pattern:
instead of using let, we can use if let. That way, if the pattern doesn’t
match, the code will just skip the code in the curly brackets, giving it a way
to continue validly. Listing 18-9 shows how to fix the code in Listing 18-8.
# let some_option_value: Option<i32> = None;
if let Some(x) = some_option_value {
println!("{}", x);
}Listing 18-9: Using if let and a block with refutable
patterns instead of let
We’ve given the code an out! This code is perfectly valid, though does now of
course mean we cannot use an irrefutable pattern without receiving an error. If
we give if let a pattern that will always match, such as x as shown in
Listing 18-10, it will error:
if let x = 5 {
println!("{}", x);
};Listing 18-10: Attempting to use an irrefutable pattern
with if let
Rust complains that it doesn’t make sense to use if let with an irrefutable
pattern:
error[E0162]: irrefutable if-let pattern
--> <anon>:2:8
|
2 | if let x = 5 {
| ^ irrefutable pattern
For this reason, match arms must use refutable patterns, except for the last
arm that should match any remaining values with an irrefutable pattern. Using
an irrefutable pattern in a match with only one arm is allowed, but isn’t
particularly useful and could be replaced with a simpler let statement.
Now that we’ve discussed where patterns can be used and the difference between refutable and irrefutable patterns, let’s go over all the syntax we can use to create patterns.