The if let syntax lets you combine if and let into a less verbose way to
handle values that match one pattern while ignoring the rest. Consider the
program in Listing 6-6 that matches on an Option<u8> value in the
config_max variable but only wants to execute code if the value is the Some
variant.
{{#rustdoc_include ../listings/ch06-enums-and-pattern-matching/listing-06-06/src/main.rs:here}}Listing 6-6: A match that only cares about executing
code when the value is Some
If the value is Some, we print out the value in the Some variant by binding
the value to the variable max in the pattern. We don’t want to do anything
with the None value. To satisfy the match expression, we have to add _ => () after processing just one variant, which is annoying boilerplate code to
add.
Instead, we could write this in a shorter way using if let. The following
code behaves the same as the match in Listing 6-6:
{{#rustdoc_include ../listings/ch06-enums-and-pattern-matching/no-listing-12-if-let/src/main.rs:here}}The syntax if let takes a pattern and an expression separated by an equal
sign. It works the same way as a match, where the expression is given to the
match and the pattern is its first arm. In this case, the pattern is
Some(max), and the max binds to the value inside the Some. We can then
use max in the body of the if let block in the same way we used max in
the corresponding match arm. The code in the if let block isn’t run if the
value doesn’t match the pattern.
Using if let means less typing, less indentation, and less boilerplate code.
However, you lose the exhaustive checking that match enforces. Choosing
between match and if let depends on what you’re doing in your particular
situation and whether gaining conciseness is an appropriate trade-off for
losing exhaustive checking.
In other words, you can think of if let as syntax sugar for a match that
runs code when the value matches one pattern and then ignores all other values.
We can include an else with an if let. The block of code that goes with the
else is the same as the block of code that would go with the _ case in the
match expression that is equivalent to the if let and else. Recall the
Coin enum definition in Listing 6-4, where the Quarter variant also held a
UsState value. If we wanted to count all non-quarter coins we see while also
announcing the state of the quarters, we could do that with a match
expression, like this:
{{#rustdoc_include ../listings/ch06-enums-and-pattern-matching/no-listing-13-count-and-announce-match/src/main.rs:here}}Or we could use an if let and else expression, like this:
{{#rustdoc_include ../listings/ch06-enums-and-pattern-matching/no-listing-14-count-and-announce-if-let-else/src/main.rs:here}}If you have a situation in which your program has logic that is too verbose to
express using a match, remember that if let is in your Rust toolbox as well.
We’ve now covered how to use enums to create custom types that can be one of a
set of enumerated values. We’ve shown how the standard library’s Option<T>
type helps you use the type system to prevent errors. When enum values have
data inside them, you can use match or if let to extract and use those
values, depending on how many cases you need to handle.
Your Rust programs can now express concepts in your domain using structs and enums. Creating custom types to use in your API ensures type safety: the compiler will make certain your functions only get values of the type each function expects.
In order to provide a well-organized API to your users that is straightforward to use and only exposes exactly what your users will need, let’s now turn to Rust’s modules.