Concise Control Flow with if let
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.
fn main() { let config_max = Some(3u8); match config_max { Some(max) => println!("The maximum is configured to be {}", max), _ => (), } }
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:
fn main() { let config_max = Some(3u8); if let Some(max) = config_max { println!("The maximum is configured to be {}", max); } }
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:
#[derive(Debug)] enum UsState { Alabama, Alaska, // --snip-- } enum Coin { Penny, Nickel, Dime, Quarter(UsState), } fn main() { let coin = Coin::Penny; let mut count = 0; match coin { Coin::Quarter(state) => println!("State quarter from {:?}!", state), _ => count += 1, } }
Or we could use an if let
and else
expression, like this:
#[derive(Debug)] enum UsState { Alabama, Alaska, // --snip-- } enum Coin { Penny, Nickel, Dime, Quarter(UsState), } fn main() { let coin = Coin::Penny; let mut count = 0; if let Coin::Quarter(state) = coin { println!("State quarter from {:?}!", state); } else { count += 1; } }
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.
Summary
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.