The Rust Programming Language

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),
        _ => (),
    }
}

Listing 6-6: A match that only cares about executing code when the value is Some

If the value is Some, we want to print out the value in the Some variant, which we do 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 as 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 get only 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.