Concise Control Flow with if let and let...else - The Rust Programming Language

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## Concise Control Flow with `if let` and `let...else`

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 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 only runs if the value matches the pattern.

Using `if let` means less typing, less indentation, and less boilerplate code. However, you lose the exhaustive checking `match` enforces that ensures that you aren’t forgetting to handle any cases. 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;
        }
    }

## Staying on the “Happy Path” with `let...else`

The common pattern is to perform some computation when a value is present and return a default value otherwise. Continuing with our example of coins with a `UsState` value, if we wanted to say something funny depending on how old the state on the quarter was, we might introduce a method on `UsState` to check the age of a state, like so:

    #[derive(Debug)] // so we can inspect the state in a minute
    enum UsState {
        Alabama,
        Alaska,
        // --snip--
    }

    impl UsState {
        fn existed_in(&self, year: u16) -> bool {
            match self {
                UsState::Alabama => year >= 1819,
                UsState::Alaska => year >= 1959,
                // -- snip --
            }
        }
    }

    enum Coin {
        Penny,
        Nickel,
        Dime,
        Quarter(UsState),
    }

    fn describe_state_quarter(coin: Coin) -> Option<String> {
        if let Coin::Quarter(state) = coin {
            if state.existed_in(1900) {
                Some(format!("{state:?} is pretty old, for America!"))
            } else {
                Some(format!("{state:?} is relatively new."))
            }
        } else {
            None
        }
    }

    fn main() {
        if let Some(desc) = describe_state_quarter(Coin::Quarter(UsState::Alaska)) {
            println!("{desc}");
        }
    }

Then, we might use `if let` to match on the type of coin, introducing a `state` variable within the body of the condition, as in Listing 6-7.

    #[derive(Debug)] // so we can inspect the state in a minute
    enum UsState {
        Alabama,
        Alaska,
        // --snip--
    }

    impl UsState {
        fn existed_in(&self, year: u16) -> bool {
            match self {
                UsState::Alabama => year >= 1819,
                UsState::Alaska => year >= 1959,
                // -- snip --
            }
        }
    }

    enum Coin {
        Penny,
        Nickel,
        Dime,
        Quarter(UsState),
    }

    fn describe_state_quarter(coin: Coin) -> Option<String> {
        if let Coin::Quarter(state) = coin {
            if state.existed_in(1900) {
                Some(format!("{state:?} is pretty old, for America!"))
            } else {
                Some(format!("{state:?} is relatively new."))
            }
        } else {
            None
        }
    }

    fn main() {
        if let Some(desc) = describe_state_quarter(Coin::Quarter(UsState::Alaska)) {
            println!("{desc}");
        }
    }

Listing 6-7: Checking whether a state existed in 1900 by using conditionals nested inside an `if let`

That gets the job done, but it has pushed the work into the body of the `if let` statement, and if the work to be done is more complicated, it might be hard to follow exactly how the top-level branches relate. We could also take advantage of the fact that expressions produce a value either to produce the `state` from the `if let` or to return early, as in Listing 6-8. (You could do something similar with a `match`, too.)

    #[derive(Debug)] // so we can inspect the state in a minute
    enum UsState {
        Alabama,
        Alaska,
        // --snip--
    }

    impl UsState {
        fn existed_in(&self, year: u16) -> bool {
            match self {
                UsState::Alabama => year >= 1819,
                UsState::Alaska => year >= 1959,
                // -- snip --
            }
        }
    }

    enum Coin {
        Penny,
        Nickel,
        Dime,
        Quarter(UsState),
    }

    fn describe_state_quarter(coin: Coin) -> Option<String> {
        let state = if let Coin::Quarter(state) = coin {
            state
        } else {
            return None;
        };

        if state.existed_in(1900) {
            Some(format!("{state:?} is pretty old, for America!"))
        } else {
            Some(format!("{state:?} is relatively new."))
        }
    }

    fn main() {
        if let Some(desc) = describe_state_quarter(Coin::Quarter(UsState::Alaska)) {
            println!("{desc}");
        }
    }

Listing 6-8: Using `if let` to produce a value or return early

This is a bit annoying to follow in its own way, though! One branch of the `if let` produces a value, and the other one returns from the function entirely.

To make this common pattern nicer to express, Rust has `let...else`. The `let...else` syntax takes a pattern on the left side and an expression on the right, very similar to `if let`, but it does not have an `if` branch, only an `else` branch. If the pattern matches, it will bind the value from the pattern in the outer scope. If the pattern does not match, the program will flow into the `else` arm, which must return from the function.

In Listing 6-9, you can see how Listing 6-8 looks when using `let...else` in place of `if let`.

    #[derive(Debug)] // so we can inspect the state in a minute
    enum UsState {
        Alabama,
        Alaska,
        // --snip--
    }

    impl UsState {
        fn existed_in(&self, year: u16) -> bool {
            match self {
                UsState::Alabama => year >= 1819,
                UsState::Alaska => year >= 1959,
                // -- snip --
            }
        }
    }

    enum Coin {
        Penny,
        Nickel,
        Dime,
        Quarter(UsState),
    }

    fn describe_state_quarter(coin: Coin) -> Option<String> {
        let Coin::Quarter(state) = coin else {
            return None;
        };

        if state.existed_in(1900) {
            Some(format!("{state:?} is pretty old, for America!"))
        } else {
            Some(format!("{state:?} is relatively new."))
        }
    }

    fn main() {
        if let Some(desc) = describe_state_quarter(Coin::Quarter(UsState::Alaska)) {
            println!("{desc}");
        }
    }

Listing 6-9: Using `let...else` to clarify the flow through the function

Notice that it stays on the “happy path” in the main body of the function this way, without having significantly different control flow for two branches the way the `if let` did.

If you have a situation in which your program has logic that is too verbose to express using a `match`, remember that `if let` and `let...else` are 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.