Enums

Like tuples, enums can also be destructured by matching:

Patterns can also be used to bind variables to parts of your values. This is how you inspect the structure of your types. Let us start with a simple enum type:

# // Copyright 2022 Google LLC
# // SPDX-License-Identifier: Apache-2.0
#
enum Result {
    Ok(i32),
    Err(String),
}

fn divide_in_two(n: i32) -> Result {
    if n % 2 == 0 {
        Result::Ok(n / 2)
    } else {
        Result::Err(format!("cannot divide {n} into two equal parts"))
    }
}

fn main() {
    let n = 100;
    match divide_in_two(n) {
        Result::Ok(half) => println!("{n} divided in two is {half}"),
        Result::Err(msg) => println!("sorry, an error happened: {msg}"),
    }
}

Here we have used the arms to destructure the Result value. In the first arm, half is bound to the value inside the Ok variant. In the second arm, msg is bound to the error message.

• The if/else expression is returning an enum that is later unpacked with a match.
• You can try adding a third variant to the enum definition and displaying the errors when running the code. Point out the places where your code is now inexhaustive and how the compiler tries to give you hints.
• The values in the enum variants can only be accessed after being pattern matched.
• Demonstrate what happens when the search is inexhaustive. Note the advantage the Rust compiler provides by confirming when all cases are handled.
• Demonstrate the syntax for a struct-style variant by adding one to the enum definition and the match. Point out how this is syntactically similar to matching on a struct.