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:
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.
This slide should take about 4 minutes.
- The
if
/else
expression is returning an enum that is later unpacked with amatch
. - 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.