Enums and optional values

A missing mark is different from a mark of zero. Option expresses either Some(value) or None, making absence visible in the type. An enum defines a finite set of variants, each of which can carry its own data.

Match handles the variants explicitly. if let handles one selected pattern when the other cases need no action. Option does not automatically unwrap itself; the program must choose what absence means. A custom enum can distinguish states such as NotSubmitted and Marked(score) more clearly than unexplained numbers.

Save the complete source as main.rs in the Rust environment. No standard input is needed.

Rust 2024 · complete program

enum Submission { NotSubmitted, Marked(u32) }
fn label(value: Submission) -> String {
  match value {
    Submission::NotSubmitted => "Awaiting work".to_string(),
    Submission::Marked(score) => format!("Mark: {score}"),
  }
}
fn main() {
  println!("{}", label(Submission::Marked(80)));
  println!("{}", label(Submission::NotSubmitted));
  let missing: Option<u32> = None;
  println!("Recorded: {}", missing.is_some());
}

Expected output

Mark: 80
Awaiting work
Recorded: false

Follow the execution

The Marked variant carries an integer that the pattern binds to score. NotSubmitted carries no mark and produces a separate message. None indicates that no integer was recorded; it does not silently become zero.

Try the complete program in the Rust compiler. For tests, Cargo commands and local files, use the specified local environment.

Your task: Should an absent submission automatically receive the recorded mark zero?

Rust 2024 · complete program

enum Submission { NotSubmitted, Marked(u32) }
fn label(value: Submission) -> String {
  match value {
    Submission::NotSubmitted => "Awaiting work".to_string(),
    Submission::Marked(score) => format!("Mark: {score}"),
  }
}
fn main() {
  println!("{}", label(Submission::Marked(80)));
  println!("{}", label(Submission::NotSubmitted));
  let missing: Option<u32> = None;
  println!("Recorded: {}", missing.is_some());
}

Worked answer

Only if the school explicitly chooses that policy. Absence and a genuine zero are different states, so keep them distinct until the reporting rule decides what to do.

Unwrap on None panics. A fallback is useful only when its meaning is valid for the domain. Exhaustive matching can make a newly added enum variant trigger a useful compiler reminder to update handling code.

Only if the school explicitly chooses that policy. Absence and a genuine zero are different states, so keep them distinct until the reporting rule decides what to do. Unwrap on None panics.

Further reading: the official Rust Book on enums and optional values.