Represent absence

Use Option[T] when a function may have no value to return. It has two variants: Some(value) holds a value of type T; Nothing has no payload.

def positive(n: i64) -> Option[i64]:
    Some(n) if n > 0 else Nothing

def show(n: i64) -> Result[(), IoError]:
    match positive(n):
        case Some(value):
            print(value)?
        case Nothing:
            print("not found")?
    Ok(())

def main() -> Result[(), IoError]:
    show(-1)?
    show(42)?
    Ok(())

not found
42

In case Some(value), the pattern binds the payload to value within that branch. A match must cover both Some and Nothing.

Some and Nothing are built in and need no type prefix. Nothing belongs to one concrete option type rather than acting as a universal null. A present zero, False, or empty string is still Some(value).

When only presence matters, ask with is_some() or is_none() instead of writing a match. Both borrow the option, so it is still usable afterwards.

def main() -> Result[(), IoError]:
    found: Option[str] = Some("key")
    missing: Option[str] = Nothing
    print(found.is_some(), found.is_none())?
    print(missing.is_some(), missing.is_none())?
    print(found.unwrap_or("default"))?
    Ok(())

True False
False True
key