34.2 Automatic propagation

The question mark operator (?) is used to easily propagate some enumerate values. The action of the operator is to:

• Either return the contained value.

• Or propagate the other case (i.e.: return the other value like Err, None, etc.).

It works with the following constructs:

• A Result.

• A Option.

• A ControlFlow.

• A Poll.

Let us redefine our custom error enum for a factorial function:

:clear

#[derive(Debug)]
enum MyError {
  ForbiddenNumber,
}

The factorial function is a bit simpler when dealing the case of the actual computing:

fn fact(n: i64) -> Result<i64, MyError> {
  if n < 0 {
    Err(MyError::ForbiddenNumber)
  } else if n < 2 {
    Ok(1)
  } else {
    Ok(n * fact(n-1)?)
  }
}

The combination function is drastically simpler, with a one-liner evaluation:

fn combination(n: i64, r: i64) -> Result<i64, MyError> {
  Ok(fact(n)? / fact(r)? / fact(n-r)?)
}

The result is exactly the same as the manual propagation:

combination(12, 15)