Options1 question

[Fr3027]

fn maybe_icecream(time_of_day: u16) -> Option<u16> {
    if time_of_day<22{
        Some(5)
    }
    if time_of_day<24{
        Some(0)
    }
    None
}

The compiler throws an error: "expected (), found enum Option".
My point is if any of the conditions match then the function will return Some(), if not, the function will return None by default.
This style is supported by java, which I can't understand why rust not, help me please!

[aleksanderkrauze]

You can either do

fn maybe_icecream(time_of_day: u16) -> Option<u16> {
    if time_of_day<22{
        return Some(5);
    }
    if time_of_day<24{
        return Some(0);
    }
    None
}

or

fn maybe_icecream(time_of_day: u16) -> Option<u16> {
    if time_of_day<22{
        Some(5)
    } else if time_of_day<24{
        Some(0)
    } else {
        None
    }
}

The compiler even suggests the first solution:

help: you might have meant to return this value
  |
3 |         return Some(5);
  |         ++++++        +

The reason for this is that in rust if statements are expressions. In your example those if branches are unrelated to each other. Therefore rust looks at them in isolation and treats them like statements which must return unit type (). You can solve this in two ways.

You can add return keywords (like compiler suggests, also my first example). This will make if expression return () as they should¹.

Or you can combine three branches in one expression (like in my second example). Then each branch of this expression will return Option<u16> so the whole expression will evaluate to Option<u16> and this will be returned from your function.

Footnotes

1. In reality this is more subtle. return is also an expression that returns never type, which will then be coerced into unit type. ↩