What are Some and None?

rust optional
Syntactic Fructose picture Syntactic Fructose · Jul 16, 2014 · Viewed 18.8k times · Source

I came across some output I don't understand using Vec::get. Here's the code:

fn main() {
    let command = [('G', 'H'), ('H', '5')];

    for i in 0..3 {
        print!(" {} ", i);
        println!("{:?}", command.get(i));
    }
}

the output is

 0 Some(('G', 'H'))
 1 Some(('H', '5'))
 2 None

I've dabbled in Haskell before, and by that I mean looked at a tutorial site for 10 minutes and ran back to C++, but I remember reading something about Some and None for Haskell. I was surprised to see this here in Rust. Could someone explain why .get() returns Some or None?

Answer

huon picture huon · Jul 16, 2014

The signature of get (for slices, not Vec, since you're using an array/slice) is

fn get(&self, index: usize) -> Option<&T>

That is, it returns an Option, which is an enum defined like

pub enum Option<T> {
    None,
    Some(T),
}

None and Some are the variants of the enum, that is, a value with type Option<T> can either be a None, or it can be a Some containing a value of type T. You can create the Option enum using the variants as well:

let foo = Some(42);
let bar = None;

This is the same as the core data Maybe a = Nothing | Just a type in Haskell; both represent an optional value, it's either there (Some/Just), or it's not (None/Nothing).

These types are often used to represent failure when there's only one possibility for why something failed, for example, .get uses Option to give type-safe bounds-checked array access: it returns None (i.e. no data) when the index is out of bounds, otherwise it returns a Some containing the requested pointer.

See also:

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