What are Some and None?

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?

The signature of get (for slices, not Vec, since you're using an array/slice) is
fn get(&self, index: usize) -> Option


That is, it returns an Option, which is an enum defined like
pub enum Option {
None,
Some(T),
}


None and Some are the variants of the enum, that is, a value with type Option 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:

• Why don't Option's Some and None variants need to be qualified?

• What is the difference between Some and Option in Rust?

Stack Overflow Q#24771655, score: 110