What are Some and None?

Question

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 1

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:

• Why don't Option's Some and None variants need to be qualified?
• What is the difference between Some and Option in Rust?