What do the keywords "Some" and "Simple" do in Rust?

Question

I'm trying to get more in the habit of programming by giving myself a directed task, and the one I found to create a toy layout engine seemed to be a good match. Since I'm focusing on learning Python, I figured it would be good practice to convert the tutorial into Python. I figured at the same time, this would teach me something about Rust, and about reading code in general. A win all around!

I'm having a difficult time understanding what the keywords (are they even keywords?) Some and Simple do. They show up in the code presented:

enum Selector {
    Simple(SimpleSelector),
}

struct SimpleSelector {
    tag_name: Option<String>,
    id: Option<String>,
    class: Vec<String>,
}

I gather that an enum is a way of storing data that may be (exactly) one of several possible types, but I don't see what this means here.

Another thing that shows up in the author's code is (for example)

match self.next_char() {
    '#' => {
        self.consume_char();
        selector.id = Some(self.parse_identifier());
    }

In this case, I have no idea what the term Some does. I have tried looking through the official Rust documentation but I cannot find a description of these terms, even though Some is used in the documentation!

What do these terms do? More generally is there a list of Rust keywords? No searching for "rust programming language keywords" seems to be helping.

Answer 1

Rust features Algebraic Data Types which in short are data types with several possible shapes, for example:

 enum OptionInt {
     None,
     Some(i32),
 }

Is a data type which is either None (a singleton value) or Some(i32) (an i32). In this case, None and Some are data constructors: when applied to a value (or without any value in the case of None) they produce a value of type OptionInt.

Those data constructors will also appear in pattern-matching:

match an_option_int {
    Some(an_integer) => 3 * an_integer,
    None => 0
}

is an expression that will produce an i32 which is either:

• 0 if an_option_int contained None
• or 6 if an_option_int contained Some(2)

This features is also known as tagged unions.

Answer 2

These are not keywords, they are giving names to the variants of the enum. Relevant section in the guide. The list of keywords is in the reference.