Enumerations

While a structure allows us to get multiple values under the same variable, enumerations allow us to choose one value from different types of values.

For example, let's write a type representing an expression:

enum Expr {
    Null,
    Add(i32, i32),
    Sub(i32, i32),
    Mul(i32, i32),
    Div { pidend: i32, pisor: i32 },
    Val(i32),
}

let quotient = Expr::Div { pidend: 10, pisor: 2 };
let sum = Expr::Add(40, 2);

The Null variant does not have a value associated with it, Val has one associated value, and Add has two. Div also has two associated values, but they are named, similar to how we define a structure.