Enums

Sometimes we need to handle multiple different types at runtime. Enums are a powerful tool that allows us to safely and robustly describe situations like this.

use std::collections::HashMap;

fn main() {
    let number = parse_json("123");
    let array = parse_json("[456, true, false]");
    let object = parse_json(r#"{ "key": "value" }"#);
}

fn parse_json(doc: &str) -> JsonValue {
    todo!("Parse the JSON string...")
}

enum JsonValue {
    Object(HashMap<String, JsonValue>),
    Array(Vec<JsonValue>),
    String(String),
    Number(f64),
    Bool(bool),
    Null,
}

The other category of polymorphism is dynamic polymorphism, where we can have different types of value at runtime, and we can’t know statically which type we’ll have at any given time.
As an example, consider parsing a JSON string. There are several different types of JSON value, and which one we return depends on the contents of the input string.
Our parse_json function has to return a single, concrete type, and that type needs to describe all of the possible types a JSON value can be. Enums are a natural way of describing this kind of situation in Rust.

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Enums