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Inspecting Enums

We can easily inspect the contents of an enum using pattern matching:

fn do_json_stuff(json: &str) {
    match parse_json(json) {
        JsonValue::Object(obj) => println!("We got an object: {obj:?}"),
        JsonValue::Array(array) => println!("We got an array: {array:?}"),
        JsonValue::String(string) => println!("We got a string: {string:?}"),
        JsonValue::Number(num) => println!("We got a number: {num}"),
        JsonValue::Bool(b) => println!("We got a bool: {b}"),
        JsonValue::Null => println!("We got a null"),
    }
}

  • Continuing with our JSON parsing example, we can easily determine which kind of value we got by pattern matching on the resulting enum.

  • This makes enums a good fit for scenarios where we want to handle different types at runtime, but want to retain type information and the ability to directly inspect the concrete value.

  • This is a big advantage enums have over dyn: With dyn we can’t easily downcast to the specific concrete type, and are generally restricted to going through the trait interface. Later we’ll see that we can support downcasting with dyn, but doing so requires extra setup that isn’t necessary with enums.