@[heap]
struct Client {
mut:
    ready_callback   ReadyHandler   = unsafe { nil }
    message_callback MessageHandler = unsafe { nil }
}

struct Ready {}

struct Message {}

type ReadyHandler = fn (mut c Client, msg &Ready)

type MessageHandler = fn (mut c Client, msg &Message)

fn (mut client Client) on[T](event string, handler fn (mut c Client, msg &T)) {
    $if T is Ready {
        if event == 'ready' {
            client.ready_callback = handler
        }
    } $else $if T is Message {
        if event == 'message_create' {
            client.message_callback = handler
        }
    }
}

fn on_ready(mut _client Client, _message &Ready) {}

fn on_message(mut _client Client, _message &Message) {}

fn main() {
    mut client := &Client{}
    client.on[Ready]('ready', on_ready)
    client.on[Message]('message_create', on_message)
}