event.swift

protocol HookEvent { }

protocol HookEventHandler {
    func handle(_ event: HookEvent)
}

protocol DiscordEventHandler: HookEventHandler {
    func handle(_ event: DiscordEvent)
}

extension DiscordEventHandler {
    func handle(_ event: HookEvent) {
        guard let event as? DiscordEvent else {
            return nil
        }
        self.handle(event)
    }
}

struct Discord {
    @propertyWrapper
    struct GuildCreate: DiscordEventHandler {
        var value: (Guild) -> ()

        func handle(_ event: DiscordEvent) {
            guard case .guildCreate(let guild) = event else {
                return
            }
            self.value(guild)
        }
    }
}

protocol MessageCreateEvent {
    var content: String { get }
}

struct Global {
    @propertyWrapper
    struct MessageCreate: HookEventHandler {
        var value: (MessageCreateEvent) -> ()

        func handle(_ event: HookEvent) {
            guard let create = event as? MessageCreateEvent else {
                return
            }
            self.value(create)
        }
    }
}

struct MyPlugins: Plugins {
    @Discord.GuildCreate
    var guildListener = { guild in
        print("created guild \(guild)")
    }

    @Global.MessageCreate
    var globalMessageListener = { message in
        print("Global: \(message.content)")
    }
}


// internal in swift hook

let event: HookEvent 

for handler in plugins.handlers {
    handler.handle(event)
}