Decoding generic JSON objects to one of many formats

Question

I am working on a general JSON based message passing protocol in Go. What I would like to do is have a BaseMessage that has general information like Type, timestamp, etc. But at the same time I want to be able to define more specific message structures for certain types of data.

For example:

type Message struct {
    Type      string `json:type`
    Timestamp string `json:timestamp`

}

type EventMessage struct {
    Message
    EventType string
    EventCreator string
    EventData interface{}
}

I have a set of handlers and to determine which handler should process the message I decode the JSON to the general Message type first to check the Type field. For this example I would get the handler associated with an "Event" message type.

I run into problems when I then want to assert the EventMessage type onto the structure.

The following code is very rough, but hopefully it displays my general idea of how I am trying to handle the messages.

type Handler func(msg Message) Message
handlers := make(map[string]Handler)

var msg Message
decoder.Decode(&msg)
handler := handlers[msg.Type]
handler(msg)

I have tried to use an interface{} but then the JSON decoder just creates a map which I then cant assert either type on. I have figured out workarounds that make it possible, but its very ugly, probably not efficient, and most likely error prone. I would like to keep things simple and straightforward so this code can be easily maintained.

Is there a method of handling generic JSON objects in Go so that the decoded JSON could be one of many struct formats?

I have also played with the idea of having more specific info in a Data interface{} in the main Message struct, but then I run into the same problem of not being able to assert any types onto the interface. There has to be a better way to handle JSON formats that I am just missing.