golang: quickly access data of maps within maps

So I've got the following bit of JSON and I want to pull out the "$t" value under "token". Continue for Go code...

{
  "@encoding": "iso-8859-1",
  "@version": "1.0",
  "service": {
    "auth": {
      "expiresString": {
        "$t": "2013-06-12T01:15:28Z"
      },
      "token": {
        "$t": "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
      },
      "expires": {
        "$t": "1370999728"
      },
      "key": {
        "$t": "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
      }
    }
}

I have the following snippet of Go code that unmarshals the json into an interface. Then I work my way down to the "$t" value of "token". This approach does work, but it's ugly.

My question: is there a faster way to access that value than by converting each map into an interface? I'm very new to Go and am not aware of many of the useful features of interfaces and maps.

var f interface{}
jerr := json.Unmarshal(body, &f)
m := f.(map[string]interface{})
ser := m["service"].(map[string]interface{})
a := ser["auth"].(map[string]interface{})
tok := a["token"].(map[string]interface{})
token := tok["$t"]
fmt.Fprintf(w, "Token: %v\n", token)

Thanks in advance!

If that's the only value you want, then how about using an anonymous struct that defines the path to your data.

var m = new(struct{Service struct{Auth struct{Token map[string]string}}})

var err = json.Unmarshal([]byte(data), &m)

fmt.Println(m.Service.Auth.Token["$t"], err)

DEMO: http://play.golang.org/p/ZdKTzM5i57

Instead of using a map for the innermost data, we could use another struct, but we'd need to provide a field tag to alias the name.

var m = new(struct{Service struct{Auth struct{Token struct{T string `json:"$t"`}}}})

var err = json.Unmarshal([]byte(data), &m)

fmt.Println(m.Service.Auth.Token.T, err)

DEMO: http://play.golang.org/p/NQTpaUvanx

OR you can use objects.Map from our stew package, it gives you dot accessors for maps:

objects.Map(data).Get("service.auth.token")

see http://godoc.org/github.com/stretchr/stew/objects