Is there a more concise way of using json objects of unknown structure in golang?

When I have a reliable rest api endpoint that returns some simple json, I can use a struct to define exactly the structure of the expected json result, but there are certain endpoints I have to use that return very large and complex json result data, and the structure of these results are not always known.

I have been using this to unmarshal into:

type JsonObj map[string]interface{}

func (jo JsonObj) GetString(name string) (string, error) {
	if val, exists := jo[name]; exists {
		if v, ok := val.(string); ok {
			return v, nil
		}
		return "", errors.New(name+" is not a string")
	}
	return "", errors.New(name+" property not found")
}

func (jo JsonObj) GetFloat64(name string) (float64, error) {
	if val, exists := jo[name]; exists {
		if v, ok := val.(float64); ok {
			return v, nil
		}
		return 0, errors.New(name+" is not a float64")
	}
	return 0, errors.New(name+" property not found")
}

and in this same way I have GetInt, GetBool, GetSlice, GetJsonObj,

but as you can see all of these functions are virtually identical in content except for the type assertion parameter. is there a way of passing in the type assertion parameter to reduce all these functions effectively to a single function?

This is the best I've got right now, as a very simple example: https://play.golang.org/p/U9WJ0bIJPp
I can't imagine you'd be able to compress/extract it much more than this:

package main
import (
"fmt"
"errors"
)
type T interface{}
type JsonObj map[string]T
func (jo JsonObj) Type(name string, defaultVal T, typeName string, typeAsserter func(val T) (T, bool)) (T, error){
if val, exists := jo[name]; exists {
if v, ok := typeAsserter(val); ok {
return v, nil
}
return defaultVal, errors.New(name+" is not of type "+typeName)
}
return defaultVal, errors.New(name+" property not found")
}
func (jo JsonObj) String(name string) (string, error) {
ret, err := jo.Type(name, "", "string", func(val T)(ret T, ok bool){ret, ok = val.(string);return})
return ret.(string), err
}
func (jo JsonObj) Float64(name string) (float64, error) {
ret, err := jo.Type(name, 0, "float64", func(val T)(ret T, ok bool){ret, ok = val.(float64);return})
return ret.(float64), err
}
func (jo JsonObj) Int(name string) (int, error) {
ret, err := jo.Type(name, 0, "int", func(val T)(ret T, ok bool){if ret, ok = val.(float64); ok {ret = int(ret.(float64))};return})
return ret.(int), err
}
func (jo JsonObj) Bool(name string) (bool, error) {
ret, err := jo.Type(name, false, "bool", func(val T)(ret T, ok bool){ret, ok = val.(bool);return})
return ret.(bool), err
}
func main() {
jo := JsonObj{
"aString": "foo",
"aFloat64": 3.142,
"anInt": 42.0, //in a json string unmarshalling all numbers are float64 even "int"s
"aBool": true,
}
fmt.Println(jo.String("aString"))
fmt.Println(jo.Float64("aFloat64"))
fmt.Println(jo.Int("anInt"))
fmt.Println(jo.Bool("aBool"))
fmt.Println(jo.String("missingString"))
fmt.Println(jo.Bool("anInt"))
}

But as mentioned in the comments there is a library which provides a much more robust means of working with arbitrary json, https://github.com/bitly/go-simplejson