why the "switch t := policy.Parameter.(type)" does not work with viper.Unmarshal()

I want to unmarshal several types from JSON and use the interface to represent the actual struct that it is different. But when I send the struct as interface{} it converts it to a map. The animal.json is:

"{"type":"cat","policies":[{"name":"kitty","parameter":{"duration":600,"percent":90}}]}"

package main

import (
	"reflect"

	log "github.com/sirupsen/logrus"
	"github.com/spf13/viper"
)

func main() {
	var err error
	animal := New()
	viper.SetConfigType("json")
	viper.SetConfigName("animal")
	viper.AddConfigPath("~/Desktop/")
	viper.AddConfigPath(".")
	if err := viper.ReadInConfig(); err != nil {
		return
	}

	if err = viper.Unmarshal(&animal); err != nil {
		return
	}
	for _, policy := range animal.Policies {
		log.Info(policy.Name)
		log.Info(policy.Parameter)
		//INFO[0000] map[duration:600 percent:90]
		log.Info(reflect.TypeOf(policy.Parameter))
		//INFO[0000] map[string]interface {}, Why is it not an interface{} and how do I get it?
		switch t := policy.Parameter.(type) {
		//why does the switch not work?
		case *CatParameter:
			log.Info("cat", t)
		case *DogParameter:
			log.Info("dog", t)
		}
	}
}

func New() *Animal {
	var animal Animal
	animal.Type = "cat"
	return &animal
}

type Animal struct {
	Type     string   `json:"type" form:"type"`
	Policies []Policy `json:"policies" form:"policies"`
}

type CatParameter struct {
	Duration int `json:"duration"`
	Percent  int `json:"percent"`
}

type DogParameter struct {
	Percent   int    `json:"percent"`
	Duration  int    `json:"duration"`
	Operation string `json:"operation"`
}

type Policy struct {
	Name      string      `json:"name"`
	Parameter interface{} `json:"parameter"`
}

It's json unmarshal feature

If you use an interface{} as a decoder, the default json object for interface{} is map[string]interface{}

You can see it here:

https://godoc.org/encoding/json#Unmarshal

bool, for JSON booleans
float64, for JSON numbers
string, for JSON strings
[]interface{}, for JSON arrays
map[string]interface{}, for JSON objects
nil for JSON null

So in t := policy.Parameter.(type), the t is map[string]interface{}

For solving your problem, you can try to define another field to distinguish CatParameter or DogParameter

Maybe:

type Policy struct {
Name      string      `json:"name"`
Parameter Parameter   `json:"parameter"`
}
type Parameter struct {
Name      string `json:"name"`   // cat or dog
Percent   int    `json:"percent,omitempty"`
Duration  int    `json:"duration,omitempty"`
Operation string `json:"operation,omitempty"`
}