Go sum type in json

What I want to achieve

I'm parsing IAM Policies in Go. In IAM Policies, most of the fields can be either a string or an array of strings. It's hard to think these decision trees in my head what I want is kind of exhaustive pattern matching.

What I did

I loeded a policy with json.Unmarshal

type Policy struct {
	Version    string      `json:"Version"`
	Id         string      `json:"ID,omitempty"`
	Statements []Statement `json:"Statement"`
}

type Statement struct {
    // ...
	Action        interface{}         `json:"Action"`              // string or array
    // ...
}

And iterating over statements.

switch ele := st.Action.(type) {
case string:
	action, _ := expandAction(ele, data)
	actions = append(actions, action...)
	setter(i, actions)
case []string:
	for _, a := range ele {
		if strings.Contains(a, "*") {
			exps, _ := expandAction(a, data)
			actions = append(actions, exps...)
		} else {
			actions = append(actions, a)
		}
		setter(i, actions)
		fmt.Println(actions)
	}
default:
	// interface{}
}

The Problem

It always goes to the default case.

Can use reflection, but don't think I really should, since runtime could know types when json.Unnarshal is called.

As you can see from the official document the type for JSON array is []interface. If you update []string to []interface then you can run the related case block. However, if you have to sure that is array of string, reflection can provide it.