Custom unmarshall to a type than input out different type

I'm learning Go and I have a question:

I have an Info type that is defined like that:

type Info struct {
    ID          ID     `json:"id,omitempty"`
    DisplayName string `json:"display_name,omitempty"`
}

I made a custom UnmarshallJSON function to unmarshall this struct because as an input I have either:

• An []interface{} with at position [0] an int and 1 a string
• A boolean always equals to false meaning that the field is null

I want that when the input is false, the Info is nil.

Here's the UnmarshallJSON function

func (i *Info) UnmarshalJSON(data []byte) error {
var v []interface{}
if err := json.Unmarshal(data, &v); err != nil {
	var v bool
	if err = json.Unmarshal(data, &v); err != nil {
		return err
	}
	return nil
}
i.ID = ID(v[0].(float64))
i.DisplayName = v[1].(string)
return nil
}

It's ugly, and I would like to know if there's a better option.
Thank you very much.

Fist you should be more defensive about unexpected types and length to avoid a panic. Then you can unmarshal into a []json.RawMessage to defer unmarshaling of the elements until you are ready. Finally you should guard against your invalid true.

Here is my best effort, please others feel free to edit (here is a playground):

func (i *Info) UnmarshalJSON(data []byte) error {
	var raw interface{}
	if err := json.Unmarshal(data, &raw); err != nil {
		return err
	}

	switch r := raw.(type) {
	case []interface{}:
	case bool:
		if r {
			return errors.New("unexpected true, must be array or false")
		}
		return nil
	default:
		return fmt.Errorf("unexpected type %T, must be array or false", r)
	}

	var v []json.RawMessage
	if err := json.Unmarshal(data, &v); err != nil {
		return err
	}
	if len(v) != 2 {
		return fmt.Errorf("unexpected length %d, must be 2", len(v))
	}

	if err := json.Unmarshal(v[0], &i.ID); err != nil {
		return err
	}
	if err := json.Unmarshal(v[1], &i.DisplayName); err != nil {
		return err
	}
	return nil
}