Why golang custom unmarshal got zero struct

Question

I can't understand why I can't to get a value

func (a *Date) UnmarshalJSON(b []byte) (err error) {
    a, err = String(string(b))
    fmt.Println("OK on unmarshal", a)
    return
}

https://play.golang.org/p/wQh7r2wJgJt

Its ok inside unmarshal but nil on result. Something wrong with pointers?

Answer 1

thats it

func (a *Date) UnmarshalJSON(b []byte) (err error) {
    bb, err = String(string(b))
    (*a) = *bb
    return
}