> Fun fact: because json is a reflective package in Go, solving your problem is quite trivial: https://play.golang.org/p/-TZ5b9Su1or .

If I understand it correctly, your code doesn't check when unmarshalling if the content of Data has the expected structure. Using interface{} it's always been possible to achieve something like you'd do with Generics but it's ugly and can't check types at compile time.