How can I implement UnmarshalJSON for a type derived from a scalar in Go?

Question

I have a simple type that implements conversion of subtyped integer consts to strings and vice versa in Go. I want to be able to automatically unmarshal strings in JSON to v

Answer 1

Use a pointer receiver for the unmarshal method. If a value receiver is used, changes to the receiver are lost when the method returns.

The argument to the unmarshal method is JSON text. Unmarshal the JSON text to get a plain string with all JSON quoting removed.

func (intValue *PersonID) UnmarshalJSON(data []byte) error {
  var s string
  if err := json.Unmarshal(data, &s); err != nil {
    return err
  }
  *intValue = Lookup(s)
  return nil
}

There's a mismatch between the JSON tags an the example JSON. I changed the JSON to match the tag, but you can change it the other way.

if err := json.Unmarshal([]byte(`{"person": "Ralph", "count": 4, "greeting": "Hello"}`), &m); err != nil {

playground example