How to Unmarshal JSON Strings into Custom Go Scalar Types?

Unmarshaling JSON Strings to Derived Scalar Types in Go

In Go, you can derive a new scalar type from an existing one. However, implementing UnmarshalJSON for such types can be challenging due to limitations in the standard library.

To correctly handle this, it is essential to use a pointer receiver for the UnmarshalJSON method. Value receivers do not retain changes made within the method.

The argument passed to UnmarshalJSON is JSON text. This can be unmarshalled into a plain string, discarding any quoting.

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
}

Additionally, ensure that the JSON tags in your code match the keys in the JSON data you are unmarshalling.

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

By following these steps, you can effectively implement UnmarshalJSON for derived scalar types, enabling automatic conversion of JSON strings to values of your custom type.

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