Minimal example of toJson/fromJson returning String

[hgiesel]

I have this simple code

@freezed
abstract class FoodAllergy with _$FoodAllergy {
  @FreezedUnionValue('SESAME')
  const factory FoodAllergy.sesame() = _Sesame;
  @FreezedUnionValue('PEANUT')
  const factory FoodAllergy.peanut() = _Peanut;

  factory FoodAllergy.fromJson(Map<String, dynamic> json) =>
      _$FoodAllergyFromJson(json);
}

Now toJson/fromJson will transform these types to { runtimeType: "SESAME" } or { runtimeType: "PEANUT" }.

What is the intended way, so that freezed generates "SESAME"/"PEANUT" when invoking fromJson/toJson, and skips the Map?

I've tried a custom JsonConverter, and then attaching it to the freezed class, but it didn't work:

class FoodAllergyConverter implements JsonConverter<FoodAllergy, String> {
  const FoodAllergyConverter();

  @override
  FoodAllergy fromJson(String json) => /* extension method on string */ json.parseFoodAllergy();

  @override
  String toJson(FoodAllergy object) => object.when(
        sesame: () => 'SESAME',
        peanut: () => 'PEANUT',
      );
}

Is there a simple way to achieve something like this?