Scala function TypeTag: T use type T in function - scala

Question

I need to parse several json fields, which I'm using Play Json to do it. As parsing may fail, I need to throw a custom exception for each field.
To read a field, I use this:
val fieldData = parseField[String](json \ fieldName, "fieldName")
My parseField function:
def parseField[T](result: JsLookupResult, fieldName: String): T = {
result.asOpt[T].getOrElse(throw new IllegalArgumentException(s"""Can't access $fieldName."""))
}
However, I get an error that reads:
Error:(17, 17) No Json deserializer found for type T. Try to implement
an implicit Reads or Format for this type.
result.asOpt[T].getOrElse(throw new IllegalArgumentException(s"""Can't access $fieldName."""))
Is there a way to tell the asOpt[] to use the type in T?

Answer 1

I strongly suggest that you do not throw exceptions. The Play JSON API has both a JsSuccess and JsError types that will help you encode parsing errors.
As per the documentation
To convert a Scala object to and from JSON, we use Json.toJson[T: Writes] and Json.fromJson[T: Reads] respectively. Play JSON provides the Reads and Writes typeclasses to define how to read or write specific types. You can get these either by using Play's automatic JSON macros, or by manually defining them. You can also read JSON from a JsValue using validate, as and asOpt methods. Generally it's preferable to use validate since it returns a JsResult which may contain an error if the JSON is malformed.
See https://github.com/playframework/play-json#reading-and-writing-objects
There is also a good example on the Play Discourse forum on how the API manifests in practice.