Generics with Play JSON
As Play JSON library uses implicits in order to know how to serialize/deserialize some object the type of that object has to be known at compile time. This makes serializing/deserializing generic classes not that straightforward - even if you know at compile time type which is substituted in generic class type parameter, you need to provide somehow reads/writes for each such type, not just for single generic class. The trick is to use def instead of val.