Type IDs

When encoding Abstract Data Types (ADTs), i.e. top types of sealed type hierarchies like Animal in this example:

copysourcesealed trait Animal
case class Dog(age: Int, name: String)                      extends Animal
case class Cat(weight: Double, color: String, home: String) extends Animal
case class Fish(color: String)                              extends Animal
case object Yeti                                            extends Animal

the encoder needs to somehow transfer the information to the decoder, which of the defined sub-types to decode into.
In the context of the example ADT above, when Json.decode(encoding).to[Animal] is called the decoder needs to know, whether the encoding holds a Dog, a Cat or any of the other (well-defined) sub-types of Animal.

This is done through a “Type ID”, which is a String or number that uniquely identifies each ADT sup-type.

By default borer will use a sub-type’s short class name as a (textual) type ID, i.e. "Dog" would be the type id of Dog instances and "Cat" would be the same for the Cat type.

If you want to customize this you can use the @key annotation to do so. Check out the @key sources here for more info on this.