1.Anatomy of a type class

• A type class is an interface or API that represents some functionality we want to implement. In Scala a type class is represented by a trait with at least one type parameter. The trait JsonWriter[A] is a type class, A is the type parameter.

• The instances of a type class provide implementations of the type class for specific types we care about, which can include types from the Scala standard library and types from our domain model. The val stringWriter and personWriter are instances of JsonWriter, the specific types are String and Person here. One is from the Scala standard library and the other from the domain model.

• A type class use is any functionality that requires a type class instance to work. In Scala this means any method that accepts instances of the type class as implicit parameters.

object Json:
  def toJson[A](value: A)(using w: JsonWriter[A]): Json =
    w.write(value)

and

object JsonSyntax:
  implicit class JsonWriterOps[A](value: A) :
    def toJson(implicit w: JsonWriter[A]): Json =
      w.write(value)

are both type class uses.