Scala, Sangria, lazies and implicits

schema.scala

trait CartTypes {
  
  implicit val ec: ExecutionContextExecutor
  val itemType: ObjectType[RequestServices, Item]
  
  lazy val cartType: ObjectType[Unit, Cart] = ObjectType[RequestServices, Cart](
  /* ... */
    fields = Field(
      "items",
      ListType(ItemType),
      resolve = c => c.ctx.getItems(c.value.id)
    )
  )
}

trait ItemTypes {
  
  implicit val cartType: ObjectType[RequestServices, ShoppingCart]
  
  lazy val itemType =  deriveObjectType[RequestServices, Item]()
}

case class Schema()(implicit val ec: ExecutionContextExecutor) 
  extends CartTypes
  with ItemTypes {
  val QueryType = ObjectType(
    "Query",
    fields[RequestServices, Unit]( /* the types are used */ )
  )
}

My impression from the above is that

For CartTypes and ItemTypes, the lazy vals are due to interdependence on each other
val itemType: ObjectType[RequestServices, Item] means the member is necessary to be implemented in the concrete Schema implementation (straightforward)
implicit val cartType: ObjectType[RequestServices, ShoppingCart]does the same but adds an additional flag to the compiler to make this val available within the deriveObjectType macro. This is because we don't refer to it directly as with an ObjectType literal. The ItemTypes is allowed to make its implemented cartType implicit.