Very basic type class example.

a-very-simple-typeclass-example.scala

//
// While testing the Elasticsearch 6.x http client works for me, using circe for json, 
// I added a silly `Entity` type class which describes how to get an ID from an entity.
//

import java.util.UUID

import org.apache.http.HttpHost
import org.elasticsearch.action.index.IndexRequest
import org.elasticsearch.client.{RestClient, RestHighLevelClient}
import org.elasticsearch.common.xcontent.XContentType

import io.circe._
import generic.auto._
import syntax._



// Type class that allows treating a type as an entity; that is, an object that is identified by
// its `primary key` in storage, as opposed to a value object where the identity is determined by it's 
// member values.
trait Entity[T] {
  def id(i: T): String
}

// Companion object defines the "interface".
object Entity {
  def id[T](i: T)(implicit tc: Entity[T]): String = tc.id(i)
}

// A Data Type
case class Project(id: String, name: String)

// And an Entity instance for that type.
// Note I don't create an anonymous class because single method traits can be treated as 
//   functions.
implicit val entityProject: Entity[Project]  = (i: Project) => i.id

//
// Create an elasticsearch client
// 

val client = new RestHighLevelClient(
  RestClient.builder(
    new HttpHost("localhost", 9200, "http"),
    new HttpHost("localhost", 9201, "http")
  )
)

// Helper
def buildProject(name: String): Project = {
  val id = UUID.nameUUIDFromBytes(name.getBytes).toString
  Project( id, name )
}

// This function is generic in the type of what it indexes, but it requires that there are instances for
// Encoder (circe json encoder) and Entity (se we can get to the id)
def buildIndexRequest[T : Encoder : Entity](doc: T): IndexRequest = {
  new IndexRequest(
    "posts",
    "doc",
    Entity.id(doc)
  ).source(doc.asJson.toString(), XContentType.JSON)
}