gistfile1.scala

  implicit def AgentCodecJson: CodecJson[Agent] =
    CodecJson(
      (agent: Agent) => jString(agent match {
        case Agent.System => "System"
        case Agent.User(name) => name
      }),

      c => c.focus.string.map {
        _ match {
          case "System" => Agent.System
          case username => Agent.User(username)
        }
      }
    )
  
  // Becomes
  implicit def AgentCodecJson: CodecJson[Agent] = {
    CodecJson(
      implicitly[EncodeJson[String]].contramap {
        _ match {
          case Agent.System => "System"
          case Agent.User(name) => name
        }
      },
    
      implicitly[DecodeJson[String]].map {
        _ match {
          case "System" => Agent.System
          case username => Agent.User(username)
        }
      }
    )
  }