circe json parsing question

gistfile1.txt

  case class SubServerConfig(
    val a: String = "inside-test",
    val b: Int = 9999
  )

  case class ServerConfig(
    val a: String = "outside-test",
    val b: Int = 8888,
    val c: List[SubServerConfig] = List()
  )

  // define some boilerplate decoders, utilizing the two constructors via decoders
  val dec1 = Decoder.instance[ServerConfig](c => for (s <- c.downField("a").as[String]; i <- c.downField("b").as[Int]; l <- c.downField("c").as[List[SubServerConfig]]) yield ServerConfig(s, i, l))
  val dec2 = Decoder.instance[ServerConfig](c => for (s <- c.downField("a").as[String]) yield ServerConfig(s))
  val dec3 = Decoder.instance[ServerConfig](c => for (i <- c.downField("b").as[Int]) yield ServerConfig("bad", i))
  implicit val serverConfigDecoder = dec1 ||| dec2 ||| dec3

  val dec11 = Decoder.instance[SubServerConfig](c => for (s <- c.downField("a").as[String]; i <- c.downField("b").as[Int]) yield SubServerConfig(s, i))
  val dec12 = Decoder.instance[SubServerConfig](c => for (s <- c.downField("a").as[String]) yield SubServerConfig(s))
  val dec13 = Decoder.instance[SubServerConfig](c => for (i <- c.downField("b").as[Int]) yield SubServerConfig("bad", i))
  implicit val subServerConfigDecoder = dec11 ||| dec12 ||| dec13

  val str1 = """{"a":"outer","b":5}"""
  val out1 = decode[ServerConfig](str1)
  println(out1)
  //Right(ServerConfig(outer,8888,List()))

  val str2 = """{"a":"outer"}"""
  val out2 = decode[ServerConfig](str2)
  println(out2)
  //Right(ServerConfig(outer,8888,List()))

  val str3 = """{"b":5}"""
  val out3 = decode[ServerConfig](str3)
  println(out3)
  //Right(ServerConfig(bad,5,List()))

  val str11 = """{"a":"outer","b":5,"c":[{"b":2221,"a":"inner1"}]}"""
  val out11 = decode[ServerConfig](str11)
  println(out11)
  //Right(ServerConfig(outer,5,List(SubServerConfig(inner1,2221))))

  val str12 = """{"a":"outer","b":5,"c":[{"a":"inner1"}]}"""
  val out12 = decode[ServerConfig](str12)
  println(out12)
  //Right(ServerConfig(outer,8888,List()))

  val str13 = """{"a":"outer","b":5,"c":[{"b":2221}]}"""
  val out13 = decode[ServerConfig](str13)
  println(out13)
  //Right(ServerConfig(outer,8888,List()))

  val str111 = """{"a":"outer","b":5,"c":[{"b":2221,"a":"inner1"},{"a":"inner2"}]}"""
  val out111 = decode[ServerConfig](str111)
  println(out111)
  //Right(ServerConfig(outer,8888,List()))