模式匹配与递归问题

Question

下面是我的ADT。需要注意的是，块可以嵌套(查看children属性)。

trait Cda {
  def format: String = this match {
    case f: Field => f.value
    case Block(fields, children) => fields.map(f => f.format).mkString("|") + "|" + children.map(b => b.format).mkString("|")
    case Record(keys, blocks) => blocks.map(b => b.format).mkString("|")
  }
}

trait Field extends Cda {
  val name: String
  val value: String
}

case class StringField(name: String, value: String) extends Field
case class DateField(name: String, value: String) extends Field
case class TimeField(name: String, value: String) extends Field
case class MatchKey(keyFields: Seq[Field]) extends Cda
case class Block(fields: Seq[Field], children: Seq[Block] = Seq()) extends Cda
case class Record(key: MatchKey, blocks: Seq[Block]) extends Cda

下面是该ADT的示例实例化

//Block - AI
val aiBlockId = StringField("blockId", "AI")
val addlFieldPos = StringField("AdditionalFieldPosition", "addlFieldPos")
val addlFieldName = StringField("AdditionalFieldName", "addlFieldName")
val AI = Block(Seq(aiBlockId, addlFieldPos, addlFieldName))

//Block - RPS
val rpsBlockId = StringField("blockId", "RPS")
val dateOfStatus = DateField("DateOfStatus", "19240811")
val timeOfStatus = TimeField("TimeOfStatus", "023829")

val rpsBlocks = Seq(rpsBlockId, dateOfStatus, timeOfStatus)

val rpsNestedBlocks = Seq(AI)

val RPS = Block(rpsBlocks, rpsNestedBlocks)

我希望格式化来返回RPS|19240811|023829|AI|addlFieldPos|addlFieldName，但是我在末尾得到了一个额外的管道\：RPS|19240811|023829|AI|addlFieldPos|addlFieldName|。

如何更改递归函数format (特别是case Block(fields,children))来纠正这一点？

Answer 1

trait Cda {
  def format: String = this match {
    case f: Field => f.value
    case Block(fields, children) => fields.map(f => f.format).mkString("|") + {if (!children.isEmpty) {"|" + children.map(b => b.format).mkString("|")} else ""}
    case Record(keys, blocks) => blocks.map(b => b.format).mkString("|")
  }
}