特征中的类型参数以在 Scala 中定义数据类型

问题描述

我是 Scala 的新手，想为消息创建一个模型类.消息由标记作为键和值组成，值可以是具有指定长度、整数或枚举的字符串

I am new to scala and want to create a model class for a message .Message are comprisedoing of tag as key with value and value can be a string with specified length,int or a enumeration

tag     value
100      a
200      b      constraint of length =45
300      0,1,2

其中 0-新1 次修正2-取消

where 0-new 1-amend 2- cancel

 trait ElementType  {
    type A
    type length
}


    case class Tag() extends ElementType {
      override type A = this.type

     }

  case class Value() extends ElementType{
     override type A = this.type
     override type length = this.type
    }

case class Message(
     messageId:MessageId
  )

case class MessageId(tag: Tag[Int], value: Value[String]){
  override def toString = tag + "=" + value + 1.toChar
 }

我在 MessageId 中遇到编译错误，有没有更好的方法来编写代码

I am getting compile error in MessageId,is there a better way to write the code

推荐答案

我猜你的 MessageId 问题是它引用了 Tag[Int] 和 Value[String]，没有泛型参数.

I guess your problem with MessageId that it refers to a Tag[Int] and Value[String], which have no generic parameters.

尝试这样的事情，但我认为您也需要重新定义 Tag 和 Value 类:

Try something like this, though I think you will need to redefine the Tag and Value classes too:

case class MessageId(tag: Tag{type A = Int}, value: Value{type A = String}){
  override def toString = tag + "=" + value + 1.toChar
}

因为您似乎想将它们用作泛型，所以我会尝试这样的操作:

As you seem to want to use them as generics, I would try something like this:

sealed trait ElementType[+A] {
    def length: Int
}

sealed trait Tag extends ElementType[Int] {
    override def length: Int = 0
}
case object Tag100 extends Tag
case object Tag200 extends Tag
case object Tag300 extends Tag

sealed trait Value[+A] extends ElementType[A] {
    override def length: Int = 0
}

final case class Value100(a: String) extends Value[String] {
    override def length: Int = a.length
}
final case class Value200(b: String) extends Value[String] {
    require(b.length <= length)
    override def length: Int = 45
}
case object Value300One extends Value[Nothing]
case object Value300Two extends Value[Nothing]
case object Value300Three extends Value[Nothing]

case class Message(
    messageId:MessageId
)

case class MessageId(tag: Tag, value: Value[String]){
    override def toString = tag + "=" + value + 1.toChar
}

+ in [+A] 表示类型是协变的，所以如果 Q 是 R 的子类型code>, ElementType[Q] 是 ElementType[R] 的子类型.(这是Tag 与任何ElementType 兼容所必需的(因此它扩展了极端的ElementType[Nothing]).)

+ in [+A] means the type will be covariant, so if Q is a subtype of R, ElementType[Q] is a subtype of ElementType[R]. (This is necessary for Tag to be compatible with any ElementType (so it extends the extreme ElementType[Nothing]).)

您的 length 用例我不清楚.如果您希望它成为编译时约束，请查看 Shapeless' 大小 和 使用示例.

Your use case for length is not clear for me. If you want it to be a compiletime constraint, take alook at Shapeless' Sized and an example on its usage.

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