从父 class 创建子实例

Question

我正在为长度操作构建一个简单的 DSL。我希望域操作是可扩展的，所以我将它们用作 mixins 以及域组件的 implicit 转换。
1. 下面是我的应用程序。

package com.shasank.funWithLengths

object LengthAdditionApp extends App{

  val length1 = 11 inches
  val length2 = 15 inches
  val length3 = 2 feet

  println(length1)
  println(length2)
  println(length3)
  println(length1 + length2) // all ok
  println(length1 + length3) // all ok
  println(length3 - length1) // all ok
  println(length1 + length2 + length2) // this breaks since object returned from first operation doesn't have adder

}

下面是我的基地class。我希望它是抽象的，但由于我找不到创建 subclass Inches 实例的方法，我将构造函数标记为受保护的，这样只有 subclasses 可以扩展它，没有其他东西可以创建实例。

package com.shasank.funWithLengths

class Length protected(val measure: Int, val unit: String) {

  private def convertToInches(length: Length)= length.unit match {
    case "feet" => length.measure * 12
    case "inches" => length.measure
  }

  protected def operateOnMeasures(other: Length, op: (Int,Int) => Int): Length ={
    val thisInches = convertToInches(this)
    val otherInches = convertToInches(other)
    val operatedMeasure = op(thisInches,otherInches)
    new Length(operatedMeasure, "inches")  // object created does not have adder & subtracter capabilities
  }

  override def toString = {
    val measureInInches = convertToInches(this)
    val (feetMeasure, inchesMeasure) = BigInt(measureInInches) /% 12
    val feetMeasureString = s"$feetMeasure feet and"
    val inchesMeasureString = s"$inchesMeasure inches"
    s"$feetMeasureString $inchesMeasureString"
  }

}

以下是我的域组件。

package com.shasank

package object funWithLengths {
  implicit class Inches(measure: Int) extends Length(measure, "inches") with Adder with Subtracter {
    def inches = this
  }
  implicit class Feet(measure: Int) extends Length(measure, "feet") with Adder with Subtracter {
    def feet = this
  }
}

以下是我的域名运营商。

package com.shasank.funWithLengths

trait Adder extends Length {
  def +(other: Length) = super.operateOnMeasures(other, _+_)
}

package com.shasank.funWithLengths

trait Subtracter extends Length {
  def -(other: Length) = super.operateOnMeasures(other, _-_)
}

问题： 有没有办法在从方法 operateOnMeasures 在我的基地 class Length?

Answer 1

我能够通过将 Length class 移动到我声明隐式 classes Inches 和 Feet 的包对象中来解决这个问题.
这是我的工作 link code

从父 class 创建子实例

Create instance of child from parent class

scala

oop

functional-programming

aop

dsl