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scala - 是否可以为伴随对象指定特征？

问题描述

假设我有这个：

trait FormData

case class DepartmentData(id: Long, title: String) extends FormData

和这个伴随对象：

object DepartmentData {
  def empty: DepartmentData = ???
  def from(value: SomeKnownType): DepartmentData = ???
}

我想要确保所有实现FormDataTrait 的类都具有这两种方法empty并from在它们的伴生对象中。

标签： scala

解决方案

我不认为我们可以直接这样做，但是尝试像这样的类型类解决方案

trait FormData
case class DepartmentData(id: Long, title: String) extends FormData
case class EmployeeData(id: Long, title: String) extends FormData

trait SomeKnownType

trait FormDataFactory[T <: FormData] {
  def empty: T
  def from(value: SomeKnownType): T
}

object FormDataFactory {
  def empty[T <: FormData](implicit ev: FormDataFactory[T]): T = ev.empty
  def from[T <: FormData](value: SomeKnownType)(implicit ev: FormDataFactory[T]): T = ev.from(value)

  implicit object fooDepartmentData extends FormDataFactory[DepartmentData] {
    override def empty: DepartmentData = ???
    override def from(value: SomeKnownType): DepartmentData = ???
  }

  implicit object fooEmployeeData extends FormDataFactory[EmployeeData] {
    override def empty: EmployeeData = ???
    override def from(value: SomeKnownType): EmployeeData = ???
  }
}

现在打电话

FormDataFactory.empty[DepartmentData]
FormDataFactory.empty[EmployeeData]

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