Scala列表与泛型的模式匹配 [英] Pattern match of scala list with generics

问题描述

我上课

case class MyClass[T](values: List[T])

并且我正在尝试创建一个函数，该函数将根据T的类型返回一个值

and I'm trying to create a function which will return a value based on the type of T

def myFunc: T = values match {
  case v: List[Boolean] => false
  case v: List[MyType] => MyType.defaultVal
  case _ => throw new Exception("unsupported type")
}

但是，我收到编译错误:

However, I get compilation errors:

布尔类型的表达不符合预期的类型T

Expression of type Boolean doesn't conform to expected type T

MyType.defaultVal.type类型的表达与预期的T类型不一致

Expression of type MyType.defaultVal.type doesn't conform to expected type T

我想我可以通过创建一个抽象类和子类来解决此问题，但我宁愿不这样做.还有其他解决方法吗?

I guess I can solve this by creating an abstract class ans subclasses, but I'd rather not do it this way. Is there any other way to solve this?

推荐答案

问题是模式匹配主要在运行时执行，类型在编译时解析.由于类型擦除，case v: List[Boolean] => ???并不比case v: List[_] => ???好.在case v: List[Boolean] => ...行中，编译器不知道T =:= Boolean.模式匹配无法在不同情况下返回不同的类型.

The thing is that pattern matching is performed mostly at runtime and types are resolved at compile time. Because of type erasure case v: List[Boolean] => ??? is not better than case v: List[_] => ???. In the line case v: List[Boolean] => ... compiler doesn't know that T =:= Boolean. Pattern matching can't return different types from different cases.

因此，您可以使用类型标签进行样式匹配并强制转换

So you can either pattern match with type tags and casting

case class MyClass[T](values: List[T]) {   
  import reflect.runtime.universe.{TypeTag, typeOf}
  def myFunc(implicit typeTag: TypeTag[T]): T = values match {
    case v: List[Boolean] if typeOf[T] <:< typeOf[Boolean] => false.asInstanceOf[T]
    case v: List[MyType]  if typeOf[T] <:< typeOf[MyType]  => MyType.defaultVal.asInstanceOf[T]
    case _ => throw new Exception("unsupported type")
  } 
}

或者使用类型类(或多态函数)更安全地进行此操作

or do this more type-safely with a type class (or polymorphic function)

case class MyClass[T](values: List[T]) {
  def myFunc(implicit myFuncInstance: MyFunc[T]): T = myFuncInstance(values)
}

trait MyFunc[T] {
  def apply(values: List[T]): T
}
object MyFunc {
  implicit val booleanMyFunc: MyFunc[Boolean] = new MyFunc[Boolean] {
    override def apply(values: List[Boolean]): Boolean = false
  }

  implicit val myTypeMyFunc: MyFunc[MyType] = new MyFunc[MyType] {
    override def apply(values: List[MyType]): MyType = MyType.defaultVal
  }
}

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