scala - 在泛型中使用asInstanceOf

Question

I'm having compilation issues with generics. The code compiles fine when I use asInstanceOf . I want to get rid of asInstanceOf .

I saw some other questions related to the usage of asInstanceOf , but I didn't help me.

trait RoundRobin[R <: Resource, F[_] <: mutable.ListBuffer[_]] {
  self: RoundRobin[R, F] =>

  // some public functions

  private def overrideMutableResourceList(original: F[R], updated: F[R]): F[R] = {
    val tempPool = original.asInstanceOf[mutable.ListBuffer[R]]
    original.indices.foreach(i => {
      val e = updated(i).asInstanceOf[R]
      tempPool.update(i, e)
    })

    tempPool.asInstanceOf[F[R]]
  }

When I remove the asInstanceOf from tempPool.asInstanceOf[F[R]] I get the below error

[error] /Users/...../RoundRobin.scala:108: type mismatch;
[error]  found   : tempPool.type (with underlying type scala.collection.mutable.ListBuffer[R])
[error]  required: F[R]
[error]     tempPool
[error]     ^
[error] one error found
[error] (clustering/compile:compileIncremental) Compilation failed
[error] Total time: 3 s, completed Oct 3, 2017 2:53:34 AM

This issue happens also for the line original.asInstanceOf[mutable.ListBuffer[R]]

• What is the reason for this issue?
• How can I avoid using asInstanceOf ?

Thanks

Answer 1

There's no relationship between F[A] and ListBuffer[A] , only that ∀A∃BF[A] <: ListBuffer[B] . This is important:

type ConstLBInt[A] = ListBuffer[Int]
val x: RoundRobin[Resource, ConstLBInt] = ??? // Legal
// Tries to manipulate ListBuffer[Int]s as if they were ListBuffer[Resources]s

Change the declaration of your type to

trait RoundRobin[R <: Resource, F[A] <: mutable.ListBuffer[A]]
//                                !                        !

This forces ∀AF[A] <: ListBuffer[A] , so that eg the updated: F[R] in overrideMutableResourceList is known to be a ListBuffer[R] .

There are probably other parts of the class that will be simplified by this.