How to implement type classes for recursive generics?

Question

I would like to use type classes to provide a specialized implementation for a generic class. The trouble is the class is recursive, and I did not find a way how to write the code so that is works. Here is my attempt:

object Compare {
  trait Compare[T <: Compare[T]] {
    def compare(that: T): Boolean
  }

  trait IsCompare[T] {
    def check: Boolean
  }

  trait LowerLevelImplicits {
    implicit def defaultCompare[T]: IsCompare[T] = new IsCompare[T] {
      def check = false
    }
  }

  trait Implicits extends LowerLevelImplicits {
    implicit def isCompare[T: Compare]: IsCompare[T] = new IsCompare[T] {
      def check = true
    }
  }

  case class MyClass(value: Int) extends Compare[MyClass] {
    override def compare(that: MyClass) = this equals that
  }
}

import Compare._

object Main extends App with Implicits {

  def matchArray[T: IsCompare](array: Array[T]) = {
    if (implicitly[IsCompare[T]].check) {
      println("Using isCompare")
    } else {
      println("Not using isCompare")
    }
  }


}

The error is:

Error:(17, 29) type arguments [T] do not conform to trait Compare's type parameter bounds [T <: Compare.Compare[T]]
implicit def isCompare[T: Compare]: IsCompare[T] = new IsCompare[T] {

Bogdan Vakulenko · Accepted Answer · 2018-10-08 13:02:36Z

2

Try to change isCompare's method signature to:

implicit def isCompare[T <: Compare[T]]: IsCompare[T] = //...

edited Oct 8, 2018 at 13:02

answered Oct 8, 2018 at 12:53

Bogdan Vakulenko

3,3901 gold badge12 silver badges29 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

How to implement type classes for recursive generics?

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related