gistfile1.scala

// Modified version of the example from http://hseeberger.github.io/blog/2013/05/31/implicits-unchained-type-safe-equality-part2/ that shows that you don't need to use implicit evidence parameters.

object TypeWiseBalancedEquality {
  implicit class Equal[A](val left: A) {
    def ===[B](right: B)(implicit equality: Equality[A, B]): Boolean =
      equality.areEqual(left, right)
  }
  trait Equality[A, B] {
    def areEqual(left: A, right: B): Boolean
  }
  object Equality extends LowPriorityEqualityImplicits {
    implicit def rightSubtypeOfLeftEquality[A, B <: A]: Equality[A, B] =
      new Equality[A, B] {
        override def areEqual(left: A, right: B): Boolean = left == right
      }
  }
  trait LowPriorityEqualityImplicits {
    implicit def leftSubtypeOfRightEquality[B, A <: B]: Equality[A, B] =
      new Equality[A, B] {
        override def areEqual(left: A, right: B): Boolean = left == right
      }
  }
}

import TypeWiseBalancedEquality._

println(Seq(1, 2, 3) === List(1, 2, 3))
println(List(1, 2, 3) === Seq(1, 2, 3))
println("a" === "a")
// println(1 === "haha") - fails, as expected