Scala: Structural type that refers to itself

so-topic-3223088.scala

// See StackOverflow: http://stackoverflow.com/questions/3201577/scala-how-to-define-a-structural-type-that-refers-to-itself/3223088#3223088

object Example {
    trait Multipliable[X] { def *(d: Double): X }

    trait Addable[X] { def +(x: X): X }

    trait Interpolable[X] extends Multipliable[X] with Addable[X]

    implicit def double2interpolable(d: Double): Interpolable[Double] = new Interpolable[Double] {
        def *(t: Double): Double = d * t
        def +(j: Double): Double = d + j
    }

    def interpolate[X](t: Double, a: Interpolable[X], b: Interpolable[X]): Interpolable[X] = a * (1.0 - t) + b * t

    def main(args: Array[String]) {
        println(interpolate(0.4, 2.0, 5.0))
    }
}

// C:\Temp>scalac Example.scala
// Example.scala:15: error: type mismatch;
//  found   : X
//  required: String
//     def interpolate[X](t: Double, a: Interpolable[X], b: Interpolable[X]): Interpolable[X] = a * (1.0 - t) + b * t
//                                                                                                                ^
// one error found