StackOverflow Question http://stackoverflow.com/questions/13897885/how-can-i-remove-this-extra-type-parameter

test1.scala

package test

class X[+T: Numeric](val x: T)
abstract class M[T: Numeric] {
  def apply(x: Int): X[T]
  final def row = (1 to 10).map(this(_))
}

class Y(x: Double, val y: Double) extends X[Double](x)
class Z extends M[Double] {
  def apply(x: Int) = new Y(0.0, 0.0)
}

object testapp {
  def main(args: Array[String]): Unit = 
    (new Z).row.map(r=>println(r.y))                   // <- y is not a member of X[Double].
}

test2.scala

package test

class X[+T: Numeric](val x: T)
abstract class M[N: Numeric, T <: X[N]] {
  def apply[A <: T](x: Int): X[A]
  final def row = (1 to 10).map(this(_))
}

class Y(x: Double, val y: Double) extends X[Double](x)
class Z extends M[Double, Y] {                         // <- Z must be abstract because it doesn't implement apply.
  def apply(x: Int) = new Y(0.0, 0.0)
}

object testapp {
  def main(args: Array[String]): Unit =
    (new Z).row.map(r => println(r.y))
}

test3.scala

// Like test2.scala, I can't implement apply properly.
package test

class X[+T: Numeric](val x: T)
abstract class M[T: Numeric] {
  def apply[A <: X[T]](x: Int): A
  final def row = (1 to 10).map(this(_))
}

class Y(x: Double, val y: Double) extends X[Double](x)
class Z extends M[Double] {                            // <- Z must be abstract because it doesn't implement apply.
  def apply[A <: X[Double]](x: Int) = new Y(0.0, 0.0)
}

object testapp {
  def main(args: Array[String]): Unit =
    (new Z).row.map(r => println(r.y))
}

test4-a_solution1.scala

package test

class X[+T: Numeric](val x: T)
abstract class M[A: Numeric, B <: X[A]] {              // <- I really don't like having to specify two type parameters.
  def apply(x: Int): B
  final def row = (1 to 10).map(this(_))
}

class Y(x: Double, val y: Double) extends X[Double](x)
class Z extends M[Double, Y] {
  def apply(x: Int) = new Y(0.0, 0.0)
}

object testapp {
  def main(args: Array[String]): Unit = {
    (new Z).row.map(r => println(r.y))
  }
}

test5.scala

package test

class X[T: Numeric](val x: T)
abstract class M[T <: X[T]] {
  def apply(x: Int): T
  final def row = (1 to 10).map(this(_))
}

class Y(x: Double, val y: Double) extends X[Double](x)
class Z extends M[Y] {                                 // <- Y doesn't conform to T <: X[T]. Why?
  def apply(x: Int) = new Y(0.0, 0.0)
}

object testapp {
  def main(args: Array[String]): Unit = {
    (new Z).row.map(r => println(r.y))
  }
}