type mystery

riddle.scala

//Riddle me this...

//This compiles:

   def plotGrowth[A,T[B] <: Seq[B]](l: T[A], fn: T[A] => T[A]): Boolean = {
    val intervals = (0 to 50).map(_ * l.size / 50)
    intervals map { i =>
      val sub = l.take(i)
      System.gc()
      val start = System.nanoTime
      val elapsed = (System.nanoTime - start)
      println(s"$i, $elapsed") }
    true }

// As does this:

  def plotGrowthA[A](l: ArrayBuffer[A], fn: ArrayBuffer[A] => ArrayBuffer[A]): Boolean = {
    val intervals = (0 to 50).map(_ * l.size / 50)
    intervals map { i =>
      val sub = l.take(i)
      val elapsed = time(sub,fn)
      println(s"$i, $elapsed") }
    true }

  def plotGrowthL[A](l: List[A], fn: List[A] => List[A]): Boolean = {
    val intervals = (0 to 50).map(_ * l.size / 50)
    intervals map { i =>
      val sub = l.take(i)
      val elapsed = time(sub,fn)
      println(s"$i, $elapsed") }
    true }

  def time[A,T[B] <: Seq[B]](l: T[A], fn: T[A] => T[A]): Long = {
    System.gc()
    val start = System.nanoTime
    fn(l)
    (System.nanoTime - start) / 1000 /*to millis*/ }
    
// But this does not:

  def plotGrowthDumb[A,T[B] <: Seq[B]](l: T[A], fn: T[A] => T[A]): Boolean = {
    val intervals = (0 to 50).map(_ * l.size / 50)
    intervals map { i =>
      val sub = l.take(i)
      val elapsed = time(sub,fn)
      println(s"$i, $elapsed") }
    true }
    
  def time[A,T[B] <: Seq[B]](l: T[A], fn: T[A] => T[A]): Long = {
    System.gc()
    val start = System.nanoTime
    fn(l)
    (System.nanoTime - start) / 1000 /*to millis*/ }
    
// call to `time` on line 45 complains about the argument `fn`
// says it expects a function of type `Seq[A] => Seq[A]`
// but got one of type `T[A] => T[A]`

// anyone got any ideas?