filterM / State

filterM.scala

scala> type RunState[A] = State[(Int, Int), A] // ((lastNum, run length), A)defined type alias RunState

scala> val ls = List(1, 1, 2, 2, 2, 3, 3, 3, 3, 1, 1)
ls: List[Int] = List(1, 1, 2, 2, 2, 3, 3, 3, 3, 1, 1)

scala> std.list.filterM[Int, RunState](ls)(x => 
  State { case (y, n) => 
    val nn = if (x == y) n + 1 else 0
    ((x, nn), nn == 0) } 
).eval((ls.head, -1))
res29: scalaz.Id.Id[List[Int]] = List(1, 2, 3, 1)