// ===

def add[R](i: Int, j: Int): Cont[R, Int] = Cont(ar => ar(i + j))
def mul[R](i: Int, j: Int): Cont[R, Int] = Cont(ar => ar(i * j))
def show[R](i: Int): Cont[R, String]     = Cont(ar => ar(s"num: $i"))

def prog[R]: Cont[R, String] =
  for {
    a <- add(1, 2)
    b <- mul(a, 3)
    s <- show(b)
  } yield {
    s.toUpperCase
  }

// ===

scala> prog.run(s => s.toList)
res18: List[Char] = List(N, U, M, :,  , 9)

scala> prog.run(s => s.length)
res19: Int = 6

scala> prog.run(s => s)
res20: String = NUM: 9

// ===

def prog[R]: Cont[R, String] =
  add(1, 2).flatMap { a =>
    mul(a, 3).flatMap { b =>
      show(b).map { s =>
        s.toUpperCase
      }
    }
  }