trait Domain{

   type  Read[A] = Reader[BoundsProblem, A]
   type Result[A] = EitherT[Read, String, A]

    def stepSize( s: State, direction: Direction): Result[Double] //depends on an injected context, can fail
    def takeStep( s: State, dir: Direction, stepSize: Double): Read[Variable] //depends on context, can't fail
    def calculate(x: Variable): (Double, Gradient) //context-independent, can't fail

     //doesn't compile:
   def iteration(s: State, dir: Direction) =  for{
          tee <- stepSize(s, dir)
         x <- takeStep(s, dir, tee)
          r <- calculate(x)
      } yield  r
 }