gistfile1.hs

type Stepper v = StepperImpl v s

data StepperImpl v s = StepperImpl { 
  initial :: s, 
  next :: s -> (v -> s -> StepState v s) -> StepState v s -> StepState v s 
}

data StepState v s = More v s | Done

step1 :: StepperImpl v s -> StepState v s
step1 k@(StepperImpl init _) = step k init

step :: StepperImpl v s -> s -> StepState v s
step (StepperImpl _ n) = f s More Done

data StepperRun v s = StepperRun (StepperImpl v s) (StepState v s)