steps.nim

type
  HalfStepUp[T] = object
    upper, step: T
  StepUp[T] = object
    lower, upper, step: T

  HalfStepDown[T] = object
    upper, step: T
  StepDown[T] = object
    lower, upper, step: T

proc `|+`[T](upper, step: T): HalfStepUp[T] =
  HalfStepUp[T](upper: upper, step: step)

proc `..`[T](lower: T; s: HalfStepUp[T]): StepUp[T] =
  StepUp[T](lower: lower, upper: s.upper, step: s.step)

proc `|-`[T](upper, step: T): HalfStepDown[T] =
  HalfStepDown[T](upper: upper, step: step)

proc `..`[T](lower: T; s: HalfStepDown[T]): StepDown[T] =
  StepDown[T](lower: lower, upper: s.upper, step: s.step)


iterator items*[T](x: StepUp[T]): T =
  var it = x.lower
  while it <= x.upper:
    yield it
    inc it, x.step

iterator items*[T](x: StepDown[T]): T =
  var it = x.lower
  while it >= x.upper:
    yield it
    dec it, x.step

for i in 5 .. 15 |+ 3:
  echo i

echo "--------------"

for i in 15 .. 5 |- 3:
  echo i