Beautiful functional programming

[rhendric]

@tomjaguarpaw

sections' :: [Section Int]
sections' = flip evalState initialCounter $ do
  for sections $ \section -> do
    when (resetLessonPosition section) $
      lessonCounter .= 1

    lessons' <- for (lessons section) $ \lesson -> do
      lessonCounter' <- use lessonCounter
      let lesson' = lesson {lposition = lessonCounter'}
      lessonCounter %= (+ 1)
      pure lesson'

    sectionCounter' <- use sectionCounter
    let section' = section {sposition = sectionCounter', lessons = lessons'}
    sectionCounter %= (+ 1)

    pure section'

Things I can only dream of: an -XRecordUpdateDo that would let someone write:

section { do
  lessons <- for (lessons section) $ \lesson -> do ...
  sposition <- use sectionCounter
}

as sugar for

do
  lessons' <- for (lessons section) $ \lesson -> do ...
  sposition' <- use sectionCounter
  pure section { lessons = lessons', sposition = sposition' }