water.hs

> import Control.Monad.Tardis
> :{
let levels hs = evalTardis (traverse 
      (\h -> do x <- min <$> getFuture <*> getPast -- Get the height of the enclosing walls
                modifyBackwards (`max` h)          -- Send the right max back to the past
                modifyForwards (`max` h)           -- Send the left max into the future
                pure (max 0 (x - h))               -- Calculate the height of the water itself
      ) hs) (0, 0)
:}
> sum $ levels [5,3,7,2,6,4,5,9,1,2]
14

@lalaithion Oh, you're right, well spotted! I think we could fix it perhaps by swapping those two lines?