# Pastebin 9bL24qdo
follow :: Dir -> [Knot] -> [Knot]
follow dir = para (\case
                  Nil -> []
                  Cons (Knot hp hs) ([], _) -> let np = fwd dir hp in [Knot np (Set.insert np hs)]
                  Cons ex@(Knot hp _) (rest@(Knot p s:_), remaining)
                    | touching hp p -> ex:remaining
                    | otherwise -> let np = towardsHead p hp in
                                         Knot np (Set.insert np s) : rest)
  where

    towardsHead tp@(tx,ty) hp@(hx,hy)
      | tx == hx || ty == hy = head [p | p <- around tp, touching hp p]
      | otherwise = head [p | p <- diags tp, touching hp p]

    diags (x,y) = [(x-1,y-1), (x-1,y+1), (x+1,y+1), (x+1,y-1)]

    touching p1 p2 = p1 == p2 || p1 `elem` aroundD p2