Step-by-step Heighway dragon fractal. Author: Pontus Granström
> import Diagrams.Backend.SVG.CmdLine
> {-# LANGUAGE FlexibleContexts #-}
> import Diagrams.Prelude
> import Diagrams.TwoD.Layout.Grid

One way to define the Heighway dragon is iteratively. If we have a dragon of a certain level of detail, we can create the next, more detailed, dragon as follows: Take two copies of the previous dragon, rotate them, invert one of them, scale them, and stick them together.

> nextDragon trail = (trail # rotateBy (-1/8)
>                     <> trail # rotateBy (5/8) # reverseTrail)
>                    # scale (1/sqrt 2)

With this, we can now generate an infinite sequence of increasingly detailed dragon curves, starting with a straight line.

> dragonCurves = map (trailLike . (at origin)) (iterate nextDragon initialTrail)
>   where
>     initialTrail = hrule 1

The above is enough to generate a Heighway dragon of arbitrary level of detail, but let’s go a little further to show the relation of successive curves in the sequence.

withPrevious combines each diagram in a list with a shadow of the previous one.

> withPrevious diagrams = zipWith (<>) diagrams (mempty : diagrams # opacity 0.2)

We remember the order of the diagrams by giving them names, so that we can lay them out and then show the order with arrows.

> rememberOrder :: [Diagram B] -> [Diagram B]
> rememberOrder = zipWith named [0::Int ..]
>
> showOrder :: Diagram B -> Diagram B
> showOrder diagram
>   = diagram # applyAll (map addArrow [0 .. length (names diagram)])
>   where
>     addArrow n = connectOutside' opts n (n + 1)
>     opts = with & gaps .~ normalized 0.005
>                 & headLength .~ tiny

Finally, we put all of the above together, with some layout tricks to make the diagrams and arrows align properly. gridSnake lays out the diagrams in a “snaking” grid, so that each diagram is adjacent to the previous one.

> example = dragonCurves # withPrevious
>                        # take 12
>                        # sameBoundingRect
>                        # rememberOrder
>                        # map (frame 0.1)
>                        # gridSnake
>                        # showOrder
>                        # lw ultraThin
>
> main = mainWith (example :: Diagram B)
> main = mainWith (example :: Diagram B)