PFDSFractal.hs

-- Draws an image similar to the cover of Okasaki's Purely Functional Data Structures

module Main where

import Diagrams.Backend.SVG.CmdLine
import Diagrams.Prelude

side = sqrt 2

triangleRect :: Diagram B
triangleRect = polygon ( with
  & polyType .~ PolySides
     [ 135 @@ deg, 90 @@ deg]
     [ 1        , side      ]
  )

sierpinski :: [Colour Double] -> Diagram B
sierpinski [c] = triangleRect # fc c
sierpinski (c:cs) =
  let s' = sierpinski cs
  in (((phantom s' ||| s') === (s' ||| s')) # center) `atop`
     (triangleRect # fc c # scale (2 ^^ (length cs)) # center)

main =
  let colors = map (\f -> blend f black white) [1, 0.95, 0.9, 0.85, 0.8, 0.75, 0.7] ++ [orange]
  in mainWith (sierpinski colors # lw 0)