Added a version of the turtle example from Seth Tisue's talk on lenses.

examples/Turtle.hs

@@ -0,0 +1,65 @@
+{-# LANGUAGE TemplateHaskell #-}
+{-# LANGUAGE DeriveDataTypeable #-}
+-- | A simple Turtle-graphics demonstration for modeling the location of a turtle.
+--
+-- This is based on the code presented by Seth Tisue at the Boston Area Scala 
+-- Enthusiasts meeting during his lens talk.
+--
+-- Usage:
+--
+-- > def & forward 10 & down & color .~ red % turn (pi/2) & forward 5
+module Turtle where
+
+import Control.Lens hiding (up, down)
+import Data.Default
+
+data Point = Point
+  { _x, _y :: Double
+  } deriving (Eq,Show)
+
+makeClassy ''Point
+
+instance Default Point where
+  def = Point def def
+
+data Color = Color
+  { _r, _g, _b :: Int
+  } deriving (Eq,Show)
+
+makeClassy ''Color
+
+red :: Color
+red = Color 255 0 0
+
+instance Default Color where
+  def = Color def def def
+
+data Turtle = Turtle
+  { _tPoint  :: Point
+  , _tColor  :: Color
+  , _heading :: Double
+  , _penDown :: Bool
+  } deriving (Eq,Show)
+
+makeClassy ''Turtle
+
+instance Default Turtle where
+  def = Turtle def def def False
+
+instance HasPoint Turtle where
+  point = tPoint
+
+instance HasColor Turtle where
+  color = tColor
+
+forward :: Double -> Turtle -> Turtle
+forward d t =
+  t & y +~ d * cos (t^.heading)
+    & x +~ d * sin (t^.heading)
+
+turn :: Double -> Turtle -> Turtle
+turn d = heading +~ d
+
+up, down :: Turtle -> Turtle
+up   = penDown .~ False
+down = penDown .~ True