Lens example

Lens.hs

{-# LANGUAGE TemplateHaskell #-}

import Control.Lens

data Ball = Ball { _position :: (Double, Double), _velocity :: (Double, Double) }
  deriving (Show)

-- create the accessors for the Ball type
makeLenses ''Ball

-- | Animates a ball's position with respect to a timestep.
-- Takes a ball's existing position and velocity and animates it
-- by the time step provided
--
animate :: Ball -> Double -> Ball
animate b t = do
  position .~ (px + vx * t, py + vy * t) $ b
 where (px, py) = b ^. position
       (vx, vy) = b ^. velocity

main :: IO ()
main = do
  -- the original ball
  let b = Ball { _position = (4.5, 6.2), _velocity = (-0.3, 1.2) }
  -- animate the ball by 1 full timestep
  let b' = animate b 1

  putStrLn $ "Initial ball : " ++ (show b)
  putStrLn $ "Animated ball: " ++ (show b')