Rotate a square in Gloss (with key event (DOWN-key))

gistfile1.hs

module Main where

-- http://www.f13g.com/%A5%D7%A5%ED%A5%B0%A5%E9%A5%DF%A5%F3%A5%B0/Haskell/GLUT/#content_1_4

import Graphics.Gloss.Interface.Game

data State
  = State { angle :: Float
          , isPositive :: Bool
          , picture :: Picture }

diffAngle :: Float
diffAngle = 14.4

main :: IO ()
main
  = gameInWindow
    "GuruGuru"
    (200, 200)
    (10,10)
    black
    10
    initialState
    draw
    handleInput
    (\_ s -> s { angle = changeAngle s
               , picture = draw s } )

changeAngle :: State -> Float
changeAngle (State t x _)
  | x         = t + diffAngle
  | otherwise = t - diffAngle

initialState :: State
initialState
  = State { angle = 0
          , isPositive = True
          , picture
              = Color white
              $ Scale 120 120
              $ Polygon [ ( 0.10,  0.10)
                        , (-0.10,  0.10)
                        , (-0.10, -0.10)
                        , ( 0.10, -0.10)
                        ] }

draw :: State -> Picture
draw (State t _ _)
  = rotate t (picture initialState)

handleInput :: Event -> State -> State
handleInput (EventKey k ks _ _) s
  | SpecialKey KeyDown <- k
    , Down <- ks
      = s { isPositive = not (isPositive s) }
  | otherwise = s
handleInput (EventMotion _) s = s