data GameState = GameState
    { gsSize :: (Double, Double) -- size of the window
    , time   :: Millisecond      -- current time of your game's state
    , player :: Player           -- your definition for a player
    , world  :: World            -- your definition for the world    
    }

es = EngineState
    { getTitle = myGetTitle
    , getW = myGetW
    , getH = myGetH
    , getTime = myGetTime
    , setTime = mySetTime
    , moveTime = myMoveTime
    , click = myClick
    , hover = myHover
    , drag = myDrag
    , resize = myResize
    , getSize = myGetSize
    , toGlInstr = myToGlInstr
    } :: EngineState GameState

data EngineState a = EngineState
    { click           :: PosX -> PosY -> a -> a -- how your game should change when clicked
    , hover           :: PosX -> PosY -> a -> a -- how your game should change when hovered
    , drag            :: PosX -> PosY -> a -> a -- how your game should change when dragged
    , resize          :: (Width, Height) -> a -> a -- how to resize your game
    , getSize         :: a -> (Width, Height) -- how to get the size of your game
    , moveTime        :: Millisecond -> a -> a -- how your game should change over time
    , getTime         :: a -> Millisecond -- how to get the current time of your game
    , setTime         :: Millisecond -> a -> a -- how to set the time of your game
    , getTitle        :: a -> String -- how to get the title of your game
    , toGlInstr       :: a -> RenderInstruction -- how to receive a render instruction to display your game
    }

data RenderInstruction = RenderNothing
                       | RenderWithCamera GlPosX GlPosY GlScaleX GlScaleY RenderInstruction
                       | RenderText String
                       | RenderLineStrip GlShape GL.GLfloat
                       | RenderTriangle GlShape
                       | RenderLineLoop GlShape GL.GLfloat
                       | RenderScale GlScaleX GlScaleY
                       | RenderTranslate GlPosX GlPosY
                       | RenderRotate Double
                       | RenderColorize GlColorRGB
                       | RenderColorizeAlpha GlColorRGBA
                       | RenderPreserve RenderInstruction
                       | RenderMany [RenderInstruction]