    {-# LANGUAGE RankNTypes #-}

    -- We export this stuff separately so we don't clutter up the
    -- API of the Graphics.Gloss module.

    -- | Simulate mode is for producing an animation of some model who's picture
    --   changes over finite time steps. The behavior of the model can also depent
    --   on the current `ViewPort`.
    module Graphics.Gloss.Interface.Pure.Simulate
     	( module Graphics.Gloss.Data.Display
            , module Graphics.Gloss.Data.Picture
    	, module Graphics.Gloss.Data.Color
    	, simulate
            , ViewPort(..))
    where
    import Graphics.Gloss.Data.Display
    import Graphics.Gloss.Data.Picture
    import Graphics.Gloss.Data.Color
    import Graphics.Gloss.Internals.Interface.ViewPort
    import Graphics.Gloss.Internals.Interface.Simulate
    import Graphics.Gloss.Internals.Interface.Backend

    -- | Run a finite-time-step simulation in a window. You decide how the model is represented,
    --      how to convert the model to a picture, and how to advance the model for each unit of time.
    --      This function does the rest.
    --
    --   Once the window is open you can use the same commands as with @display@.
    --
    simulate :: forall model
            .  Display                      -- ^ Display mode.
            -> Color                        -- ^ Background color.
            -> Int                          -- ^ Number of simulation steps to take for each second of real time.
            -> model                        -- ^ The initial model.
            -> (model -> Picture)           -- ^ A function to convert the model to a picture.
            -> (ViewPort -> Float -> model -> model) -- ^ A function to step the model one iteration. It is passed the
                                                     --     current viewport and the amount of time for this simulation
                                                     --     step (in seconds).
            -> IO ()

    simulate display backColor simResolution
             modelStart modelToPicture modelStep

     = simulateWithBackendIO defaultBackendState
             display backColor simResolution
             modelStart
             (return . modelToPicture)
             (\view time model -> return $ modelStep view time model)