    {-# 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.Data.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
            :: 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

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