Encapsulated applications

Definitely a speculative proposal, but it might lead somewhere.

The proposal is to create an abstract type to encapsulate and build up simulations, interactions, etc. Something like:

data Application state -- abstract

defaultApplication :: state -> Application state

withTimeStep :: (Double -> state -> state) -> Application state -> Application state
withEventHandler :: (Event -> state -> state) -> Application state -> Application state
withPicture :: (state -> Picture) -> Application state -> Application state

applicationOf :: Application state -> IO ()

The various event handlers, time steps, and visualization functions would all be run, in some defined order. Pictures would be stacked with the outermost picture on top.

Advantages:

1. No need to add a time step if it's not needed.
2. Ability to abstract different effects that span events, time, and pictures (such as UI controls).
3. Ability to incorporate more components (for example, sound?) without making the top-level entry point more complex.

The API is probably not right yet, but that's the idea.

Start by reviewing the existing top-level application flow and how event handlers, time steps, and pictures are currently composed. Decide whether the proposed abstraction and API are appropriate, define the ordering and stacking semantics, and confirm the design with maintainers before implementation.

Written by the indexing model from the issue text.