You need a lot of discipline in the overall design of your application so that you can add code and data at any time without messing up the design. It's easy t

You need a lot of discipline in the overall design of your application so that you can add code and data at any time without messing up the design. It's easy to add code in most languages, even for unexpected features which were not considered at first place. In Haskell it can be painful to do that.