[tikhonj]

The lesson here should not be "make illegal states representable", but rather "the state of the form widget in the UI" and "the state of the value the form represents" are conceptually distinct. We should explicitly consider the state of the form, the state of what the form represents/controls and the mapping between them. We should think about these separately and represent them in our code separately.

[dvdkon]

I think you often have an opportunity to model the UI closely after the internal state, in data entry UIs mostly, and you should take that opportunity. It means less code complexity, and the users will expect the UI to closely match whatever database entity you're storing anyway.