Disjoint forms in graphical user interfaces

Forms are parts of a graphical user interface (GUI) that show a (struc- tured) value and allow the user to update it. Some forms express a choice between two or more (structured) values using radio buttons or check boxes. We show that explicitly modelling such a choice leads to a cleaner separation of logic and lay- out. This is done by extending the combinator library FunctionalForms with dis- joint form combinators. To implement these, we have generalized the technique of compositional functional references which underlies the library.