The “data is the app” people are right too, because from a user’s perspective, the UI is the data model as well. Different controls map in users’ minds to the different data types – check box = Boolean, dropdown list or radio-button group = enum, spinner = integer, slider = floating point, text box = string, window with icons = unsorted set, list view = 1D array, grid view = 2D array, tree view = tree. To prove this, add a tree view to your app’s save-file dialog, and watch how much deeper they start nesting their folders.