My Haskell way of thinking of it would be that Square and Rectangle are distinct (non-inheriting) data types, Square and Rectangle are both instances of the typ

My Haskell way of thinking of it would be that Square and Rectangle are distinct (non-inheriting) data types, Square and Rectangle are both instances of the type-class Rectangular (which inherits and provides default functions for Closed and Polygonal), Square has a transformation into Rectangle, and Rectangle has a transformation into Maybe Square.