With the FP approach you can just write "show book as a widget" function yourself, without the whole inheritance hodge-podge. In case of real libraries the chan

With the FP approach you can just write "show book as a widget" function yourself, without the whole inheritance hodge-podge. In case of real libraries the change of data structure probably won't be a problem as author would probably supply a function Book -> (Title, Chapters, Sentences, Strings) which would be guaranteed not to change and discourage the use of actual constructors.