Apples and oranges, chalk and cheese: choose the metaphor you prefer.
Everyone knows Excel is an enormously popular tool. However Excel is also infamous and has been implicated in many stories of financial mismanagement. Excel is attractive because it is easy to use. But as news stories and groups like the European Spreadsheet Risks Interest Group (EUSprIG) highlight, it is also easy to abuse and this abuse is not just or even mainly by novice users. Rather some of the most serious abuse is by seasoned professionals.
We identify many types of thing: animals, plants, concepts, pictures. Any one type is usually a member of a hierarchy (such as clothing-> outdoor-> footwear-> boot or Painting-> Landscape-> Rembrandt). In this way types allow us to workout how things can be accumulated, grouped and compared.
Using these relationships make any programming more reliable – including Excel formulas.
This article will review relevant ideas from general programming that should also apply to a type system for Excel. If you program using a language that uses a type system, you’ll probably have a good idea of the likely topics.