I think the way some dynamically typed languages use maps is interesting. In most languages if you have a hashmap, all values have to have the same type. In a dynamic language you can have some members be methods, some members be values of potentially different types and so on. Of course, depending on what you want to achieve, you might be able to use a struct for example. A map is more flexible though. You can union two different maps, or you can have a function that takes a map that has either a or b. It's not necessarily impossible to express this in static types either, but there are many things here that quickly become tedious to do with types that Just Work in dynamically typed languages.