difference between modules inside a class and class inside a modules in ruby

Question

I think both are used for namespace. Modules are generally mixed into classes, right? So, what would be the purpose of defining a module inside a class?

Answer 1

Generally speaking modules have two uses:

• Namespacing modules. When you nest stuff here and the module is intended only for specifying paths.
• Functional modules. When the module has actual functionality that is intended to be called directly on the module or the module is intended to be included/extended.

Classes should be used for functionality only, even though Ruby doesn't enforce it.

Trying to do something but the above (like use module for both namespacing and functionality ⁽¹⁾ or use a class for namespacing ⁽²⁾) will generally confuse you.

---

^{(1) Some will disagree pointing to the rails' module with instance methods that also holds another module, called ClassMethods. I think it would have been cleaner if there was a module with two modules - ClassMethods and InstanceMethods instead.}

^{(2) Some will disagree. A probably valid case is if you try to emulate private classes from other languages (where the private class will be nested inside your public class).}