Ruby.markdown

Core ideas

• An Object is a set of instance variables and a pointer to a 'singleton class'.
• Properties are looked up in the instance variables, methods are dispatched via the singleton class.
• Module is a subtype of Object. A Module is a set of methods and an ordered list of zero-or-more 'parent' modules.
• Module A becomes a parent of module B via B.include(A).
• Method lookup works by doing a depth-first right-to-left search of a module tree.
• Class is a subtype of Module. A Class is a Module that can be instantiated.
• A Class has only one 'superclass'. A class includes its superclass as its first parent module for the purposes of method dispatch. A class's singleton class includes the superclass's singleton class as its first parent.
• The default superclass of all classes is Object.
• Object includes the Kernel module as a parent.

Warts

• Although Class is a subtype of Module, you cannot call B.include(A) if A is a Class.
• Singleton classes are not really classes. You cannot instantiate or inherit from them.

Sugar

• obj.extend(M) is sugar for obj.singleton_class.include(M).