Ruby module instance and class methods

A very common idiom especially when code starts to become complicated is to put functions in modules and mix in that behavior to several classes.  And a common expansion is to have a single "include" add both instance and class methods.

Here is how to do it:

"include" is used to add a module's instance methods as instance methods to the including class, but it doesn't traverse sub modules.  "extend" is used to add a module's instance methods as class methods to a class.

When a module is "included" it can register a callback which is passed the class that is doing the including.  And calling "extend" on the base and passing a submodule will cause it to put those methods at the class level.  This causes a chain reaction of loading both instance and class methods using a single "include".

Pro Tip: It is possible to cause the same behavior using "extend" by using the "extended" callback; however, this is not common and "include 'x'" reads better then "extend 'y'".