Library

• Ruby comes with a ton of functionality that is not available right away, so we need to require it as it stored in libraries.

  require digest

• Normally require statements should be placed at the very top of the file, so it is easy to see what libraries a particular piece of code (class) uses

Modules

• Modules are somewhat similar to classes: they are things that hold methods, just like classes do but it can not be instantiated => no method new

Mixin

• With modules you can share methods between classes: Modules can be included into classes, and this makes their methods available on the class, just as if we'd copied and paste these methods over to the class definition
• This is useful if we have methods that we want to reuse in certain classes, but also want to keep them in a central place, so we do not have to repeat them everywhere
• NOTES: Notice the keyword include, that is to make everything under a module available to the current context