What are some good examples of Mixins and or Traits?

Question

I was reading up on Ruby, and learned about its mixins pattern, but couldn't think of many useful mixin functionality (because I'm not used to thinking that way most likely). So I was wondering what would be good examples of useful Mixin functionality?

Thanks

Edit: A bit of background. I'm Coming from C++, and other Object languages, but my doubt here is that Ruby says it's not inheriting mixins, but I keep seeing mixins as Multiple inheritance, so I fear I'm trying to categorize them too soon into my comfort zone, and not really grok what a mixin is.

Answer 1

They are usually used to add some form of standard functionality to a class, without having to redefine it all. You can probably think of them a bit like interfaces in Java, but instead of just defining a list of methods that need to be implemented, many of them will actually be implemented by including the module.

There are a few examples in the standard library:

Singleton - A module that can be mixed into any class to make it a singleton. The initialize method is made private, and an instance method added, which ensures that there is only ever one instance of that class in your application.

Comparable - If you include this module in a class, defining the <=> method, which compares the current instance with another object and says which is greater, is enough to provide <, <=, ==, >=, >, and between? methods.

Enumerable - By mixing in this module, and defining an each method, you get support for all the other related methods such as collect, inject, select, and reject. If it's also got the <=> method, then it will also support sort, min, and max.

DataMapper is also an interesting example of what can be done with a simple include statement, taking a standard class, and adding the ability to persist it to a data store.