[arxanas]

I wouldn't say mix-ins are based on the concept of composition. The mix-in is not a discrete object that's part of the composition of the class.

[nahname]

Their effect of adding a mixin is similar to macros, but they provide the same abilities as composition. I wouldn't be surprised to find that mixins were based off of macros. They work really well in places where I would compose behaviour into an object.