The Ruby Way: Solutions and Techniques in Ruby Programming, Second Edition

8.3. Enumerables in General

What makes a collection enumerable? Largely it is just the fact of being a collection. The module Enumerable has the requirement that the default iterator each should be defined. Sequence as such is not an issue since even an unordered collection such as a hash can have an iterator.

Additionally, if the methods min, max, and sort are to be used, the collection must have a comparison method (<=>). This is fairly obvious.

So an enumerable is just a collection that can be searched, traversed, and possibly sorted. As a rule of thumb, any user-defined collection that does not subclass an existing core class should probably mix in the Enumerable module.

Bear in mind that what we say about one enumerable applies in effect ...