The Enumerable Contract

One method in, fifty methods out

Enumerable is the best deal in Ruby. Define one method — each — include Enumerable, and your class instantly gains map, select, reject, find, sort_by, group_by, sum, min_by, count, to_a, and ~40 more. All of them are implemented once, in the module, in terms of the each you wrote. This is the ancestor chain (module 24) paying off: Enumerable sits in your class's lookup path and every method drives your each.

In an interview, say: "Enumerable is a mixin that builds ~50 collection methods on top of a single each. Any class that can iterate can include Enumerable and get the whole query vocabulary for free — that's how Array, Hash, Range, and ActiveRecord relations all share the same API."