Objects with small behavior contracts

The idea

Ruby objects combine state with methods that operate on it. Instance variables belong to an individual object, while a class describes how new instances behave. Keep state private when callers should request operations instead of directly changing fields. A small public surface helps maintain invariants, such as a counter never decreasing or a record keeping a stable identifier.

A module can supply reusable methods without requiring a single inheritance hierarchy. Duck typing means code often depends on an object supporting the needed methods rather than having a particular class name. Make that expected behavior explicit in documentation and tests. Composition is useful when an object delegates a task to another collaborator, allowing a test implementation to be supplied without changing the caller.

module Named
  def label
    "Item #{@name}"
  end
end

class Card
  include Named
  def initialize(name)
    @name = name
  end
end

puts Card.new("A").label

Item A

The included module supplies label as an instance method. The method uses the instance variable initialized separately for each Card.