Metaprogramming: send, define_method and method_missing

Code that writes code

Metaprogramming means code that defines or changes code at run time, and Ruby makes it unusually easy. send(:method_name, *args) (or public_send, which respects visibility) calls a method chosen at run time. define_method creates methods programmatically, for example generating paid?, shipped? and cancelled? from a list of statuses. method_missing catches calls to undefined methods, enabling dynamic finders or proxies; when you use it, also define respond_to_missing? so respond_to? stays accurate. instance_variable_get/set, class_eval and instance_eval inspect and modify objects and classes. Hooks such as included, inherited and method_added let modules react when they are used. Frameworks such as Rails use metaprogramming heavily (for example has_many :orders defines a whole set of methods). In application code, use it sparingly: dynamic methods are harder to find with search, confuse IDEs and static analysis, and can be slower. Prefer explicit code until repetition clearly justifies generation.

Generating predicate methods and a careful method_missing

define_method for known names; method_missing with respond_to_missing?.

class Order
  STATUSES = %i[placed paid shipped cancelled].freeze
  attr_reader :status, :attributes

  def initialize(status:, **attributes)
    @status = status
    @attributes = attributes
  end

  STATUSES.each do |s|
    define_method(:"#{s}?") { status == s }          # placed?, paid?, shipped?, cancelled?
  end

  def method_missing(name, *args)
    key = name.to_s.delete_prefix("attr_").to_sym
    return attributes.fetch(key) if name.start_with?("attr_") && attributes.key?(key)
    super                                              # keep NoMethodError for everything else
  end

  def respond_to_missing?(name, include_private = false)
    (name.start_with?("attr_") && attributes.key?(name.to_s.delete_prefix("attr_").to_sym)) || super
  end
end

o = Order.new(status: :paid, city: "Pune", total: 1200)
puts o.paid?                     # true
puts o.attr_city                 # Pune
puts o.respond_to?(:attr_total)  # true
puts o.public_send(:shipped?)    # false

A receptionist with a rulebook

method_missing is a receptionist who handles visitors asking for rooms that do not exist on the directory, using a rulebook. Useful, but if the rulebook is vague, nobody can tell which rooms really exist.

Quick check: What should you define alongside method_missing?

• initialize
• to_s
• each
• respond_to_missing? so respond_to? reports dynamic methods correctly