Ruby Language Method Definitions are Expressions


Example

Defining a method in Ruby 2.x returns a symbol representing the name:

class Example
  puts def hello
  end
end

#=> :hello

This allows for interesting metaprogramming techniques. For instance, methods can be wrapped by other methods:

class Class
  def logged(name)
    original_method = instance_method(name)
    define_method(name) do |*args|
      puts "Calling #{name} with #{args.inspect}."
      original_method.bind(self).call(*args)
      puts "Completed #{name}."
    end
  end
end

class Meal
  def initialize
    @food = []
  end
  
  logged def add(item)
    @food << item
  end
end

meal = Meal.new
meal.add "Coffee"
# Calling add with ["Coffee"].
# Completed add.