special_attr_accessor.rb

class ClassToAddMethodsTo
  def self.special_attr_accessor(attr, set: ->(val){val}, get: ->(val){val})
    define_method attr do
      get.(instance_variable_get "@#{attr}")
    end
    define_method "#{attr}=" do |rhs|
      instance_variable_set "@#{attr}", set.(rhs)
    end
  end
  
  special_attr_accessor :x, set: ->(val){val*3000}, get: ->(val){val.fdiv(300)}
  
  def initialize(x)
    self.x = x
  end
end

foo = ClassToAddMethodsTo.new(1)
puts "starting x: #{foo.x}"
foo.x = 30
puts "new x: #{foo.x}"