One-time use setters

one_time_use_setters.rb

class Module
  def attr_accessor(name, once: false)
    ivar = "@#{name}"
    define_method "#{name}=" do |value|
      once && instance_variable_defined?(ivar) &&
        raise(ArgumentError, "Already initialized #{name}!")
      instance_variable_set ivar, value
    end

    attr_reader name
  end
end

class Whatever
  attr_accessor :resettable
  attr_accessor :set_only_once, once: true
end

w = Whatever.new
w.resettable = 1
w.resettable # => 1
w.resettable = 2
w.resettable # => 2

w.set_only_once = 1
w.set_only_once # => 1
w.set_only_once = 2  # ~> ArgumentError: Already initialized set_only_once!
w.set_only_once

# ~> ArgumentError
# ~> Already initialized set_only_once!
# ~>
# ~> program.rb:6:in `block in attr_accessor'
# ~> program.rb:27:in `<main>'