Making Ruby Strings Immutable by Default

immutable_strings.rb

module BeforeMethodHook
  def before(*names)
    names.each do |name|
      m = instance_method(name)
      define_method(name) do |*args, &block|  
        yield self
        m.bind(self).call(*args, &block)
      end
    end
  end
end

class MutableString < String
  def mutable?
    true
  end

  def freeze
    raise "Can't freeze a mutable string, silly."
  end

  def frozen?
    false
  end
end

class String
  extend BeforeMethodHook

  before(*instance_methods) do |obj|
    unless obj.mutable?
      unless @freeze_mutex
        @freeze_mutex = true
        obj.freeze unless obj.frozen?
        @freeze_mutex = false
      end
    end
  end

  def mutable?
    false
  end

  def frozen?
    true
  end
end

str = "hello"

begin
  str.upcase!
  puts "Something went wrong, the previous operation should have caused an exception"
rescue => e
  puts "Error: #{e.message}"
end

str = MutableString.new("hello")
str << "world"
puts str