Question

I’m trying to track every class variable’s history by metaprogramming. I’m not a fan of asking such questions but It took me 5 hours to be able to write these and from now on I have no idea how to proceed (I’m new to ruby, and this is the first time I’m playing with metaprogramming).

In my understanding; when attr_accessor_with_history initializes in a class, it should execute the code it is containing. Thus, every time this method initializes, by the merits of metaprogramming every class is going to have its own method for the problem I described.

In the code I submitted, readers are initialized properly but I can’t say the same about the code in class_eval part. I need clarification about why the code isn’t working, and metaprogramming in general.

class Class
  def attr_accessor_with_history(attr_name)
    attr_name = attr_name.to_s
    attr_reader attr_name
    attr_reader attr_name + "_history"

    class_eval "%Q{
    @#{attr_name}_history=[nil] 
    def #{attr_name}=(value)
        #{attr_name}=value
        #{attr_name}_history.push(value)
    end
    }
    "
  end
end

class Klass
  attr_accessor_with_history :kamil
  def initialize(value)
    kamil = value
  end
end

a = Klass.new(5)
a.kamil = 1
puts "#{a.kamil_history}"

Answer 1

To call a setter method on self, you’ll need to write self.foo = bar. If you just write foo = bar it will just create local variable named bar and not call any method. So you’ll need to change lines 11 and 23 accordingly.

Also by using %Q{} inside quotes, your whole eval will actually just evaluate to a string. You should use %Q{} or quotes – not both. In fact you probably shouldn’t use a string at all, but call class_eval with a block and use define_method inside the block.