Meta-programming ruby for

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}"

I don't believe you should be wrapping your %Q in quotes for the class_eval bit. %Q{...} evaluates to an interpolated string. — leemeichin
– leemeichin, Commented Mar 3, 2012 at 17:54

ДМИТРИЙ МАЛИКОВ · Accepted Answer · 2012-07-23 05:21:10Z

4

Saas course, huh? You should know that instance variables should start with the @ sign. So for example in your initialize method what's kamil? If it is a instance variable it should be @kamil. I would also suggest that you revise your class_eval argument with respect to this consideration.

EDIT:

@#{attr_name}_history=[nil].

I would also put this code to some method because it's not very good to initialize your instance variable out of any method.

edited Jul 23, 2012 at 5:21

ДМИТРИЙ МАЛИКОВ

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answered Mar 3, 2012 at 17:54

Sergey

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Max Ivak · Accepted Answer · 2012-03-05 09:20:30Z

The code inside your method 'def attr_accessor_with_history(attr_name)' is invoked every time you call this method. You call it in your class when wrote class Klass attr_accessor_with_history :kamil ..

When Ruby process this line 'attr_accessor_with_history :kamil' it will actually run the code from the method Class.attr_accessor_with_history. A string inside class_eval is interpreted as code as it was written by you directly.

Finally, your interpreted code will be like this:

class Klass ..

@kamil_history=[nil]

def kamil=(value)
    kamil=value
    kamil_history.push(value)
end

See the problem? it must be @kamil=value, otherwise it will call the method 'kamil=' again, not accessing the instance variable @kamil.

Similarly, it must be '@kamil_history.push(..)'.

You can find the working code here: http://maxivak.com/ruby-metaprogramming-and-own-attr_accessor/

sepp2k · Accepted Answer · 2012-03-03 17:51:48Z

0

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.

answered Mar 3, 2012 at 17:51

sepp2k

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