gistfile1.txt

Define a method attr_accessor_with_history that provides the same functionality as
attr accessor but also tracks every value the attribute has ever had:
class Foo
 attr_accessor_with_history :bar
end
f = Foo.new            # => #<Foo:0x127e678>
f.bar = 3              # => 3
f.bar = :wowzo         # => :wowzo
f.bar = 'boo!'         # => 'boo!'
f.bar_history #        => [nil, 3, :wowzo, 'boo!']
We'll start you off. The first thing to notice is that if we define
attr_accessor_with_history in class Class, we can use it as in the snippet
above. This is because, as ELLS mentions, in Ruby a class is simply an object of
class Class. (If that makes your brain hurt, just don't worry about it for now. It'll
come.) The second thing to notice is that Ruby provides a method class_eval
that takes a string and evaluates it in the context of the current class, that is, the
class from which you're calling attr_accessor_with_history. This string will
need to contain a method definition that implements a setter-with-history for the
desired attribute attr_name.
        Don't forget that the very first time the attribute receives a value, its history
    •
        array will have to be initialized.
        Don't forget that instance variables are referred to as @bar within getters and
    •
        setters, as Section 3.4 of ELLS explains.
        Although the existing attr_accessor can handle multiple arguments (e.g.
    •
        attr_accessor :foo, :bar), your version just needs to handle a single
        argument.
        Your implementation should be genreal enough to work in the context of any
    •
        class and for attributes of any (legal) variable name
        History of instance variables should be maintained separately for each object
    •
        instance. that is, if you do
        f = Foo.new
        f.bar = 1
        f.bar = 2
        f = Foo.new
        f. bar = 4
        f.bar_history
        then the last line should just return [nil,4], rather than [nil,1,2,4]
Here is the skeleton to get you started:
class Class
  def attr_accessor_with_history(attr_name)
     attr_name = attr_name.to_s                 # make sure it's a string
     attr_reader attr_name                      # create the attribute's
getter
     attr_reader attr_name+"_history" # create bar_history
getter
     class_eval "your code here, use %Q for multiline strings"
  end
end
class Foo
  attr_accessor_with_history :bar
end
f = Foo.new
f.bar = 1
f.bar = 2
f.bar_history # => if your code works, should be [nil,1,2]