From: 7stud -- <bbxx789_05ss@...>
Date: 2011-04-13T03:14:44+09:00
Subject: Re: Copying parameters to singleton class

Lars Olsson wrote in post #992249:
> Hello again!
>

I was fooling around with your original requirement:

> I want to create a class method that takes a bunch
> of options and returns a singleton class with those
> options set,

...and I think I was probably running into the same problems you were:

class Object
  def singleton_class
    class << self
      self
    end
  end

end

class Opportunities
  def self.using(options)
    # Store options in singleton class variable
    # @options and then return singleton class

    obj = self.new
    singleton = obj.singleton_class
    singleton.instance_variable_set(:@options, options)

    singleton.class_eval do  #<----PROBLEM HERE
      def options
        @options
      end
    end

    singleton

  end

end


Foo = Opportunities.using(one: 1, two: 2, three: 3)
Bar = Opportunities.using(four: 4, five: 5, six: 6)

p Foo.options
p Bar.options

--output:--
prog.rb:37:in `<main>': undefined method `options' for 
#<Class:#<Opportunities:0x88cfef8>> (NoMethodError)

The problem is that options() is an instance method of the singleton 
class, but in this line:

Foo.options

Foo is the singelton class, and therefore options() is being called as a 
class method of the singleton class.  A class method of a singleton 
class??!!  You can actually create such a thing in ruby:

class Object
  def singleton_class
    class << self
      self
    end
  end
end

class Opportunities
  def self.using(options)
    # Store options in singleton class variable
    # @options and then return singleton class

    obj = self.new
    singleton = obj.singleton_class
    singleton.send(:instance_variable_set, :@options, options)

    singleton.singleton_class.class_eval do  #<---SOLUTION***
      def options
        @options
      end
    end

    singleton

  end


end


Foo = Opportunities.using(one: 1, two: 2, three: 3)
Bar = Opportunities.using(four: 4, five: 5, six: 6)

p Foo.options
p Bar.options

--output:--
{:one=>1, :two=>2, :three=>3}
{:four=>4, :five=>5, :six=>6}

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