From: Robert Klemme <shortcutter@...>
Date: 2012-08-06T06:18:44+09:00
Subject: Re: Redefining constants for a given instance only

On Sun, Aug 5, 2012 at 7:48 PM, Andrea Dallera <andrea@andreadallera.com> wrote:
> Unfortunately, I'd like to avoid storing the class in a variable and then
> referring to it because then the code inside what I called Evaluator would
> have to be aware of the mechanics.

> The method Evaluator#evaluate might also
> refer to more than one constant

From the rest of your posting it's clear that you mean "classes" and
not "constants".

> and, ideally, shouldn't be at all aware that
> one of the constant it's using is not what it originally was defined:

I think you should step back a bit and rethink.  Then, what is the
logic you actually want to implement?

> mind that the swap should happen only for a given instance of Evaluator, not
> at the class level, in a way such as

Why then are you insisting on constants?  Constants are definitively
the wrong mechanism in this case.

> ev1 = Evaluator.new
> ev2 = Evaluator.new
> ev1.set(A).to(B) #something like this, or worse named ev1.set(A,B) for
> simplicity
> ev1.evaluate #here, A.is_a?(B)#true.
> ev2.evaluate #here, not so
>
> I'm not even sure if that's possible to do in ruby at this point. Opening
> the singleton class (class << ev1) and defining the constant there works (in
> the sense that the singleton class has the constant correctly  changed) but
> the methods defined on the class correctly ignore it.

If you want something constantish then I suggest you use a Hash with
Symbols as keys in Evaluator like

class Evaluator
  def initialize
    @classes = {
      :a => A,
      :b => B,
    }
  end

  def evaluate
    a_instance = @classes[:a].new
    ...
  end

  def set_class(label, class_object)
    raise "Wrong arguments" unless label.class == Symbol &&
class_object.class == Class
    @classes[label] = class_object
  end
end

Cheers

robert

-- 
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/