Re: Redefining constants for a given instance only

 Hi Robert,

    thanks for your reply. 

> That can lead to confusion with regard to the class names.

I know, but that shouldn't cause me problems in this particular case.

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

class A; end
class B; end
class C; end

class Evaluator
  def evaluate
     #complex expression using one or more among A,B,C, like for example
     A.method1 == :test && @myvar.is_a?(B) && C.method2 == :other_test
  end
end

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

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.

In any case, thanks for your time :-)

Andrea
--

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From: "Robert Klemme" [e-mail]
To: [e-mail]
Cc: 
Date: Mon, 6 Aug 2012 02:28:09 +0900
Subject: Re: Redefining constants for a given instance only

> On Sun, Aug 5, 2012 at 7:19 PM, Andrea Dallera  wrote:
> > Hello,
> >
> >     let's say we have two empty classes:
> >
> > class ClassA
> >
> > end
> >
> > class ClassB
> >
> > end
> >
> > then we have a third one, which has a method that references the first
> > class:
> >
> > class Evaluator
> >
> >    def evaluate
> >        return ClassA
> >    end
> >
> > end
> >
> > What I'd like to do is to be able to redefine what ClassA is for a given
> > instance of Evaluator, like so:
> >
> > ev = Evaluator.new
> > 
> > ev.evaluate #ClassB
> >
> > I found out how to do this, but only class-wide:
> >
> > ev = Evaluator.new
> > ev.evaluate #ClassA
> > class Evaluator
> >   ClassA = ClassB
> > end
> > ev.evaluate #ClassB
> 
> That can lead to confusion with regard to the class names.
> 
> > Ideas?
> 
> Well, the simplest is to just store it in a member.
> 
> irb(main):001:0> Evaluator = Struct.new :other_class do
> irb(main):002:1* def evaluate; other_class; end
> irb(main):003:1> end
> => Evaluator
> irb(main):004:0> class A;end
> => nil
> irb(main):005:0> class B;end
> => nil
> irb(main):006:0> e1 = Evaluator.new A
> => #
> irb(main):007:0> e1.evaluate
> => A
> irb(main):008:0> e2 = Evaluator.new B
> => #
> irb(main):009:0> e2.evaluate
> => B
> 
> Note: you do not necessarily need the Struct, I just picked it because
> that saved me a bit of typing.
> 
> Kind regards
> 
> robert
> 
> 
> -- 
> remember.guy do |as, often| as.you_can - without end
> http://blog.rubybestpractices.com/
>