Re: Redefining constants for a given instance only

module Ctx1
  class A
    def foo; 1; end
  end
  class B
    def bar; 2; end
  end
end

module Ctx2
  class A
    def foo; 4; end
  end
  class B
    def bar; 5; end
  end
end

class Evaluator
  def run(a)
    a ? Ctx1 : Ctx2
  end
end

code = "A.new.foo + B.new.bar"
puts Evaluator.new.run(true).module_eval code  # 3
puts Evaluator.new.run(false).module_eval code # 9

Here, Evaluator#run returns a module, and that sets the namespace for 
the constant resolution in the eval of code.

However, as Robert points out, classes are just regular objects in Ruby, 
and so can be passed around and used as such. It's perfectly normal and 
reasonable to return classes in variables, so you don't have to mess 
around with eval (which is often a security risk anyway)

class Evaluator
  def run(a)
    a ? [Ctx1::A,Ctx1::B] : [Ctx2::A,Ctx2::B]
  end
end

a, b = Evaluator.new.run(true)
puts a.new.foo + b.new.bar
a, b = Evaluator.new.run(false)
puts a.new.foo + b.new.bar

That is, the variables a and b contain classes, and you use them 
directly, independent of the fact that they happen to be bound to 
constants like Ctx1::A.

In fact, you can also have completely anonymous classes (using 
Class.new)

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