Changing self class from inside a method??

Let's start off with the assumption I want a method that allows an
object to transform itself into another object.  In other words:


begClass = a.class
a.changeToSomethingElse(42)
endClass = a.class

# Where begClass != endClass

As a more specific example, consider some hopeful code for allowing
auto-creation of arrays from nil, perl-ish style:


class Nil
  def push(*els)
    a = Array.new
    a.push(*els)
    #self = a             # Nope!
    #self.class = Array   # Nope!
  end
end

This would allow:
  a = nil
  a.push(1,2,3)
  puts "#{a.class}"

I know we have things like to_a, but that requires an assigment instead
of just the method call.  Let's just assume we need a way to do it like
this.

Is this just considered impossible, or is there a way to change/convert
the class of self from inside a method?  I can imagine it would cause
complications the moment the class changes, since we'd suddenly be in
the method of the wrong class, but if any language had a way to do it, I
figure it'd be ruby.

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