From: "Jan E." <lists@...>
Date: 2012-08-17T02:59:46+09:00
Subject: Re: A Ruby class is never closed

What you do mean? A sealed class like in C# which doesn't allow 
subclasses? Or the singleton pattern where a class can only be 
instantiated once?

You can actually do both. For a sealed/final class you can use the 
"inherited" hook which gets called every time a new class is derived 
from this class. If it raises an error, the class is "sealed" (the 
question is if this makes sense).

class A
  def self.inherited subclass
    raise "cannot derive from sealed class #{self}"
  end
end

# this doesn't work
class B < A; end

For the singleton pattern, there's the singleton module in the standard 
library.

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