From: James Coglan <jcoglan@...>
Date: 2008-09-03T16:14:16+09:00
Subject: Re: How can I prevent include one module multi-times?

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2008/9/3 Thomas B. <tpreal@gmail.com>

> Zhao Yi wrote:
> > I am looking for a method to check whether this module has been
> > included. Does Ruby have this feature?
>
> What's your question? In the subject you ask 'How can I prevent include
> one module multi-times?' and this is checked automatically without any
> special handling.



That's partially correct, but you'll find that the module's included() hook
gets called repeatedly:

module M
  def self.included(base)
    puts "included M"
  end
end

class C
  # prints "included M" 3 times
  include M
  include M
  include M
end

To check whether M is already mixed into C, the expression 'M > C' returns
true if C includes M. You could put this check inside M.included and throw
an exception if it's true.

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