From: James Coglan <jcoglan@...>
Date: 2008-09-03T16:15:27+09:00
Subject: Re: How can I prevent include one module multi-times?

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2008/9/3 James Coglan <jcoglan@googlemail.com>

>
>
> 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.
>

I should have mentioned that including a module multiple times will not have
any nasty side effects on the inheritance tree of the including class --
this is checked for you.

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