From: John Mair <jrmair@...>
Date: 2010-10-07T21:29:02+09:00
Subject: Re: Why does Module#include exclude the module's metaclass?

@Urabe,

sorry if my reply seemed rude or snarky, i did not mean it that way :)

yeah #ancestors behave strange with real_include, I think this is due to 
the fact that normal T_ICLASS store the actual module in the klass 
pointer but real_include store a singleton class there, which #ancestors 
does not expect.

I think the best approach (if real_include or something like it accepted 
as a patch) would be to store the actual module in a hidden __module__ 
instance variable on the ICLASS (just like __attached__ for singleton 
class) and #ancestors can look at this instead of klass pointer.

I can see some other arguments for not bringing in module singleton 
methods with #include, one for example is that the hook methods 
included() and extended() will be accessible to the class too, which is 
ugly.

I think there are OK arguments either way; but it's not such a serious 
issue really - to be honest i was just curious why it is that way :)

Thanks for your time:)

John

Urabe Shyouhei wrote:
> (2010/10/07 10:59), John Mair wrote:
>> 1) I agree that singleton class of the module is actually a 'class' but 
>> (if we allowed this behavior) it would only be included when and only 
>> when the module is included. We can just consider this a special 
>> exception, no?
>> 
>> This exception would only apply to singleton class of modules, of 
>> course, normal classes cannot be included in this way.
> 
> So you were actually not asking why, but claiming this is wrong.  Not 
> sure for
> that.  Your real_include gem breaks Class#ancestor in a strange way so I 
> guess
> there are some pitfalls, though noting particular.
> 
>> 2) Given that class inheritance does include the singleton, do you not 
>> think it 'surprising' that module inclusion does not? I found it 
>> surprising.
> 
> Tips: principle of least surprise is a wrong way for you to persuade 
> matz
> because at any moment he can say "I don't think so".  Try a different 
> route.
> 
>> 3) Many people get around this limitation in module inclusion by abusing 
>> the included() hook and then extending a ClassMethods submodule, I think 
>> this is ugly.
> 
> Maybe.  But I think a module's having class method is itself ugly.  It 
> tends
> to smell like a API design mistake.  I would recommend you to rethink 
> why you
> need such things.
> 
>> 3) Out of curiosity, are there any other reasons (besides the one you 
>> gave) for why modules do not include the singleton? What reason did Matz 
>> give?
> 
> No, I do not know other reasons.  It is rather obvious to me.  Of course 
> that
> does not ensure there is not reason though.

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