From: Devin Mullins <twifkak@...>
Date: 2005-11-16T23:42:03+09:00
Subject: Re: The subtleties of const_missing

Hi,

Rails does this very thing. You should check out their const_missing and 
see if it suffers from the self-same problem.

Devin

Nicholas Seckar wrote:

> Hello all,
>
> I've run into an issue regarding the use of const_missing. This issue  
> revolves around the fact that these two cases are supposed to behave  
> differently from each other:
>
> irb(main):001:0> B = 10
> irb(main):002:0> module A; B; end
> => 10
> irb(main):003:0> A::B
> NameError: uninitialized constant A::B
>         from (irb):3
>
> However it seems impossible to detect which case we are in within the  
> const_missing method. A more detailed explanation follows:
>
> Suppose we are lazy and avoid typing "require 'my_module.rb'" by  
> defining a const_missing that performs that require when MyModule is  
> first referenced. Now, suppose we are writing my_class, and wish to  
> include MyModule. So, we put this code in my_class.rb:
>
> class MyClass
>   include MyModule
>   ...
> end
>
> Our const_missing handler is called, and it loads 'my_module.rb',  
> finds the now present MyModule constant, and returns it. Everything  
> is good.
>
> However, let's say we're slightly misguided and do MyClass::MyModule.  
> Since this is not defined, our const_missing handler is once again  
> called. 'my_module.rb' is thus loaded again, (usually to no effect,)  
> and MyModule returned.
>
> This is a pretty clear violation of the semantics of ::. Indeed, Ruby  
> will produce a warning regarding our misbehaved const_missing. That  
> said, there does not seem to be a way to behave correctly; from  
> inside the const_missing handler there is no way to tell which case  
> the constant is missing in.
>
> One way to 'fix' this is to have our const_missing look in it's  
> parent modules for the missing constant. If we find it there, we know  
> this is a case of A::B. However this only works if the missing  
> constant has been defined in one of the parents. Thus, we can still  
> load ::B using A::B, but afterwards A::B will fail.
>
> So, sorry for rambling on. The real question is: Can const_missing  
> detect if we are in a "A::B" case rather than "module A; B; end" ?
>
> Thanks for your time and any replies,
> Regards,
> Nicholas Seckar
>
>