From: Peter McLain <peter.mclain@...>
Date: 2010-03-04T06:57:27+09:00
Subject: Re: Class vs Module in constant lookup


On Mar 3, 2010, at 12:15 PM, Caleb Clausen wrote:

> On 3/3/10, Peter McLain <peter.mclain@gemstone.com> wrote:
>>
>> Constant lookup behaves differently if there is a class versus a
>> module in the name:
>
> Extending your example a little to actually look up the constants
> instead of just checking if they're defined?:
>
>  XYZ = 10
>
>  module M
>  end
>
>  class C
>  end
>
>  p defined? M::XYZ  # => nil
>  p defined? C::XYZ  # => "constant"
>
>  C::XYZ
> #(irb):13: warning: toplevel constant XYZ referenced by C::XYZ
> #=> 10
>
>  M::XYZ
> # raises NameError
>
>
> Note the warning for C::XYZ. You really shouldn't look up constants
> this way, so I think a little inconsistency like this is tolerable.

   Well, the code isn't mine, but I'd still like to understand if  
there is a rationale for the difference in behavior, or if this is  
just another MRI implementation detail that's leaked out.  Currently,  
MagLev returns "constant" for both classes and modules, but that  
breaks third party code.  E.g,:

     return unless defined? RDoc::VERSION # RDoc 1 does not have VERSION
     # code that blows up if RDoc 1 being used...

One could argue that the proper test, which works for both classes and  
modules in all ruby implementations, is:

     return unless RDoc.const_defined? :VERSION

But I'd still like to understand if the difference in behavior is  
intended or an implementation quirk, and if it is intended, what the  
rationale is.

--  
Peter McLain
peter.mclain@gemstone.com