From: Wilbert Berendsen <wilbert@...>
Date: 2003-01-27T02:34:31+09:00
Subject: Re: "exclude" (was: Re: cascading configuration variables)

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On Sunday 26 January 2003 17:56, ts wrote:
> >>>>> "W" == Wilbert Berendsen <wilbert@oswf.org> writes:
>
> W> well, I would think B and all ancestors of B. But the stuff from A
> W> would still be available, because it also got included in C.
>
>  Unfortunately no : ruby never include twice the same module. If you
>  remove A and B, the stuff from A will not be available for the class D

I understand. But if every module has a list of (pointers to) different 
ancestors, then this would be easily done.

# e.g.: (assume A,B,C,D exist)
module E
  include A, B, C, D
end
# now module E has A,B,C and D mixed in.
module F
  include E
end

Now F has 1 ancestor. E has 4.  When ruby starts to search for a constant 
(or method or whatever) it first looks in F itself. Then in the ancestor 
that first got added, etc.

When I would do "exclude E" from within F, it would simply delete the 
internal pointer to the E module. Now E would not be accessible any more.

But this is all imaginary. In reality, is a mixed-in module really like a 
superclass? because a module can mix-in (include) another module in at 
any time of its life, but classes cannot inherit a superclass later on.

to illutrate all my musings:

module A
  KDE=true
end

module B
  KDE=false
  include A
end

B.module_eval "KDE"
>>> false

so at the moment A gets included in B, Ruby finds that there is already a 
constant with the name "KDE" and does not update it. But is this constant 
KDE now in B or in A?

Because:

module A
  KDE=true
end
module B
  include A
  KDE=false
end

issues a warning. Apparently, the "include A" from within B really sets 
the constant in B.

Let me make clear, I understand (I hope:) how Ruby does it, but I'm just 
curious.

all the best,
Wilbert

- -- 
Wilbert Berendsen (http://www.xs4all.nl/~wbsoft/)
To understand recursion, one must first understand recursion.
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