From: David Flanagan <dflanagan@...>
Date: 2009-05-14T07:05:03+09:00
Subject: Re: Superclass of eigenclass

On May 7, 9:21 am, Rick DeNatale <rick.denat...@gmail.com> wrote:
> On Thu, May 7, 2009 at 11:50 AM, Rick DeNatale <rick.denat...@gmail.com> wrote:
> > On Thu, May 7, 2009 at 8:35 AM, Danny O cuiv <danny.oc...@gmail.com> wrote:
> >> On page 261 of The Ruby Programming Language, they state:
>
> >> "Class objects are special: they have superclasses. The eigenclasses of
> >> class objects are also special: they have superclasses, too. The
> >>eigenclassof an ordinary object stands alone and has nosuperclass."
>
> > I'm pretty sure that this is a typo.
>
> Just for the record, I just submitted the following erratum report to
> the O'Reilly site for the book:

Rick,

It wasn't a typo, and it correctly describes how method inheritance
works, but it doesn't reflect the reality of the superclass method of
eigenclasses.

Your suggested replacement text won't work as a replacement--the point
of the paragraph in question is to explain method inheritance, not to
get into the nitty gritty about eigenclasses.

Having said that, I should perhaps try to fit some of these eigenclass
details into my next edition, as well as your argument for why the
term "singleton class" makes sense.

    David Flanagan


> Page 261 2nd Paragraph:
> The paragraph says:
>
> "Class objects are special: they have superclasses. The eigenclasses of
> class objects are also special: they have superclasses, too. Theeigenclassof an ordinary object stands alone and has nosuperclass."
>
> Both theeigenclassof a class object and theeigenclassof an
> ordinary object have asuperclass.  If not then sending a message to
> an ordinary object would end up invoking method_missing.
>
> I think that the paragraph should read:
>
> "Class objects are special: they can have subclasses. The eigenclasses of
> class objects are also special: they can have subclasses, too. Theeigenclassof an ordinary object can have neither a subclass, nor any
> other instance except that single ordinary object.  If an ordinary
> object with aneigenclassis duplicated with the Object#dup method,
> the duplicate will not acquire any of it's singleton methods. If it is
> cloned with the Object#clone method, the result will get aneigenclass
> which is a duplicate of the original objectseigenclass, so that eacheigenclasshas but a single instance."
>
> The following ruby code (which behaves identically on Ruby 1.8.6,
> 1.8.7 and 1.9.1) illustrates the above:
>
> module Kernel
>   defeigenclass
>     class << self;self;end
>   end
> end
>
> class TestClass
>
>   def bar
>     :bar
>   end
>
>   def self.foo
>     :foo
>   end
> end
>
> t = TestClass.new
> def t.gorp
>   :gorp
> end
>
> p t_eigenclass = t.eigenclass# => #<Class:#<TestClass:0x232d0>>
> p t_eigenclass.superclass# => #<Class:TestClass>
>
> p t.bar # => :bar
> p t.gorp # => :gorp
>
> p t_dup = t.dup
> p t_clone = t.clone
> p t_clone.respond_to?(:gorp) # => true
> p t_clone.eigenclass== t_eigenclass # => false
> p t_dup.respond_to?(:gorp) # => false
>
> p t.class.foo # => :foo
> p t.eigenclass.foo # => :foo
>
> --
> Rick DeNatale
>
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