From: dblack@...
Date: 2007-09-03T02:59:18+09:00
Subject: Re: Shouldn't it be class >> self , or am I dyslexic?

Hi --

On Mon, 3 Sep 2007, Ben Tompkins wrote:

> I like it when people agree with me, and according to the diagram on
> page 384 of Programming Ruby, you are basically correct to say that
> "self inherits from the singleton class." The diagram shows that a
> "virtual" (singleton) class is inserted into the inheritance hierarchy
> for self (or whatever the RHS happens to be) as an immediate subclass of
> the most derived, non-singleton, class of which the RHS is an instance
> whenever 'class << RHS ...' is executed. So even if we think of the '<<'
> as an "inheritance operator," it is still pointing in the wrong
> direction! But is inheritance really the right concept here? Notice that
> the singleton class is inserted below the most specific class of the RHS
> term.  If the RHS were the same as its class (which is impossible), then
> Marcin would be right, because the singleton class would be subclassing
> the RHS.  But no instance is ever the same as its class. What is really
> going on is that we are
> augmenting the interface of the RHS by decorating its most derived class
> with an anonymous class to which RHS has exclusive access, thus
> effectively adding the methods of the singleton class to the RHS. As the
> recipient of these new methods, RHS is on the receiving side of the
> operation, which is why 'class >> RHS ...' is more intuitive than 'class
> << RHS ... .'

I'm sorry, but I had to chuckle at the use of the word "intuitive"
after that explanation :-)

I wouldn't think of class << obj as having anything at all to do with
inheritance. It's more like this, in terms of how the class keyword
works:

   class expr

where expr can be:

   A
   A < B
   << arbitrary_object

So the inheritance < is more like:

   class (A < B)

and the singleton notation is more like:

   class (<< obj)

Remember, also, that the class keyword isn't itself a class
identifier; you're not looking for "the class that is >> object", but
the class that is, so to speak, yielded from object. (Or something.)
It's not a notation that relates very closely to any other.


David

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