From: Joel VanderWerf <vjoel@...>
Date: 2003-11-30T17:02:41+09:00
Subject: Re: class << x


The 'class << x' notation has some weaknesses:

* It looks a bit like a heredoc, but there is no analogy.

* '<<' does not have any other role as an operator or special form 
associated with singleton classes.

* '<<' suggests an append or shift operation, or perhaps a "much less 
than" comparison, or even some kind of bracket ('<<x,y,z>>') notation.

* It looks too much like 'class Y < x' in which the new class Y is less 
than x in the ancestral sense. However, the analogy is false--there is 
no class below x.

* How do you read it? "class under x"? "class singleton of x"?

It has bothered me for the last 3 years, but I simply accept it as an 
idiom and get on with life. We don't always have to construct meaning 
for the symbols we push around.

Still, I'd feel better if there were some method on objects that 
returned the singleton class and you could use that in a class 
definition, something like this:

   def singleton(x); class << x; self; end; end

   x = [0,1,2,3]

   # class singleton(x); end # parse error!

   singleton(x).class_eval do
     def last_elt; at(-1); end
   end

   p x.last_elt   # ==> 3

But this is a poor workaround, since do..end and class..end have 
different scoping rules.