From: Peter Vanbroekhoven <calamitates@...>
Date: 2005-10-19T22:50:10+09:00
Subject: Re: [RCR] Cut-based AOP

On Wed, 19 Oct 2005, Dave Burt wrote:

> Peter Vanbroekhoven explained:
>> On Wed, 19 Oct 2005 dave.burt@gmail.com wrote:
>>
>> Did you mean the following? (Otherwise the class definition of Bar is
>> useless.)
>>
>>    Foo = Bar
>
> Quite. Let's imagine that's what I said.

Done :-)

> OK, great. I'm starting to see the usefulness of this. It occurs to me that
> there is some similarity between this and selector namespaces (targeted for
> Ruby 2) -- a lot of what I imagined namespaces being useful for (i.e. making
> changes to a core class that only affects code within a particular limited
> context) is accomplished easily using cuts.

There are definitely parallels, but cuts are at this moment not 
context-aware by themselves. Something like namespaces can be done as 
follows:

   cut FileLogging < File
     def initialize(*args, &blk)
       do_some_logging if Thread.current[:LOGGING_ENABLED]
       super
       do_some_more_logging if Thread.current[:LOGGING_ENABLED]
     end
   end

   def with_logging
     old_logging = Thread.current[:LOGGING_ENABLED]
     Thread.current[:LOGGING_ENABLED] = true
     yield
     Thread.current[:LOGGING_ENABLED] = old_logging
   end

   f1 = File.new('some_file') # => no logging
   with_logging do
     f2 = File.new('some_other_file') # => with logging
   end

I think though that this is different from selector namespaces, as my 
example provides different behavior in a dynamic scope, and selector 
namespaces are about static scope. That's provided I understand about 
selector namespaces myself.

> I still don't know anything, really, about Ruby namespaces. Something else
> to look forward too, and something complementary to this, I guess.
>
> Last thing, a criticism: shouldn't cut be a module, but not a class, because
> classes are instantiable?

First things first: when you talk about cut, is that Cut, or an instance 
of Cut? If it's the latter, then the answer is that we had long 
discussions about whether Cut should be a subclass of Class or of Module. 
There are some features that make it class, and some features that make it 
a module:

* a module because:
   - Not instantiable (though singleton classes aren't instatiable either)
* a class because
   - Not includable (though neither is Class which is a subclass of Module
     too)
   - It has a superclass, namely the class it cuts

So you see, there are arguments for both, and in the end we settled on it 
being a class.

Peter