From: Peter Vanbroekhoven <calamitates@...>
Date: 2005-10-19T20:26:16+09:00
Subject: Re: [RCR] Cut-based AOP

On Wed, 19 Oct 2005 dave.burt@gmail.com wrote:

> What is a cut? Is it a subclass that hides its parent's class? Are the
> following two snippets the same?
>
>  cut Bar < Foo; baz() end
>
>  class Bar < Foo; baz() end
>  Bar = Foo  # (except for this warning)

Did you mean the following? (Otherwise the class definition of Bar is 
useless.)

    Foo = Bar

> If not, what is the difference?

There are a few subtle differences:
* In the second snippet, Foo and Bar are the same. This means that code
   that changes Foo changes Bar as well. In the first snippet, Bar is
   invisible to someone using Foo. Changing Foo only changes Foo and leaves
   Bar alone.
* In the second snippet, existing instances of class Foo are not affected,
   while they are in the first case.

So the basic idea is the same, but a cut is designed to keep existing code 
from breaking. It's supposed to be non-intrusive. That's what a cut does 
and your second snippet does not.

Actually, in the beginning we considered something like your second code
snippet, but we decided it was not sufficient for our purposes as we 
wanted to allow sneaking in a cut into an existing class hierarchy without 
breaking the code that uses and changes these classes.

Peter