From: Austin Ziegler <halostatue@...>
Date: 2006-06-15T00:20:13+09:00
Subject: Re: Cut-based AOP

On 6/14/06, Ruby Newbie <rubynewbie@gmail.com> wrote:
> Looking around, you have hundreds of posts, and you're involved as a
> mentor in Google Summer of Code "Automated Wrapper Generation for
> Information Extraction" arena. I take it you have the experience to
> back up what you're saying.

I like to think I do, at least. I will freely admit that I don't "get"
AOP. I *understand* the reasons for AOP (adding logging for something
you no longer necessarily have source access to, for example), but I
have *long* felt that AOP is a pure hack.

> Can you provide references to better and more-established ways of
> offering such extensibility?  I looked briefly at traits which you'd
> alluded to earlier, but it requires changes to the core code for
> trait/hook insertion.  I'm interested to read about other mechanisms
> which are more maintainable and extensible than AOP in Ruby if you
> could please share them.

If you control the source, write the source better. If you *want*
extensibility points, provide clear hooks.

  class Foo
    def bar
      pre_baz if respond_to? :pre_baz
      baz
      post_baz if respond_to? :post_baz
    end
  end

Matz will be providing (somewhat) cleaner ways to do this, but that's
the essential nature of the beast. It's a form of plugin-style
programming and that's well understood, while AOP is a poorly defined
and easily misunderstood beast.

Similarly, mixins provide the main capability that Ruby needs -- but the
original programmer has to write the code to be able to deal with this
properly. And if they don't, you can always inherit or compose.

I think that it's important to note that *even now* (five years on from
the PARC research) you can't find many clearly-written, concise
explanations of AOP and why you'd want it. It's all buzzwords.

AOP may be valuable. But I have yet to see anything that convinces me
that it's worth all the bother. It's sort of like Inversion of Control.
Interesting, but not relevant to most (98%+) needs.

-austin
-- 
Austin Ziegler * halostatue@gmail.com * http://www.halostatue.ca/
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               * austin@zieglers.ca