From: Michael Neumann <mneumann@...>
Date: 2004-11-22T21:49:24+09:00
Subject: Re: Typical/idiomatic examples of dynamic code generation with Ruby?

Iwan van der Kleyn wrote:
> Hi there,
> 
> I'm writing a paper on the "rediscovery of dynamic languages".
> Definition issues aside (what *is* a "dynamic" language, anyway?), in
> it I try to show how "old" (Smalltalk, Lisp) and "new" (Python, Ruby)
> programming languages are vastly superior for the cost-effective
> development of scalable and secure applications compared with static
> languages like C/C++ and half-breeds like Java and C#. Nothing
> original for sure, but then I again I'm trying to postulate some
> convincing arguments for a non-geek, managerial audience.
> 
> In the paper I'm comparing "idiomatic" examples of how these languages
> deal with common problems. Problem is, I'm quite new with Ruby and I'm
> a bit at a loss for examples on how to show Ruby's strengths with
> dynamic code generation (metaprogramming). That is often mentioned as 
> one of Ruby's strengths. Don't get me wrong, I've found that Ruby has 
> got the necesarry nuts and bolts but I couldn't really figure out what I 
> could do with it, what I could not easily do with Python as well.
> 
> So my question: do you know of any examples on the subject of dynamic
> code generation which would be typical for Ruby and could not be
> easily implemented in languages like Python?

I think stuff like "attr_accessor" is typical for Ruby. It could be 
implemented like this:

   class Module
     def attr_accessor(*attrs)
       attrs.each do |attribute|
         class_eval "def #{ attribute }() @#{ attribute } end"
         class_eval "def #{ attribute }=(val) @#{ attribute } = val end"
       end
     end
   end

   # and use it

   class MyClass
     attr_accessor :a
     def initialize
       @a = "test"
     end
   end

   m = MyClass.new
   p m.a # => "test"
   m.a = 2
   p m.a # => 2

You can do the same for defining abstract methods:

   class Module
     def abstract(*meths)
       meths.each do |meth|
         class_eval "def #{ meth }(*args, &block) raise 'abstract 
method' end"
       end
     end
   end

And for much more else. Have a look at ActiveRecords, which uses quite a 
lot of metaprogramming, I guess.

Regards,

   Michael