From: Aleks Kissinger <aleks0@...>
Date: 2006-07-26T06:28:09+09:00
Subject: Automagic class prototyping

Anybody know any slick techniques for automatically prototyping
classes so they can interact with each other in top-level code?  I've
run through two methods, but I'm not real wild about either of them.

Say for example, classes represent some security label and I want to
allow things with one label to access those of another label.  So,
these hypothetical classes all descent from Sec....

class Sec
  class <<self
   def allow(c)
    (@allowed_classes||=[]) <<c
   end
  end
end


And I want to ultimately just indescriminately run stuff like this:

class Documents < Sec
  allow Downloads
end

class Downloads < Sec
  allow Documents
end

But this is a run-time error unless the classes are prototyped somehow.  Like:

class Downloads < Sec; end

class Documents < Sec
  allow Downloads
end

class Downloads < Sec
  allow Documents
end

As a framework, this would be so much nicer for the end-user if it
does that automatically, so she doesnt have to thing about declaration
order.  So thats the problem.  Possible solutions I've tried:

1. Grep-prototyping:
File.grep out all class defs and eval them before I actually require
the code.  I don't like this because it chokes on classes defined
inside of modules or other classes.  Grep has no idea about the
structure of the ruby code.

2. Delayed execution of the 'allow' statements.
I dump allow statements off into proc's then after all the definitions
are loaded, I run them.  This works great, but syntactically its
clunky.

class Documents < Sec
   later do
     allow Downloads
   end
end

class Downloads < Sec
  later do
    allow Documents
  end
end

with:
class Sec
  class << self
    def later(&code)
      (@code_for_later||=[]) << code
   end
end

And I go back and exec all the @code_for_later blocks when all the
files are require'd.  So, if any of you have questions about this code
or have any ideas for a prettier way to do it, I'd love to hear it.