From: Knut Franke <knut.franke@...>
Date: 2010-09-13T06:10:23+09:00
Subject: Declarative relations between object attributes

Some time ago I stumbled over Cells[1], a Common Lisp extension allowing
one
to declare relations between instance variables (called slots in CL);
i.e. a
change to one variable will automatically recompute other variables
depending
on it. I think it's pretty neat, but using it in any interesting context
is
somewhat hampered by the fact that few people go through the
considerable
trouble involved in learning Common Lisp.

Just today it occurred to me that it should be pretty easy to do
something
similar in Ruby. Indeed, a couple of codelines later, I had a Ruby
version of
the standard Cells example (well, a simplified version) running:

class Motor
   cell :temperature, :status
   def initialize
      self.temperature = 0
      calculate :status do
         if self.temperature < 100
            :on
         else
            :off
         end
      end
   end
end
m = Motor.new
m.observe(:temperature) { |old, new| puts "temperature: #{old} ->
#{new}"  }
m.observe(:status) { |old, new| puts "status: #{old} -> #{new}"  }
m.temperature = 80
m.temperature = 110

=>

temperature: 0 -> 80
temperature: 80 -> 110
status: on -> off

That's certainly not dramatically new; you can do similar things with
the
observer pattern or Qt signals, for example. However, I like the idea of
just
declaring that one variable (status) is a certain function of one or
more
other variables (temperature) and have the library take care of all the
rest.
Makes for cleaner code, particularly if you do model/view programming
(see
model-view.rb in [2]).

I'm not yet sure what to do with this. Possibly similar/better solutions
already exist. Certainly the code needs some work. So I figured that
before
going further I'd try to get some feedback; particularly

* Do you think this is useful?
* Do you know related projects?
* What could be improved?

I've put the code on github[1]. Any input appreciated. Thanks.


Knut

[1] http://common-lisp.net/project/cells/
[2] http://github.com/nome/ruby-cells
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