From: Tom Sawyer <transami@...>
Date: 2002-07-05T09:53:36+09:00
Subject: Re: self_parent

hi david,

yes the REXML isn't really a good example, it was just a passing
thought. you may a good point. using a "Factory" would add a
consideration for self_parent's use. but i don't see how it could
possibly "constrain code and code growth significantly." when one
designs a program it is exactly that: designed. no what i'm saying?
self_parent is just another tool.

~transami


On Thu, 2002-07-04 at 17:45, David Alan Black wrote:
> Hi --
> 
> On Fri, 5 Jul 2002, Tom Sawyer wrote:
> 
> > class Customer
> >   attr_reader :name, :address
> >   def initialize(name, address)
> >     @name = name
> >     @address = Address.new(address)
> >   end
> >   def pretty_name
> >     return @name.upcase
> >   end
> > end
> >
> > class Address
> >   def initialize(address)
> >     @address = address
> >   end
> >   def print_label
> >     puts self_parent.pretty_name
> >     puts @address
> >   end
> > end
> >
> > c = Customer.new("John Doe", "111 Test Ln.\n Santa Fe, NM 87501")
> > c.address.print_label
> 
> This introduces a dependency between objects, though, based on a kind
> of "self" stack, which won't necessarily mesh with the actual design
> and behavior of the classes and objects.
> 
> For example, let's say you wanted to refactor/rewrite Customer like
> this:
> 
>   class Factory
>     def Factory.make(thing,*args)
>       const_get(thing).new(*args)
>     end
>   end
> 
>   class Customer
>     def initialize(name, address)
>       @name = name
>       @address = Factory.make("Address", address)
>     end
>     ...
>   end
> 
> Not a very elegant example -- but the point is, now you're two "self"s
> away, instead of one, but your Address objects are hardcoded to look
> one "self" away.
> 
> I know Ruby allows for a lot of introspection... but I think relying
> on this would constrain code and code growth significantly.
> 
> > no need to pass self and explicitly store it. so there's a fictional
> > example for you. in the real world there are many cases of such a
> > parent-child relationship. in fact i think REXML has an explicit Parent
> > class it uses to glue such realtionships together. hiearchy structures
> > too (like a tree) are another area in which a parent-child relationships
> > of this sort are used. and i'm sure there are many others.
> 
> REXML is specifically modeling the parent/child relationships of XML
> in that class, though (I think).  That's different from a
> language-level (I mean Ruby, not XML) description of one object as the
> "parent" of another because the second object is created by a method
> of the first object.
> 
> 
> David
> 
> -- 
> David Alan Black
> home: dblack@candle.superlink.net
> work: blackdav@shu.edu
> Web:  http://pirate.shu.edu/~blackdav
> 
> 
-- 
~transami

"They that can give up essential liberty to obtain a little
 temporary safety deserve neither liberty nor safety."
	-- Benjamin Franklin