From: Robert Klemme <shortcutter@...>
Date: 2007-02-17T00:45:17+09:00
Subject: Re: class design issues

On 16.02.2007 14:35, Spitfire wrote:
> Robert Dober wrote:
>>> class LifeForm
>>>    attr_reader :age, :name, :foo
>>>
>>>    def initialize(age,name,foo)
>>>      @age = age
>>>      @name = name
>>>      @foo = foo
>>
>>        freeze
>>
>> and you might add freeze here, now it becomes quite tough to change
>> the LifeForm object.
>> Personally I do not know any way to modify it now, but someone will
>> show us soon, I am quite sure ;)
>>
>   Sorry, I'm no expert in Ruby. So you have to explain what 'freeze' 
> does???

It prevents further manipulation of an instance:

irb(main):001:0> %w{foo bar bx}.shift
=> "foo"
irb(main):002:0> x=Struct.new(:name).new("foo")
=> #<struct #<Class:0x7ef96ce8> name="foo">
irb(main):003:0> x.name = "bar"
=> "bar"
irb(main):004:0> x.freeze
=> #<struct #<Class:0x7ef96ce8> name="bar">
irb(main):005:0> x.name = "foo"
TypeError: can't modify frozen Struct
         from (irb):5:in `name='
         from (irb):5
         from :0
irb(main):006:0> x
=> #<struct #<Class:0x7ef96ce8> name="bar">
irb(main):007:0> s="foo"
=> "foo"
irb(main):008:0> s << "bar"
=> "foobar"
irb(main):009:0> s
=> "foobar"
irb(main):010:0> s.freeze
=> "foobar"
irb(main):011:0> s << "xxx"
TypeError: can't modify frozen string
         from (irb):11:in `<<'
         from (irb):11
         from :0

>   Let me add more hypothetical requirements to my problem (sorry for not 
> stating these initially!)

Right.  Your new set of requirements rules out "freeze" as that won't 
allow for adding of children etc.

>   Lets consider that LifeForm has a property called 'Rank'. Now, this is 
> a very critical property that I must make sure to retain consistent. 
> LifeForm can also have offsprings, which are tied to it, say by a 
> instance variable that points any LifeForm to its list of offspring 
> LifeForm objects.
> 
>   Now, the rank of a LifeForm is its distance from all its ancestors. I 
> want a functionality such that whenever you create a LifeForm, the rank 
> is set to '0'. Next, I want to make sure that when I add offsprings to 
> an existing LifeForm, its depth gets updated automagically, without my 
> intervention. More specifically, I want to have a feature by which 
> LifeForm has a mechanism in the class, which allows to it set by itself 
> the 'rank' of its instances. And, when I add a child, say through a 
> method 'add_Child' (don't know if this is the ideal solution, but this 
> is what I can think of!), it does something like this,
> 
>   for each child in new_children_added
>       child.depth = child.depth + current.depth
>         # current refers to parent or current object
>   end

You have a tree data structure here (if you have multiple roots it's 
called a "forest").  This is one of the well researched and understood 
structures.  You'll find plenty of implementations and information on 
the web.

Actually when adding a child, I would go up recursively to the root of 
the tree and update the rank.  Kind of:

def add_child(ch)
   delta = rank + 1

   # BFS because Ruby is not good at recursion
   q = [ch]

   until q.empty?
     obj = q.shift
     obj.rank += delta
     q.concat obj.children
   end
end

You could make method rank= protected to prevent accidental invocation 
from the outside.

>   actually I want this to be carried out to all children newly added, 
> their children and so on. So that the ranks of a LifeForm is always 
> consistent! Hope I've conveyed exactly what I want.

Yes.  One solution is to calculate the rank on demand and only optimize 
this to a local variable if you have performance issues.  That solution 
is much easier because then you can simply do

def rank
   obj, r = self, 0
   while obj
     r += 1
     obj = obj.parent
   end
   r
end

This is slow but always consistent.

>   Now I want to be able to only 'read' this rank, not modify it from 
> outside the LifeForm class. Is this possible? If so, how do you design it?

See above for ideas.  Although I would probably not spend too much 
efforts in making it impossible to change the value from the outside. 
There is always a way.

Kind regards

	robert