Re: NEWBIE QUESTION: pattern with nil

On Thu, 11 Aug 2005, Austin Ziegler wrote:

> I didn't go into detail there, John, but it's worth noting that
> originally Codd thought that there should be several different NULL
> values for relational databases. Later in life, he came to believe
> that NULL values (both the single NULL value in SQL databases and
> the multiple NULL values in his original relational papers) were
> bad.


No, as I remember it, his problem wasn't with NULL, but 
rather the DB design.

ie. All instances where you have NotApplicable or NoThing null IN A 
DB TABLE can be refactored to a better relational design that didn't 
require them.

However, having factored the DB into more tables so neither variety of 
NULL occurred, there is still a need for NULL. There is still a need on 
occasion to construct reports for user display which are joins of various 
tables. And in those "views" there may be some fields are going to be 
NULL. And again, it is still necessary to disambiguate between 
NotApplicable and NoThing.

> Your proposal of multiple types of nil makes life harder. Instead
> of:
>
>  if foo.nil?
>    # blah
>  end
>
> I would have to do:
>
>  if foo.nothing? or foo.undefined? or foo.not_applicable?
>    # blah
>  end

How about starting with the following in the core

class Object

# This is as is already
   def nil?
     false
   end

   def uninitialized?
     false
   end

   def not_applicable?
     false
   end


   def nothing?
     false
   end
end

class NilClass
   # No change here
   def nil?
     true
   end

   def to_s
     raise "No method, old definition inconsistent."
   end
end

class NoThing < NilClass
   def nothing?
     true
   end

   def empty?
     true
   end

   def to_s
     ""
   end

   def to_i
     0
   end
end

class NotApplicable < NilClass
   def not_applicable?
     true
   end
end

class Uninitialize < NilClass
   def uninitialized?
     true
   end
end

Now if the rest of the system chose the right version of nil when it 
assigned nil to something, the problem would go away and we can extend the 
NoThing in a sane harm free fashion.

> The problem, as others have stated, is that adding #empty? to
> NilClass implies a containment. I'd have as big a problem if we
> added #empty? to Fixnum so that 0 returned true and everything else
> returned false. (Indeed, would that be correct? Would -3.empty? be
> true or false?)

Tut! Tut! I'm having this same problem with my son at school, they teach 
him the rote mechanics, but forget to teach the principles. (To be fair, I 
only started learning these things when I start reading foundations of 
mathematics type books from the university library...)

Come now, what is 3? 3 is 1+1+1. What do we mean by 1+1+1 we mean three 
contains a 1 and a 1 and a 1! So if I extract the contents of three I get 
1 and 1 and 1 back.

And -3? Surprise surprise it contains three minus ones!

There is a strong overlap between core system design and axiomatic 
mathematics.

class FixNum
   def empty?
     self == 0
   end

   def length
     self
   end

   def each
     if self < 0
       # -3 contains three minus ones...
       (1..-self).each{|i| yield -1}
     else # And 3 contains three ones.
       (1..self).each{|i| yield 1}
     end
   end
end







John Carter                             Phone : [phone]
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New Zealand

Carter's Clarification of Murphy's Law.

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From this principle, all of life and physics may be deduced.