From: Xavier Noria <fxn@...>
Date: 2007-09-21T02:28:25+09:00
Subject: Re: Idiomatic Ruby for Array#extract / Range#length?

On Sep 20, 2007, at 6:56 PM, Rick DeNatale wrote:

> You lost me, so what's the rational which is the successor to 1/3?

OK, the point is that we are not using the everyday order of Q. Our  
target are Ruby ranges, and I took custom orders and succ on a subset  
of Q to show a (theoretic) property of Ruby ranges that follows from  
the fact that they only assume #<=> and #succ.

In Ruby we are defining a class[*]

   class MyReorderedRationalsBetween0and1
     attr_reader :q

     def initialize(q)
       @q = q
     end

     def <=>(p)
       ...
     end

     def succ
       ...
     end
   end

such that we have a range r

   my0 = MyReorderedRationalsBetween0and1.new(0)
   my1 = MyReorderedRationalsBetween0and1.new(1)
   r   = my0..my1

which is well-defined and infinite. That is what I meant when I said  
that an enumeration of the rationals in [0, 1] with suitables #<=>  
and #succ would give an example of infinite range.

I demonstrated that is possible in theory, taking a function f that  
set theory guarantees it exists.

Now, I didn't give a computable f you can encode in Ruby, but I am  
sure there's one. Of course it wouldn't really work in practice  
because you can't represent all rationals in a real computer. So in  
the first place there's no way you can go over the rationals in a  
real computer, no matter whether you have an f or not.

I think I could come with a simpler example that had the naturals  
plus infinity plus perhaps something else technically needed, and use  
standard orderings and succs for the finite part.

-- fxn

[*] There's a detail about 1.succ that I won't address to keep it  
simple.