From: David Alan Black <dblack@...>
Date: 2002-08-04T23:28:47+09:00
Subject: Re: Assoc Class (Hash Pairs)

Hi --

On Sun, 4 Aug 2002, Tom Sawyer wrote:

> On Sun, 2002-08-04 at 07:38, David Alan Black wrote:
> > It's an interesting thing to think about, in part because it raises
> > the question of what "everything" is.  For example, an array is an
> > ordered list of elements, where each element can be of any type.  So
> > -- should there be an ArrayElement type?  I tend to think of "hash
> > pair" as sort of like "array element" -- I don't mean I think of them
> > as the same construct, but rather that, in both cases, I take the
> > object-hood of the underlying elements to be sufficient (in terms of
> > "everything is an object"), together with the object-hood of the
> > containers themselves.
>
> i think i understand what your getting at.
>
> but on particulars, there is such a thing as ArrayElement. it is called
> Object. for an array's elements are objects (100% O), while a hashes are
> not. rather they are some sort of syntatical-construct-of-association
> between two objects (nil-O)

You could argue that an array is basically a hash whose keys are
constrained to be Integers.  So that would leave you needing a
AssocWhereKeyIsAlwaysAnInteger class, I guess :-)

Another way to think of it is that hashes are just arrays which have
the ability to return odd-numbered elements from a query based on the
previous even-numbered element:

  class MyHash < Array
    alias :oldget :[]

    def [](k)
      ind = (0...size).detect {|i| i % 2 == 0 and self.at(i) == k}
      ind ||= size
      self.oldget(ind+1)
    end
  end

  m = MyHash.new.concat %w{ a b c d e f }
  p m['c']  #  =>  "d"

Dinky example, but you see the point: if arrays are OK, and methods
are OK, then if hashes can be defined in terms of arrays and their
methods [Q: is this CS'ly sound?], then hashes are OK.


David

-- 
David Alan Black
home: dblack@candle.superlink.net
work: blackdav@shu.edu
Web:  http://pirate.shu.edu/~blackdav