From: "Aleksi Niemelä" <aleksi.niemela@...>
Date: 2000-09-27T07:02:44+09:00
Subject: [ruby-talk:5143] Re: Types and ===

Hal asks:
> # Are there any two types X and Y such that  x===y and y===x both have
> # meaning, and mean different things?

Conrad helps:
> Do relations like inside ... count as === case candidates?

I was thinking much harder cases, but this one is really easy. 

Think that regexp.===(other) actually delegates to self.match(other) so the
meaning of === is thought to be purely useful in context of String (or a
class which implements to_s). 

If we take a Bag, and put stuff in it, we could argue that the Bag has other
functions that merely containing something. So let's go one step higher -
let's take a Container. Container merely has some stuff and it's main
function is to contain. Thus Container#=== maps to Container#contains?.

<friendly_note>
Now comes a lot of meaningless discussion, so if you're busy you should skip
to the last paragraphs.
</friendly_note>


So it makes sense to write

  case bought_object
  when shopping_bag
    puts "dull old you"
  when bag_of_tricks
    puts "hmm...you seem quite interesting person to me"
  else
    log "seems we have a hacker here, well, he can't misuse our systems"
  end

Remember also there's a this weird cube with infinite surface and zero
volume. So nothing could be inside it. Still you could easily stuff that
thing in bag_of_tricks. There's no point, however, to claim it's an
container - no it's purely an entertainer. It's main function is to
surprise. There's entire group of entertaining wonders of world, and
ZeroVolumeCube "implements" Entertainer interface, which among all fun
things has a method to query if this entertainer is surprising.

On the other hand, the cube surprise only people pondering it's essence, and
ordinary data structures couldn't care less (even though the Hash would be
compeletely green because of envy to other type of hash, which could contain
infinite number of elements but take only constant space :).

  module Entertainer
    def surprising?; end
  end

  class ZeroVolumeCube
    def surprising?(target)
      case target
      when PeopleWare
        true
      when Hardware
        false
      when Software
        false
      when AlienFormOfLife
        true
      else
        false
      end
    end
    def ===(other)
      surprising?(other)
    end
  end

Now, while my examples are purely made, and might not make any sense I could
argue the functionality in above bought_object case should claim me as
interesting person if I buy ZeroVolumeCubes. At the same time I might query
of all the candidates in my space what I should do with them:

tiny_zvc  = ZeroVolumeCube.new(size => miniature)
circus    = Circus.new
colosseum = Circus.new(size => CIRCUS::ENORMOUS)

girl_candidate_space.each do |girl|
  case girl
    when tiny_zvc
      puts girl.details + " is just like I want it"
    when circus
      puts girl + " is fun company too"
    when colosseum
      puts girl + " is nice but simply can't afford her"
    when /ann/i
      puts girl.name + ", had too much of these"
    when Cook
      sure_choices << girl  end
end

(OT note, few of my ex-girlfriends have 'nn' in their names, like Anna,
Anne, Anne, Anne, Hanna, Hanna. B-} )

I hope I made clear few points. In any case I'd like to summarize them even
if they just got more blurred:

1) the classes might not be interested in reproducing functionality of == in
===.
2) the classes will probably implement their only, sole 
   interesting usage in ===.
3) the x == y is commutative, the x === y might not be

The third point needs more argumentation. Take an example of the case of
container (like a bag) which implements === as bag.contains?(object) and if
object does not do === like a container does the results differ.

Here's the catch. Even if the objects does the go of container
(ZeroVolumeCube#contains? is trivial and really uninteresting for anything
but claiming it's a special case of a container) we might not have same
results.

So, finally, as an example of (x===y) != (y===x) I can conclude 

  bag.===(zero_volume_cube)   #=> true
  zero_volume_cube.===(bag)   #=> false
	
Uh, let me be excused... :)

  - Aleksi