From: Csaba Henk <csaba@..._for_avoiding_spam.org>
Date: 2004-08-06T06:36:27+09:00
Subject: Sets are not extensional

(FYI: the phenomena described in the sequel were seen produced by the following
ruby instances: a vanilla 1.8.1, a Debianish 1.8.2 and a cvs snapshot
from the 10th of May, identifying itself as 1.9.0.)

require 'set'
#the empty set:
Set[]
 => #<Set: {}>
#the singleton of the empty set:
Set[ [Set[]] ]
 => #<Set: {[#<Set: {}>]}>
#if sets were extensional as God ordered, this would give the same, but...
Set[ [Set[], Set[]] ] 
 => #<Set: {[#<Set: {}>, #<Set: {}>]}>
#and also...
Set[Set[]].member? Set[]
 => false
[Set[], Set[]].uniq
 => [#<Set: {}>, #<Set: {}>]
Set[ [Set[]] ] - Set[ [Set[]] ]
 => #<Set: {[#<Set: {}>]}>
#anyway...
Set[] == Set[]
 => true
Set[ [Set[]] ] == Set[ [Set[]] ]
 => false

(Set.new behaves the same way as Set[])

Now I don't see what's the purpose of having a set class which is not
extensional (ie., two sets behave as the same one in all respect if their
elements are the same). But, given it's non-extensional, why isn't it
consequently non-extensional ("Set[] == Set[]" evaluates in an extensional
manner, that is, to "true")? And how does Array#uniq work? -- by what rule
does it dispose elements if not by getting "true" by a "==" comparison with
an other member of the array?

-- 
Csaba

"There's more to life, than books, you know but not much more..."
[The Smiths]
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