From: Charles Mills <cmills@...>
Date: 2004-08-06T11:34:53+09:00
Subject: Re: Sets are not extensional


On Aug 5, 2004, at 6:41 PM, George Ogata wrote:

> Kristof Bastiaensen <kristof@vleeuwen.org> writes:
>
>> {}.hash == {}.hash
>> => false
>> Set[].hash == Set[].hash
>> => false
>> [].hash == [].hash
>> => true
>>
>> I have no idea for the reason why different hashes have different
>> hashvalues, while different arrays may get the same value (based on 
>> the
>> contents of the array).
>
> This seems like a blatant bug to me.  `a.eql? b' must imply `a.hash ==
> b.hash', yet:

I agree:
 > a = {} #=> {}
 > a.hash #=> 1650134
 > a.id #=> 1650134
 > b = {} #=> {}
 > b.hash #=> 1639814
# So hash on an empty hash returns the objects id....
 > a = {"a"=>"b"} #=> {"a"=>"b"}
 > a.hash #=> 1682094
 > a.clear #=> {}
 > a.hash #=> 1682094
# Looks like no two hashes hash to one another...
# Here is the reason
/* in object.c */
     rb_mKernel = rb_define_module("Kernel");
     rb_include_module(rb_cObject, rb_mKernel);
/* ..snip.. */
     rb_define_method(rb_mKernel, "object_id", rb_obj_id, 0);
/* in hash.c the "hash" method is never overridden.  So rb_obj_id() is 
the "hash" method for hashes. */

-Charlie