Hi Chris,
On Fri, 2 Nov 2018 07:44:00 -0700, Chris Cunningham <cunningham.cb@gmail.com> wrote:
>ParcPlace-Digitalk VSE 3.1 (roughly 1999):
>
>(0 to: 1) = (0 to: 5/3). "true"
>(0 to: 1) hash = (0 to: 5/3) hash. "true"
>
>So, ancient VSE and current VisualWorks are consistent, and agree on where
>they want to be. This is also the direction I want to take Squeak.
>VA is also consistent, but #= doesn't match any other Smalltalk varient that we've looked at.
>Squeak, Pharo, Dolphin all currently have the same answer, but are not
>consistent.
>Interesting indeed.
I have been talking to the VA Smalltalk guys about this and they are thinking about it but haven't decided what to do
yet. It turns out that the way collections (like Set) that use #hash in VA Smalltalk work, because of the #= test
failing for intervals that cover the same range and have the same hash, that it overrides the equal hash value and adds
the interval to the collection. I find this troubling.
Lou
The rules for = and hash are that if two object are #=, then their hash values have to be equal as well.
There is no statement about if two objects hashes are the same, what this means for equality. This, I believe, is intentional.
The collection objects in (most?all?) smalltalks behave similarly to VA's - if objects have the same hash but are not equal, then they will both be in the hashed collection (such as Set). The squeak implementation is described in Set>>scanFor: . This method also shows why having objects equal but their hash not equal is so dangerous - if you had two objects that are supposed to be one and the same and are in fact #= but don't have the same hash, they can both show up in a Set together, or as keys in a Dictionary together, which breaks what we would expect.
But getting back to VA's collection issue that you have issues with - they are undoubtedly doing something similar in their collections that we do in Squeak, which is what is expected (although not necessarily obvious).
A long time ago, I took advantage of this and just hard-coded the hash for some of my classes to 1. This actually did work, but is a horrible (I mean HORRIBLE) idea - it really, really slows down the system when you have more than a couple instances of an object, but it does work.
-cbc
>thanks,
>cbc
>
>On Thu, Nov 1, 2018 at 5:40 AM Louis LaBrunda <Lou@keystone-software.com>
>wrote:
>
>> Hi Benoit,
>>
>> On the latest version of VA Smalltalk:
>>
>> VA Smalltalk V9.1 (32-bit); Image: 9.1 [413]
>> VM Timestamp: 4.0, 10/01/18 (100)
>>
>> I see:
>>
>> (0 to: 1) = (0 to: 5/3). "false"
>> (0 to: 1) hash = (0 to: 5/3) hash. "true"
>>
>> Very interesting.
>>
>> Lou
>>
>>
>> On Thu, 1 Nov 2018 02:40:00 +0000 (UTC), Benoit St-Jean via Squeak-dev <
>> squeak-dev@lists.squeakfoundation.org> wrote:
>>
>> >Interesting!
>> >
>> >As a comparison:
>> >Squeak 5.2
>> >(0 to: 1) = (0 to: 5/3). "true"(0 to: 1) hash = (0 to: 5/3) hash. "false"
>> >Dolphin 7(0 to: 1) = (0 to: 5/3). "true"(0 to: 1) hash = (0 to: 5/3)
>> hash. "false"
>> >VisualWorks 8.1.1(0 to: 1) = (0 to: 5/3). "true"
>> >(0 to: 1) hash = (0 to: 5/3) hash. "true"
>> >Pharo 5.0(0 to: 1) = (0 to: 5/3). "true"
>> >(0 to: 1) hash = (0 to: 5/3) hash. "false"
>> >
>> >I don't have VAST installed on the PC I'm using right now. I'd be
>> curious to see how other Smalltalk and/or GemStone handle this? So far
>> (according to what I could test, only VW is right (according to the ANSI
>> standard and just plain logic!)
>> >
>> >I wonder how much code relies on this "behavior" out there!
>> >But the ANSI Smalltalk draft is very clear on this (revision 1.9, page
>> 53, http://wiki.squeak.org/squeak/uploads/172/standard_v1_9-indexed.pdf):
>> >"If the value of receiver = comparand is true then the receiver and
>> comparand *must* have equivalent hash values."
>> >That's what I always thought (or was taught or even read in the Blue
>> Book). Was this something that was changed at some point???
>> >
>> >----------------
>> >Benoît St-Jean
>> >Yahoo! Messenger: bstjean
>> >Twitter: @BenLeChialeux
>> >Pinterest: benoitstjean
>> >Instagram: Chef_Benito
>> >IRC: lamneth
>> >Blogue: endormitoire.wordpress.com
>> >"A standpoint is an intellectual horizon of radius zero". (A. Einstein)
>> --
>> Louis LaBrunda
>> Keystone Software Corp.
>> SkypeMe callto://PhotonDemon
>>
>>
>>
--
Louis LaBrunda
Keystone Software Corp.
SkypeMe callto://PhotonDemon