Hi Eliot,
I really shouldn't speak for Instantiations but since I brought them into this conversation I will say this:
(0 to: 1) = (0 to: 5/3). "false" (0 to: 1) hash = (0 to: 5/3) hash. "true"
The comparison of the intervals answers false. I argued strenuously that two intervals that cover the same range should compare as equal. Unfortunately the ANSI standard is I think ambiguous on this point. It says that if two things compare equal their hashes should be equal but here the two intervals don't compare equal. The VA Smalltalk code has been this way for over 20 years. Changing it could impact an unknown amount of customer code. I eventually concluded that even though the ranges were equal the objects were not and that their definition of equal was as valid as any other. If this came up 20+ years ago, maybe they could be convinced to change their definition. Now I agree with them, it is too late and too dangerous.
Since this is Smalltalk, if one is really interested in intervals that cover the same range comparing equal, there are simple ways to make that work. Yes, moving code from Squeal to VA Smalltalk would need a little love but probably not much.
Lou
On Thu, 15 Nov 2018 07:50:31 -0800, Eliot Miranda eliot.miranda@gmail.com wrote:
On Nov 15, 2018, at 5:55 AM, Louis LaBrunda Lou@Keystone-Software.com wrote:
Hi Tim,
After thoroughly discussing this with the VA Smalltalk guys I have concluded that the developer should be responsible for creating intervals that have equal ranges that compare equal. For example intervals with mixed integers and real values can have the same range but don't compare equal. If comparing equal is desired, the values should be made to make that work.
IMO thats a cop out. An implementation which compares two intervals as equal if their elements are equal makes perfect sense and is easy to implement. All thats needed is that the implementation access self last instead of stop.
Implementing newHash as one that uses self last in place of stop then in my image
| insts s | insts := Interval allInstances. { insts size. s := (insts select: [:i| i hash ~= i newHash]) size. s * 100.0 / insts size } #(3267 0 0.0)
So there's minimal risk in breaking anything simply redefining hash (I would also reformat #= as per my suggestion ;-) ).
Lou
On Thu, 15 Nov 2018 07:44:01 -0600, Tim Olson tim.olson.mail@gmail.com wrote:
Interval >> size does the correct thing with the stop value, so maybe Interval >> = could use:
isInterval and: [start = anInterval start and: [step = anInterval step and: [self size = anInterval size]]]
tim
On Nov 2, 2018, at 12:53 PM, Louis LaBrunda Lou@Keystone-Software.com wrote:
Hi Chris,
I know about and understand everything you have said about the spec and the way #= and #hast should work and relate to each other. My problem with VA Smalltalks implementation of #= is that it doesn't consider what an interval is and how it is used and therefor what equals should mean. I would interpret two intervals being equal if they span the same range and not worry about how they got there. In the case where the increment (by) is an integer the start and end values map down to integers and if those integers are the same in two intervals then the intervals span the same range and should be considered equal. Any program using those intervals would expect them to work the same. In VA Smalltalk they would work the same but you can't tell that with #=.
I have no problem with VA's hash based collections, they work the way they should given the way #hash and #= work. But from a higher level, if I put a bunch of intervals in a Set and only wanted one entry for each range spanned, I would be out of luck and probably confused as to why. Sure, this is Smalltalk and there are ways around this, if you know you need to work around it. One could always add a method to intervals to "fix" the start and end values if the increment is an integer.
I have heard from Instantiations and they plan to leave well enough alone at this point, since the #= method has been this way since 1996. They are concerned that changing #= may break existing user code. I doubt it but I understand the concern.
Lou
On Fri, 2 Nov 2018 10:01:38 -0700, Chris Cunningham cunningham.cb@gmail.com wrote:
All of that said, I too find the VA troubling a bit in this case. I rely on this (0 to: 1) = (0 to: 5/3) being true. VA not supporting is limits cross-dialect portability, although I don't (personally) use VA, other folks at work do and we do occasionally share code.
However, this implementation is internally consistent and obeys the = and hash rule in this case. Its just not what I would want.
-cbc
On Fri, Nov 2, 2018 at 9:52 AM Chris Cunningham cunningham.cb@gmail.com wrote:
Hi Louis, On Fri, Nov 2, 2018 at 9:12 AM Louis LaBrunda Lou@keystone-software.com wrote:
> 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 > > >
-- Louis LaBrunda Keystone Software Corp. SkypeMe callto://PhotonDemon
-- Louis LaBrunda Keystone Software Corp. SkypeMe callto://PhotonDemon