The interval
1 to: (10 raisedTo: 100)
can be created just fine, yet hashing its elements won't compute.
A generous interpretation of the intent of #=, where any wisp of
equivalence is promoted to full fledged equality, is problematic in the
long run. Here's another one:
17/20 = 0.85, therefore (17/20) hash = 0.85 hash
Never mind there's no floating point value that is equal to 17/20 in the
first place. It just snowballs from there. For instance, should
collections like these be equal?
(0.0 to: 1.0 by: 0.3) = #(0.0 0.3 0.6 0.9)
Sometimes it's just better if different things stay different.
Andres.
On 7/30/18 8:27 , Henrik Sperre Johansen wrote:
jgfoster wrote
Werner,
I would say that you are right, this is a problem. A (not un-common)
source of subtle bugs in Smalltalk is missing this rule that equivalent
objects must have the same hash. In GemStone the objects are not
equivalent (I’m not arguing that this is right, just that it avoids the
problem you identify).
I wonder what would happen if the hash comparison were added to the
equivalence operator (#’=‘)!
James
On Jul 30, 2018, at 5:07 AM, werner kassens <
wkassens@
> wrote:
Hi,
i guess i can subsume almost everything i know about hashes in one
sentence:
it is my understanding that two objects that are equal (obj1=obj2.
-->true) have to have the same hash value (which is used for some
collection types), whereas objects where obj1=obj2 returns false should
have different hash values although it may happen that they have the
same one.
now here things don't turn out exactly like that:
(1 to:4) = #(1 2 3 4). "true"
(1 to:4)hash = #(1 2 3 4)hash. "false"
well ok, actually these results make - pfffh, in a certain way - sense
to me, but i wonder what arguments the people in the know would use to
defend that result, if i would have another opinion?
werner
+1, this is a bug.
Either Interval >> #hash needs to change*, or the equivalency be broken**.
Fun fact:
#(1 2 3 4) = #[1 2 3 4] false
You'd think they might be more similar than an Interval and an Array, or a
LinkedList containing ValueLinks and an Interval, but no ;)
Cheers,
Henry
*Interval >> hash
"Hash is reimplemented because = is implemented.
Since we are equivalent to other collections of our species, we must
also
hash equivalently"
| hash |
hash := self species hash.
start to: stop by: step do: [:element | hash := (hash + element hash)
hashMultiply].
^hash
**both ways, which is challenging, if one neither wants to:
- change species of Interval (which has bad consequences)
- add hash comparison to sequenceablecollection = (which slows it down even
further)
--
Sent from: http://forum.world.st/Pharo-Smalltalk-Users-f1310670.html
.
