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)




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