+1 on what Nils said; I only find it slightly surprising initially. Compare
this with comparing a list to a Partition as well. Equality is a bit
different as a programming concept than a mathematical one. When you
implement a custom __eq__, then you are separating that class from the
coercion-based equality.
+1 on removing the parent from the hash of ClonableArray and its family
tree.
Best,
Travis
On Tuesday, December 13, 2022 at 1:56:46 AM UTC+9 Nils Bruin wrote:
> On Monday, 12 December 2022 at 00:22:30 UTC-8 axio...@yahoo.de wrote:
>
>> Ah, thank you so much!
>>
>> Just to make sure that I understand the philosophy: if two elements are
>> supposed to compare equal, they should have parents which can be coerced to
>> a common parent, right? For example, the following (which is current
>> behaviour) is not really what we want:
>>
>> sage: A = SetPartitions(3)([[1,2],[3]])
>> sage: B = SetPartition([[1,2],[3]])
>> sage: A == B
>> True
>> sage: coercion_model.common_parent(A, B)
>>
> TypeError: no common canonical parent for objects with parents: 'Set
>> partitions of {1, 2, 3}' and 'Set partitions'
>>
>
>> I think that is a bit surprising, but if SetPartitions don't really occur
> much in binary operations I think it will be a hardly visible quirk. One
> has to be careful generalizing principles about how to implement
> coercion/equality/hashing because we a;ready know that some of the things
> we want lead to inconsistencies.
>
> When in doubt, the safe thing is definitely to NOT install coercions (it
> can always be done later), so I'd hesitate to recommend to change the
> behaviour above. I think you'd want at least a practical use case to
> justify it.
>
> Note that the relevant conversions: A.parent()(B) and B.parent()(A) do
> work.
>
> It seems to me the converse implication, if for two elements the coercion
> framework is happy to find a common parent in which they compare equal,
> then they should be equal in the first place, sounds a little more
> probable, but I suspect one can get problematic situations from that too.
>
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