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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