#15303: Coercion discovery fails to be transitive
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Reporter: nbruin | Owner:
Type: defect | Status: new
Priority: major | Milestone: sage-5.13
Component: coercion | Resolution:
Keywords: | Merged in:
Authors: | Reviewers:
Report Upstream: N/A | Work issues:
Branch: | Commit:
Dependencies: #14711 | Stopgaps:
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Comment (by SimonKing):
Replying to [comment:3 nbruin]:
> > For example `B._coerce_embeddings_from[A] = phi`, ''in addition'' to
`A._coerce_embedding = phi`. Since it is a `MonoDict` and thus has weak
references to the keys, B would not prevent A from being garbage collected
(but of course A would still keep B alive, because we registered the
coerce embedding).
>
> Yes, something along those lines. In fact, since
`_coerce_embeddings_from` and `_coerce_from_list` would serve the same
purpose from the perspective of coercion discoveries, I think the entries
in `_coerce_from_list` should also be added to `_coerce_embeddings_from`,
because it would simplify the code. By then it would make more sense to
call the attribute `_coerce_from_to_be_used_in_backtracking` and the list
that is now `_coerce_from_hash` could be `_coerce_from_cached_results`.
Or shorter: `_coerce_from_backtracking` and `_coerce_from_cache`.
> By then we should probably be storing ALL maps there in weakened form.
Probably.
> The only function of `_coerce_from_list` now is to keep domains alive,
so it would be more economical to replace that by a list
`_domains_to_keep_alive` where we can store strong references to the
domains of the corresponding weakened maps that are now in
`_coerce_from_to_be_used_in_backtracking`
I have introduced `Parent._registered_domains` for exactly this purpose,
at #14711. So, we should extend its use.
> - `P._codomains_to_keep_alive`: List of codomains that should be kept
alive as long as P lives. Normally, such codomains Q would have an entry
in `Q._coercion_maps[P]`. Just storing a map in P._coerce_embedding would
also have this effect.
Yes, and that's why I don't think we need `P._codomains_to_keep_alive`
(I'd prefer the name `P._registered_codomains`). Even in weakened maps, we
have a strong reference to the codomain.
--
Ticket URL: <http://trac.sagemath.org/ticket/15303#comment:8>
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