#14711: Weak references in the coercion graph
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Reporter: jpflori | Owner: davidloeffler
Type: defect | Status: needs_review
Priority: critical | Milestone: sage-5.13
Component: number fields | Resolution:
Keywords: memleak, number | Merged in:
field, QuadraticField | Reviewers:
Authors: Simon King | Work issues:
Report Upstream: N/A | Commit:
Branch: | 364b9856b28d7060e3ea9825144de66c8f11ca2a
u/SimonKing/ticket/14711 | Stopgaps:
Dependencies: |
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Comment (by nbruin):
I'm not so sure that the coercion system was designed with embeddings as
an ''alternative'' to registered embeddings: If you register a map only as
an embedding (and not also as a coercion on the codomain) you can end up
with the coercion system yielding non-transitive results:
{{{
class pA(Parent): pass
class pB(Parent): pass
class pC(Parent): pass
A=pA()
B=pB()
C=pC()
BtoA=Hom(B,A)(lambda x: A(x))
AtoC=Hom(A,C)(lambda x: C(x))
A.register_coercion(BtoA)
A.register_embedding(AtoC)
C.coerce_map_from(A) #finds the right map
A.coerce_map_from(B) #finds the right map
C.coerce_map_from(B) #returns none!
}}}
(other combinations of using `register_coercion` and `register_embedding`
do lead to the appropriate discovery)
so, if we want to view the coercion framework as a digraph and valid
coercions as paths in this digraph, then an arbitrary combination of
`register_embedding` and `register_coercion` may lead to invalid
manipulations of the graph (i.e., leading to a state where the system
fails to provide consistent (transitive) results).
This wasn't such a problem before, but since we are now tying lifetime
implications to how a coercion is registered, I think it now becomes
apparent.
--
Ticket URL: <http://trac.sagemath.org/ticket/14711#comment:134>
Sage <http://www.sagemath.org>
Sage: Creating a Viable Open Source Alternative to Magma, Maple, Mathematica,
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