#11239: Incorrect coercion of polynomials over finite fields
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Reporter: johanbosman | Owner: robertwb
Type: defect | Status: needs_info
Priority: major | Milestone: sage-6.1
Component: coercion | Resolution:
Keywords: finite fields, | Merged in:
polynomials, coercion, sd53 | Reviewers: Jean-Pierre Flori
Authors: Peter Bruin | Work issues:
Report Upstream: N/A | Commit:
Branch: | 236effb6198c6192dce0cedc0e53423c68743e3e
u/jpflori/ticket/11239 | Stopgaps:
Dependencies: #8335 |
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Comment (by SimonKing):
See my answer on sage-devel. In a nutshell: I would recommend against
doing expensive tests when talking about conversion, and I would be rather
permissive in conversions.
Conversions have no defined properties, and thus there is nothing that one
could test. They don't need to be composable. They aren't even maps (only
partially defined). It is perfectly fine to have a conversion for all
elements of a ring R1 into a ring R2 and of all elements of a ring R2 into
a ring R3, but only some elements (e.g.: polynomials of degree zero) of R1
convert into R3 without raising an error.
Please clearly distinguish between coercion and conversion. Coercions are
morphisms. They are unique (for any pair of parents). They can be
composed. The identity is a morphism (which together with the uniqueness
and composability can imply the non-existence of certain coercions).
I only know of one rule for conversion: If there is a coercion then
conversion must coincide with it. And since the rules for conversions are
so relaxed, I don't see why one should do expensive tests before applying
a conversion (in particular when one thinks that conversions are not
defined parent-by-parent, but element-by-element).
--
Ticket URL: <http://trac.sagemath.org/ticket/11239#comment:25>
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