While trying to subclass QuotientRing, I noticed that this class does
not follow the coercion model: It overrides __call__ and _coerce_impl,
which the reference manual frowns upon. So I tried to change
QuotientRing to work via _coerce_map_from_ and _element_constructor_
(see first_attempt.patch at http://trac.sagemath.org/sage_trac/ticket/9429).
That ends up working for QuotientRings, but apparently messes up
coercion for everything else:
sage: FF = FiniteField(7)
sage: P.<x> = PolynomialRing(FiniteField(7))
sage: x+1
---------------------------------------------------------------------------
TypeError Traceback (most recent call
last)
/home/vbraun/opt/sage-4.5.alpha1/devel/sage-main/<ipython console> in
<module>()
/home/vbraun/Sage/sage/local/lib/python2.6/site-packages/sage/
structure/element.so in sage.structure.element.RingElement.__add__
(sage/structure/element.c:10876)()
/home/vbraun/Sage/sage/local/lib/python2.6/site-packages/sage/
structure/coerce.so in
sage.structure.coerce.CoercionModel_cache_maps.bin_op (sage/structure/
coerce.c:6966)()
TypeError: unsupported operand parent(s) for '+': 'Univariate
Polynomial Ring in x over Finite Field of size 7' and 'Integer Ring'
Note that I neither touched finite fields nor polynomial rings, I only
changed QuotientRing. Moreover, the coercion model believes that it
should work and no coercion to any quotient ring is involved:
sage: cm = sage.structure.element.get_coercion_model()
sage: cm.explain(P, ZZ, operator.add)
Coercion on right operand via
Composite map:
From: Integer Ring
To: Univariate Polynomial Ring in x over Finite Field of size
7
Defn: Conversion map:
From: Integer Ring
To: Finite Field of size 7
then
Polynomial base injection morphism:
From: Finite Field of size 7
To: Univariate Polynomial Ring in x over Finite Field
of size 7
Arithmetic performed after coercions.
Result lives in Univariate Polynomial Ring in x over Finite Field of
size 7
Univariate Polynomial Ring in x over Finite Field of size 7
Can somebody who is more familiar with the coercion tell me what I am
doing wrong?
Volker
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