Hi
In my case "0 + MyElement" does find a common parent,
namely the modular forms _ring_.
So basically in my case "0 + some_element" is the same
as "some_element.as_ring_element()", unless the weight of
some_element (resp. its parent) is 0 with multiplier 1
in which case ZZ coerces into the space.
In any case, it's ok. I can work around these issues.
Best
Jonas
On 21.02.2015 23:44, Eric Gourgoulhon wrote:
Le samedi 21 février 2015 23:09:24 UTC+1, Jonas Jermann a écrit :
Hi
It's a good idea but it won't work (in fact the _element_constructor_
does accept zero correctly in my case). The coercion framework _first_
tries to find a common parent and _then_ the rest happens.
At least in the cases I've tried (free modules as implemented in
#15916), if no common parent is found, there is then a conversion of 0
via _element_constructor_, followed by the addition via the _add_
(single underscore) of the module elements.
Do I understand correctly that 0 + whatever first calls
sage.structure.element.RingElement.__add__, which starts by
if have_same_parent(left, right):
(line 1570 of sage/structure/element.pyx); if this fails, it performs
return coercion_model.bin_op(left, right, add)
(line 1579)
and the latter shall invoke your parent._element_constructor_ to convert
0 to your zero element. I am not completely sure about this... (this is
my understanding why it works in my case).
Eric.
Best
Jonas
On 21.02.2015 22:59, Eric Gourgoulhon wrote:
> Hi,
>
> Le samedi 21 février 2015 21:59:05 UTC+1, Jonas Jermann a écrit :
>
>
> The parent is a vector space / module not a ring. Every
vector space
> contains zero, so in my opinion from a conceptual point of view
> adding zero should not change the parent space.
>
> However since it views 0 as an integer and not as (an
element) the zero
> subspace (unless we forcefully map it there) it seems hard to
change
> this behavior. :-(
>
>
> It seems that a possible way to have 0 + MyElement work even if
ZZ does
> not coerce to MyElement.parent() is to have
> MyElement.parent()._element_constructor_ accept 0 (i.e.
Integer(0)) as
> an argument and return the zero element of your vector space /
module
> for such an argument. Am I correct?
>
> Eric.
>
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