#14084: Wrong domain of the fraction field construction functor
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Reporter: SimonKing | Owner: nthiery
Type: defect | Status: new
Priority: major | Milestone: sage-5.7
Component: categories | Resolution:
Keywords: | Work issues:
Report Upstream: N/A | Reviewers:
Authors: | Merged in:
Dependencies: | Stopgaps:
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Comment (by SimonKing):
Replying to [comment:3 nbruin]:
> ... but
> {{{
> sage: k=Qp(7)
> sage: k.category()
> Category of commutative rings
> sage: k in Fields()
> True
> sage: k.category()
> Category of fields
> }}}
> which is a little uncomfortable in its own right.
No, this is very important for making the test `k in Fields()` fast
without caching.
> You'd think that a category is part of the defining properties of the
parent, so changing it seems to fly into the face of immutability of
parents.
No. It is just "learning more and more about immutable properties of the
parent".
> If we have to keep it like this, we'd have to be very clear that one
should only test if a parent is IN a given category; never rely on the
category reported by "<parent>.category()".
Sure. It could always be that the reported category is a subcategory of
what one wants to have. `R in C` is recommended, but `R.category() is C`
is not.
> It certainly flies in the face of what I thought sage did: I thought
specifying a parent implied specifying the category in which you want to
consider it, and that if you want to consider a number field as a
`QQ`-vector space instead, one should explicitly apply a functor and use a
map (or perhaps conversion if you want to be implicit about it) to go
between the two.
I think this is currently not supported.
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/14084#comment:4>
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