On Sep 16, 2007, at 6:20 PM, William Stein wrote:
> I'm implementing a subfields command that returns
> a list of all subfields of a number field K. It should be possible
> to make it so each subfield canonically coerces into K.
> How do I do that with your new coercion code? Is enough
> written to make that possible?
>
> For now, I'm just writing a lot of functions that return
> object, map from object to ambient
> and we can change it to just set a canonical coercion when
> you explain the above.
On 9/17/07, Robert Bradshaw <[EMAIL PROTECTED]> wrote:
>
> Every parent has a _coerce_from_list (cdef) attribute. To set up
> coercion, all you need to do is create a Morphism object that does
> subfield -> K and append it to K. _coerce_from_list. Perhaps this
> should be made into a(n underscore) method.
Is there one example in the entire SAGE codebase that actually does this?
I suspect not, since:
sage: search_src('_coerce_from_list')
----------------------------------------------------------------------
| SAGE Version 2.8.4.2, Release Date: 2007-09-13 |
| Type notebook() for the GUI, and license() for information. |
----------------------------------------------------------------------
structure/parent.pyx: self._coerce_from_list = list(coerce_from)
structure/parent.pyx: other._coerce_from_list.append(mor)
structure/parent.pyx: self._coerce_from_list = []
structure/parent.pyx: for mor in self._coerce_from_list:
structure/parent.pyx: i = self._coerce_from_list.index(R)
structure/parent.pyx: self._coerce_from_list[i] = mor # cache
in case we need it again
structure/parent.pxd: cdef _coerce_from_list
One issue is that might not affect the __call__ method of K being
> able to interpret things correctly. It might be a good thing to re-
> write __call__ methods to try coercion first, and only do non-
> canonical things if that fails.
We really need a __call__ infrastructure. It's been suggesting a few
times before (most recently by Joel Mohler, I think), and I hope it
will get some discussion now.
- Robert
>
>
>
>
--
William Stein
Associate Professor of Mathematics
University of Washington
http://wstein.org
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