On Thu, Sep 17, 2015 at 2:53 AM, Simon King <simon.k...@uni-jena.de> wrote:
> Hi!
>
> On 2015-09-17, Jeroen Demeyer <jdeme...@cage.ugent.be> wrote:
>> On 2015-09-16 16:43, Jean-Pierre Flori wrote:
>>> Hi,
>>>
>>> I guess one of the issue is that there is no canonical map between two
>>> different representations of the same finite field (so no coercion).
>>
>> The question is really: is the map between representations of the same
>> finite field, which differ only in variable name, "canonical"?
>
> I think that it is in the same way canonical as we have a
> name-preserving map between polynomial rings.
>
> The real problem here is that *conversion* gives rise to an error
> that mentions *coercion*.
>
> sage: K.<x> = GF(25)
> sage: L.<y> = GF(25)
> sage: K(y)
> Traceback (most recent call last):
> ...
> TypeError: unable to coerce from a finite field other than the prime
> subfield
>
> That's clearly a bug. Conversion should work, even if it isn't canonical
> and thus doesn't qualify as coercion!
That may be because K(y) used to actually be called "coercion" in
Sage, and it didn't get changed in this one place when Robert Bradshaw
(?) introduced the name "conversion" for non-canonical coercions.
When I wrote the first coercion model I had two notions:
- coercions -- like K(y)
- canonical coercion -- like x + 5, which converts the integer 5 to GF(25)
I think Robert renamed these to:
- coercion --> conversion
- canonical coercion --> coercion
which would lead to confusion like you're seeing.
>
> Is there no ticket for it already? I think I have seen that issue
> before.
>
> Cheers,
> Simon
>
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William (http://wstein.org)
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