On 19/12/2017 07:58, Jeroen Demeyer wrote:
On 2017-12-18 18:31, Simon King wrote:
One way to answer the question whether a coercion from A to B exists is
by implementing the method B._coerce_map_from_.
Right, I was asking the question because I need to implement such a
method for RIF and CIF.
If that method returns "True" then the coercion system knows that
it is right to choose _real_mpfi_() for coercion.
1. Check for a coercion map.
2. If coercion is possible, check for a NamedConvertMap. If so, use that
instead.
No. If there is a coercion map, then (by definition) coercion is
possible,
and the coercion map is cached. So, do not use something else instead.
I think it should be the other way around: if a _real_mpfi_() method
exists, it should always be picked as conversion map. The existence of
that method means that somebody explicitly implemented a specific
conversion to RIF. So any other map that the coercion framework can come
up with cannot be as good as this NamedConvertMap using _real_mpfi_().
+1
In #24285, I did implement a _arb_ and _acb_ methods on number field
elements. In order for the coercion model to use it, I had to fight with
coercions declared in the constructor (self._populate_coercion_lists_)
versus has_coerce_map_from. Depending on what you choose, different
coercions are chosen. Namely, there was a predefined coercion RLF -> RBF
in the constructor. As a consequence, any object with a coercion to RLF
would go through this map. It is the case for number fields.
And you can see that I left a comment in real_mpfi.pyx (line 1223)
elif hasattr(x, '_real_mpfi_'):
# TODO: this is a stupid useless copy!
# this case should be handled by coercion
If you start making it better, please doctest which coercion map is used
to avoid regressions
sage: K = QuadraticField(-2, embedding=AA(-2).sqrt())
sage: CBF.coerce_map_from(K)
Conversion via _acb_ method map:
...
Vincent
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