#813: forced coercion vs. automatic coercion
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Reporter: nbruin | Owner: roed
Type: defect | Status: needs_review
Priority: major | Milestone: sage-4.7.1
Component: coercion | Keywords:
Work_issues: | Upstream: N/A
Reviewer: | Author: Simon King
Merged: | Dependencies: #9944
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Changes (by newvalueoldvalue):
* status: new => needs_review
* dependencies: => #9944
* author: => Simon King
Comment:
I think I was able to solve the problem. With my patch applied on top of
sage-4.7.1.rc1, all tests seem to pass, and one can do
{{{
sage: P = QQ['x','y']
sage: Q = Frac(QQ['x'])['y']
sage: Q.has_coerce_map_from(P)
True
sage: P.0+Q.0
y + x
}}}
In order to avoid bidirectional coercions (that would break a lot of
tests), I have
{{{
sage: Q = QQ['x']['y']
sage: Q.has_coerce_map_from(P)
False
sage: Q.base_ring() is P.remove_var(Q.variable_name())
True
}}}
The rule is: If `Q.base_ring() is P.remove_var(Q.variable_name())` then
there can not be a coercion from the multivariate ring P to the univariate
ring Q; in fact, there is a coercion in the opposite direction. But
otherwise, there is a coercion if `Q.base_ring()` has a coercion from
`P.remove_var(Q.variable_name())`.
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/813#comment:12>
Sage <http://www.sagemath.org>
Sage: Creating a Viable Open Source Alternative to Magma, Maple, Mathematica,
and MATLAB
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