#813: forced coercion vs. automatic coercion
--------------------------------+-------------------------------------------
Reporter: nbruin | Owner: roed
Type: defect | Status: new
Priority: major | Milestone: sage-4.7.1
Component: basic arithmetic | Keywords:
Work_issues: | Upstream: N/A
Reviewer: | Author:
Merged: | Dependencies:
--------------------------------+-------------------------------------------
Changes (by SimonKing):
* upstream: => N/A
Comment:
I accidentally came accross this stone age ticket.
For the record: Current behaviour as of sage-4.7.rc2 is
{{{
sage: P1.<x>=QQ[]
sage: L=P1.fraction_field()
sage: x=L(x)
sage: P2.<y>=P1[]
sage:
sage: f=x+y
sage:
sage: P3.<x,y>=QQ[]
sage:
sage: P3(f)
x + y
sage:
sage: 0*P3.0+f
---------------------------------------------------------------------------
TypeError Traceback (most recent call
last)
/home/king/<ipython console> in <module>()
/mnt/local/king/SAGE/sage-4.7.rc2/local/lib/python2.6/site-
packages/sage/structure/element.so in
sage.structure.element.RingElement.__add__
(sage/structure/element.c:11959)()
/mnt/local/king/SAGE/sage-4.7.rc2/local/lib/python2.6/site-
packages/sage/structure/coerce.so in
sage.structure.coerce.CoercionModel_cache_maps.bin_op
(sage/structure/coerce.c:7382)()
TypeError: unsupported operand parent(s) for '+': 'Multivariate Polynomial
Ring in x, y over Rational Field' and 'Univariate Polynomial Ring in y
over Fraction Field of Univariate Polynomial Ring in x over Rational
Field'
sage: f.parent()
Univariate Polynomial Ring in y over Fraction Field of Univariate
Polynomial Ring in x over Rational Field
sage: P3
Multivariate Polynomial Ring in x, y over Rational Field
}}}
The question thus is: Should there be a coercion from P3 to the parent of
f? Then one should implement it. Otherwise, the ticket should be closed as
invalid.
Let fP be the parent of f, hence, the polynomial ring in y over the
fraction field of the polynomial ring in x over the rationals.
There ''is'' a coercion to fP from the polyomial ring in y over the
polynomial ring in x over the rationals:
{{{
sage: from sage.structure.element import get_coercion_model
sage: cm = get_coercion_model()
sage: fP = f.parent()
sage: cm.explain(fP, QQ[x][y])
Coercion on right operand via
Conversion map:
From: Univariate Polynomial Ring in y over Univariate Polynomial
Ring in x over Rational Field
To: Univariate Polynomial Ring in y over Fraction Field of
Univariate Polynomial Ring in x over Rational Field
Arithmetic performed after coercions.
Result lives in Univariate Polynomial Ring in y over Fraction Field of
Univariate Polynomial Ring in x over Rational Field
Univariate Polynomial Ring in y over Fraction Field of Univariate
Polynomial Ring in x over Rational Field
}}}
However, there is no such coercion from the bivariate polynomial ring in x
and y over the rationals:
{{{
sage: cm.explain(fP, QQ[x,y])
Will try _r_action and _l_action
Unknown result parent.
}}}
I think there should be such coercion! What do you think? I guess I'll
also ask sage-devel.
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/813#comment:6>
Sage <http://www.sagemath.org>
Sage: Creating a Viable Open Source Alternative to Magma, Maple, Mathematica,
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