Dear sage-devel,
I'm a bit confused about what the recommended way to implement a free
module is. In particular, I would like a custom parent and elements
classes and coercion into the module. For simplicity, lets assume that
speed is not an issue. It seems like there are at least three
options:
1) Derive from one of the various classes in sage/modules/
free_module.py. Apparently requires cython if you want a custom
element class as you have to override "cdef _new_c()". Fun fact:
"FreeModule_generic_field.__init__()" does not work because of a
base_ring vs. base_field typo.
2) Derive from CombinatorialFreeModule and
CombinatorialFreeModuleElement from /sage/combinat/free_module.py.
3) Build your own using the categories framework. It seems to me that
making something in the FiniteDimensionalModulesWithBasis category
should be very concise, but I can't get it to work.
The options 1) and 2) derive from ParentWithGens, which is deprecated
(see comment at the top of sage/structure/parent_gens.pyx). Related is
the following error:
{{{
sage: F = CombinatorialFreeModule(QQ, ['a','b','c'])
sage: F.gens()
---------------------------------------------------------------------------
RuntimeError Traceback (most recent call
last)
/home/vbraun/opt/sage-4.5/devel/sage-main/<ipython console> in
<module>()
/home/vbraun/Sage/sage/local/lib/python2.6/site-packages/sage/
structure/parent_gens.so in
sage.structure.parent_gens.ParentWithGens.gens (sage/structure/
parent_gens.c:2674)()
/home/vbraun/Sage/sage/local/lib/python2.6/site-packages/sage/
structure/parent_gens.so in
sage.structure.parent_gens.ParentWithGens.ngens (sage/structure/
parent_gens.c:2489)()
/home/vbraun/Sage/sage/local/lib/python2.6/site-packages/sage/
structure/parent_gens.so in
sage.structure.parent_gens.check_old_coerce (sage/structure/
parent_gens.c:1190)()
RuntimeError: Free module generated by {'a', 'b', 'c'} over Rational
Field still using old coercion framework
}}}
In 3) tried to derive from Parent but then how do I get
MyClass.gen(int) and MyClass.gens()? Am I supposed to implement them
manually?
Best,
Volker
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