Dear all,
Following the discussion with Paul-Olivier, I became convinced that
the very tolerant conversions provided by CombinatorialFreeModule are
*harmful*, and that we have to disable them. And then, if needed,
think seriously about which mathematically meaningful conversions we
really want, and how to implement them safely. Here is a piece of the
current doc of _element_constructor, after a patch I just wrote:
Conversions between distinct free modules are not allowed any
more::
sage: F = CombinatorialFreeModule(ZZ, ["a", "b"]);
F.rename("F")
sage: G = CombinatorialFreeModule(QQ, ["a", "b"]);
G.rename("G")
sage: H = CombinatorialFreeModule(ZZ, ["a", "b", "c"]);
H.rename("H")
sage: G(F.term("a"))
Traceback (most recent call last):
...
TypeError: do not know how to make x (= B['a']) an element of self
(=G)
sage: H(F.term("a"))
Traceback (most recent call last):
...
TypeError: do not know how to make x (= B['a']) an element of self
(=H)
Here is a real life example illustrating that this yielded
mathematically wrong results::
sage: S = SymmetricFunctions(QQ)
sage: s = S.s(); p = S.p()
sage: ss = tensor([s,s]); pp = tensor([p,p])
sage: a = tensor((s[5],s[5]))
sage: pp(a) # used to yield p[[5]] # p[[5]]
Traceback (most recent call last):
...
NotImplementedError
Extensions of the ground ring should probably be reintroduced
at some point, but via coercions, and with stronger sanity
checks (ensuring that the codomain is really obtained by
extending the scalar of the domain; checking that they share
the same class is not sufficient).
Comments, thought?
Cheers,
Nicolas
--
Nicolas M. Thiéry "Isil" <nthi...@users.sf.net>
http://Nicolas.Thiery.name/
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