#12667: category root lattice realization issue: infinite loop while trying to
reflect to the positive chamber
-----------------------------+----------------------------------------------
Reporter: mshimo | Owner: sage-combinat
Type: defect | Status: new
Priority: major | Milestone: sage-5.0
Component: combinatorics | Keywords: root system
Work_issues: | Upstream: N/A
Reviewer: | Author: Mark Shimozono
Merged: | Dependencies:
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Comment(by nthiery):
Hi Mark,
> What is the right sage-combinat way to handle a situation
> where the function input will lead to an infinite loop?
> Can I just assert conditions that will ensure things like that
> don't happen, or should I emit a more precise error?
> Pointing me to example code will suffice.
Sorry for my delayed answer. In the affine case, please add a test on
level; something like:
{{{
assert not self.parent().is_affine() or self.level() >= 0, "This
element is not in the orbit of the fundamental chamber"
}}}
For the general Kac-Moody case: I think we can assume that a user
playing with those has some non trivial background. In that case, and
unless there is a cheap and easy to implement test as in the affine
case, it would be perfectly fine to write in the documentation
something like:
{{{
INPUT:
- ``self`` -- an element in the orbit of the fundamental chamber
.. warning::
The behavior of this method is not specified if ``self`` is
not in the orbit of the fundamental chamber (the algorithm may
run in an infinite loop). This never occurs for finite root
systems. For affine Weyl groups, an element is in the orbit of
the fundamental chamber iff it is of positive level (see
:meth:`level`), and this is currently checked, but do not rely
on it::
sage: ...
}}}
Otherwise put: as long as it is well documented, it is preferable for
power users to have a feature that needs to be used with care, than no
feature. Sage should be usable as a race car for those who wants one.
Cheers,
Nicolas
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/12667#comment:4>
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