#15456: fix bug in has_right/left_descents in Weyl group code
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Reporter: | Owner:
zabrocki | Status: needs_review
Type: | Milestone: sage-6.2
defect | Resolution:
Priority: major | Merged in:
Component: | Reviewers:
combinatorics | Work issues:
Keywords: | Commit:
Authors: | 549f238c87e446116457d645c2b0171b423c02df
Frédéric Chapoton | Stopgaps:
Report Upstream: N/A |
Branch: |
u/chapoton/15456 |
Dependencies: |
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Comment (by sdenton):
tscrim: That's what WAS happening, but if you instantiate an abstract Weyl
Group or Coxeter Group it leads to an infinite loop, which is a bug. What
we need is a NotImplemented error if neither side is implemented, and the
appropriate call to the implemented 'side' if one side or the other is
concretely implemented but not the other.
I see two possible ways forward:
1) The default Coxeter/Weyl Group 'has_x_descent' methods just give a
NotImplemented error, pushing it to the person creating a concrete
realization to actually implement them correctly and not give an infinite
loop, or
2) Introduce some kind of '_is_implemented' flags for checking left and
right descent, which are false by default, and set to true if there's a
concrete implementation on one side that the other can use. Then if it's
true, we call the other side, and otherwise raise a NotImplemented error.
Option 2 seems doable but introduces an Obscure Thing for the implementers
of Coxeter groups to keep track of. As such, I would advocate for Option
1.
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Ticket URL: <http://trac.sagemath.org/ticket/15456#comment:10>
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