#15303: Coercion discovery fails to be transitive
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Reporter: nbruin | Owner:
Type: defect | Status: needs_work
Priority: major | Milestone: sage-5.13
Component: coercion | Resolution:
Keywords: | Merged in:
Authors: Simon King | Reviewers:
Report Upstream: N/A | Work issues: Implement
Branch: | backtracking properly
u/SimonKing/ticket/15303 | Commit:
Dependencies: #14711 | 74821fe5409c3104b5d6eb7407a8287d54170df9
| Stopgaps:
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Comment (by nbruin):
Replying to [comment:63 SimonKing]:
> Well, the point I wanted to make is: One needs to construct the maps,
because only the map know about their costs.
Don't we almost always have the map in hand? When we find the map as a
registered coercion or embedding, we can readily read off the cost. We can
also do that when we get a map back from `_coerce_map_from_`. The only
case where we have to estimate the cost is when we get the existence of a
map via `K._coerce_map_from_(L)==True`, but in that case I expect the cost
is a fixed number anyway, because it's a generically constructed map.
I assume that the cost of a composition is the sum of the costs of the
composed maps, because otherwise we are not working on a weighted digraph.
That means we can simply sum the costs of the edges to get the cost of a
path. No need to compute the "composition" of the maps explicitly until we
actually need it.
Your "don't consider other maps coming into `K` if we're getting an actual
map back from `K._coerce_map_from_(L)`" is then simply an assumption that
`K._coerce_map_from_(L)` returns the cheapest map available. I think it
helps making such assumptions about the coercion discovery algorithm
explicit.
--
Ticket URL: <http://trac.sagemath.org/ticket/15303#comment:64>
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