Hi folks,
In the process of dealing with
http://trac.sagemath.org/sage_trac/ticket/4246 , I ran into some issues
involving coercion of variables. I quickly realized that I don't really
know what the desired behavior should be, hence this post.
On one hand, William gave a very compelling example of something going quite
wrong:
sage: var('x y')
sage: S = QQ['x']
sage: S(x+y)
2*x
What happens is that the symbolic variables x and y both coerce to x, hence
we get 2*x. It's hard to imagine any situation in which you would want this
to be the right answer, and in fact it is hard to see what a "right answer"
would be -- hence the idea of throwing a TypeError here.
However, this clashes with some current behavior that might be viewed as
correct/useful:
sage: var('x')
sage: S = QQ['t']
sage: S(x)
t
It's a bit harder to discard this as wrong. Also, it's not just an issue
with symbolic variables:
sage: R = QQ['x']
sage: S = QQ['t']
sage: S(x)
t
I don't know what to do. If we want to keep the last two behaviors, it
seems that we're stuck with the first one as well.
Best,
Alex
--
Alex Ghitza -- Lecturer in Mathematics -- The University of Melbourne --
Australia -- http://www.ms.unimelb.edu.au/~aghitza/
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