Burcin Erocal wrote:
> On Fri, 7 Nov 2008 01:14:26 -0800 (PST)
> Simon King <[EMAIL PROTECTED]> wrote:
>
>> Dear Team,
>>
>> the impression that I got from this thread is the following:
>> -------
>> Commutative:
>> 1. If f is a *commutative* polynomial in x,y,z,..., then everybody
>> would at least correctly guess that f(1,2,3,...) has the intended
>> meaning "evalutation of f at x=1, y=2, z=3,..."
>> 2. Some people would actually *expect* (not just accept) that this
>> meaning is intended.
>>
>> =>
>> It is ok that multivariate polynomials in Sage are callable in the
>> sense described above. Currently, symbolic variables commute with each
>> other, and thus it is acceptable that they are callable as well.
>>
>> Hence, no need to change!
>> Do we agree on this?
>
> I was already unhappy about symbolic expressions being callable, this
> discussion convinced me that they shouldn't be.
>
> One thing we should not forget is that, polynomials and symbolic
> expressions in Sage are different things. I really appreciate
> polynomials being callable, and I don't see the need to differentiate
> polynomial functions and polynomials.
>
> Note that for polynomials, it is clear what to do when it is called.
> There is an ordered list of variables that will be substituted. We
> don't need to guess what the user intends.
>
> Things are not so simple for symbolic expressions. I find the current
> behavior confusing, and many people on sage-support seem to be confused
> by the different ways of constructing functions in Sage. (i.e.,
> python function definition, f(x) = ... syntax, just use a symbolic
> expression)
>
> I think the python function definition and f(x) = syntax are more than
> enough, and we don't need to make symbolic expressions callable.
>
> The advantage of the f(x) syntax is that it gives a clear and ordered
> list of variables to substitute for. One could say that this exists for
> any symbolic expression, since we have an alphabetically ordered list of
> variables, but then we have the following problem:
>
> sage: var('x,y,a,b')
> (x, y, a, b)
> sage: f = a*x + b*y
> sage: f(3,5)
> 5*y + 3*x
>
> If I was trying to second guess what the user intended, I would expect
> a and b to be constants, and x and y to be replaced. This is just
> impossible to get right.
>
> Keeping the "explicit is better than implicit" motto in mind, I think
> this should change. This syntax is much better, IMHO.
>
> sage: f(x,y) = a*x + b*y
> sage: f(3,5)
> 5*b + 3*a
>
> And I think this should raise an error:
>
> sage: f(5)
> b*y + 5*a
>
I think it is very handy to be able to partially evaluate an
expression. Do you propose a syntax that lets you effectively do f(5)
and get a function back? For example, if I want to plot a level curve
of f at x=5 presuming that f(x,y)=2*x+3*y, say. Here are some
possibilities:
plot( f(x=5), (y, -10,10))
plot( f(x=5,y=y), (y, -10,10))
plot( f(5,None), (y, -10,10))
plot( f(5,y), (y, -10,10))
g(y) = f(5,y)
plot(g, (y, -10,10))
That last one seemed too verbose
Jason
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to sage-devel@googlegroups.com
To unsubscribe from this group, send email to [EMAIL PROTECTED]
For more options, visit this group at http://groups.google.com/group/sage-devel
URLs: http://www.sagemath.org
-~----------~----~----~----~------~----~------~--~---
