On Thu, Feb 26, 2009 at 2:35 PM, Robert Bradshaw
<rober...@math.washington.edu> wrote:
>
> On Feb 26, 2009, at 2:28 PM, YannLC wrote:
>
>> Forgive my stubborness, but you answered only half of my question :)
>> do you think the following is a sane behavior?
>>
>> sage: var('f x')
>> (f, x)
>> sage: f(x+3)
>> x+3
>>
>> I would prefer a NotImplementedError...
>
> This has come up many times, and it's unclear exactly what the best
> behavior should be. What about
>
> sage: var('x')
> sage: f = x
> sage: f(3)
> 3
To add a little more detail here: I'm not sure you're understanding
what's going on in your example.
Consider this:
sage: E = (x+3)*sin(x)
sage: E(5)
8*sin(5)
sage: E.subs(x=5)
8*sin(5)
So E(5) is treated as a shorthand for E.subs(x=5).
Now if we consider:
sage: var('f x')
(f, x)
sage: f(x+3)
x + 3
sage: f.subs(f=x+3)
x + 3
it's the exact same thing.
This causes trouble in other situations, as well; if you wanted
multiplication, but you left off the '*' symbol:
sage: x(x+1)
x + 1
sage: x.subs(x=x+1)
x + 1
I've argued before that this shorthand should be removed because it's
too confusing, but I haven't managed to convince enough people, so
it's still there. (The shorthand isn't there in the new, Pynac-based
symbolics, so when we switch over to that system, this particular
source of confusion will be gone.)
Carl
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