[GiNaC-devel] Implicit derivation of functions (re-submission)

[Richard B. Kreckel]

Dear Vladimir,

On 01/02/2015 04:19 PM, Vladimir V. Kisil wrote:
> 	Dear Richard,
> 	
>>>>>> On Wed, 31 Dec 2014 19:41:11 +0100, "Richard B. Kreckel" <kreckel at in.terlu.de> said:
> 
>     RK> Your proposal is to have two ways to implement a function's
>     RK> derivative: 1) by registering the existing derivative_func (pure
>     RK> formal derivative) 2) or your new impl_derivative_func (smart
>     RK> derivative, has to care about chain rule)
> 
>     RK> I wonder if there is a way to make this more uniform. What do
>     RK> you think?
> 
>     Of course, the chain rule may be included as a particular case in
>   the more general method. This will require a user slightly revise
>   their custom function definitions for the new version of GiNaC. Do you
>   consider this to be a reasonable price for a more straightforward code?

We shouldn't change the semantics of how the chain rule is applied to
derivative_func because this would break existing code out there.

And you're probably right that the not having to code the chain rule
manually in most cases makes the code so much more straightforward that
it's worth maintaining this more general case in the long run.

I think this settles it: we should support both functions.

In hindsight, what made your patch a little bit hard to understand to me
may have been the function's naming _impl. What is the reason you named
it like this? Isn't it applying the chain rule explicitly, rather than
implicitly/automatically? Could we name it differently?

>     RK> derivatives, your patch has three (very minor) problems which
>     RK> should be fixed before committing:
> 
>     You are right. I may do this within a couple of weeks, I hope.

Thanks.

   -richard.
-- 
Richard B. Kreckel
<http://in.terlu.de/~kreckel/>