dolfin team mailing list archive

["Martin Sandve Alnæs" <martinal@xxxxxxxxx>]

2008/10/21 Anders Logg <logg@xxxxxxxxx>:
> On Tue, Oct 21, 2008 at 06:01:53PM +0100, Garth N. Wells wrote:
>>
>>
>> Anders Logg wrote:
>> > On Tue, Oct 21, 2008 at 04:45:01PM +0100, Garth N. Wells wrote:
>> >> I have a few questions and thoughts regarding the new Function design
>> >>
>> >> * It's not clear to me what the intention is with user-defined
>> >> functions. The functions Function::interpolate(...) never call eval(..),
>> >> so they can't pick up user-defined values. Should Function::interpolate
>> >> test for the presence of a GenericVector to decide whether or not the
>> >> Function is discrete or user-defined?
>> >
>> > Yes, sorry. I've missed this. I'll fix it.
>> >
>> >> * It would be useful to declare user-defined functions without
>> >> associating a FunctionSpace. If we want to interpolate the function, a
>> >> FunctionSpace must then be provided. Anyone see any problems with this?
>> >
>> > The reasoning here is that all Functions must always be associated
>> > with a FunctionSpace so that they may be correctly interpreted in
>> > forms and correctly plotted. When a Function is created in PyDOLFIN,
>> > it must always be associated with a certain FiniteElement (and in a
>> > while FunctionSpace). It would simplify the handling of Functions if
>> > they are always associated with a FunctionSpace.
>> >
>>
>> I agree that is makes life simple if every function has a space, but it
>> is a bit clunky for declaring user-defined functions. The forms must be
>> declared first to extract the finite element to create the function
>> space. Could look nasty when a lot of functions are involved.
>>
>> We have a function Function::interpolate which takes a function space V
>> as an argument and it interpolates the function u in V. What if we
>> permit undefined function spaces (which perhaps only have a domain)? We
>> would then interpolate the user defined function u in the provided space V.
>>
>> Garth
>
> Are user-defined functions ever used without being related to a
> particular element/function space?
>
> It don't think it will be very clumsy. The clumsy thing will be to (in
> C++) get from something compiled by a form compiler to a FunctionSpace.
>
> If we can make that operation smooth, then creating (user-defined)
> functions will be very simple and convenient. One just needs to supply
> the variable V holding the function space.
>
> The current way of extracting function space data from the form is not
> very nice (in C++). What would be the optimal way to initialize a
> FunctionSpace in C++? We could think of extending the code generation
> to generate code that makes this convenient.
>
> --
> Anders

The current way of extracting function space data from the form is not
very nice in Python either, since it doesn't work with compiled functions.
(Never mind that the current code is FFC-specific, this will be the
same with UFL).

Using Python functors can easily make the assembly slower than solving
the linear system, so it's not really interesting to do in real applications...

To make a function object that is both of a C++ subclass of dolfin::Function
and of the Python class ufl.Function, we can't use the fixed multiple
inheritance
solution in the current PyDOLFIN.

We would have to define a new class dynamically in python, inheriting from
both ufl.Function and the freshly compiled C++ Function subclass.
After all this work cleaning up the Function class hierarchy, is that really
something you want?

I'm not sure if that is even possible to do while maintaining efficiency,
with cross-language inheritance and SWIG directors and all that.

If anyone has another solution, I'm very interested in hearing it!
Otherwise, I'm all for keeping the ufl.Function objects used in form
definition separated from dolfin.Function objects used in assembly.

--
Martin