dolfin team mailing list archive

[Johan Hake <hake@xxxxxxxxx>]

On Thursday 12 March 2009 12:58:33 Garth N. Wells wrote:
> Anders Logg wrote:
> > On Thu, Mar 12, 2009 at 10:46:14AM +0100, Martin Sandve Alnæs wrote:
> >> On Thu, Mar 12, 2009 at 10:32 AM, Anders Logg <logg@xxxxxxxxx> wrote:
> >>> On Thu, Mar 12, 2009 at 10:25:36AM +0100, Martin Sandve Alnæs wrote:
> >>>> I have serious problems with the idea of letting user-defined
> >>>> functions change nature to discrete functions as a side effect
> >>>> of other operations, effectively hiding the user-defined
> >>>> implementation of eval.
> >>>>
> >>>> (I know this concept wasn't introduced in this discussion, but it's
> >>>> related).
> >>>
> >>> You mean by calling u.vector()? Yes, I agree it's problematic.
> >>>
> >>> But I don't know how to handle it otherwise. Consider the following
> >>> example:
> >>>
> >>>  u = Function(V)
> >>>  solve(A, u.vector(), b)
> >>>
> >>> First u is a user-defined function since it doesn't have a
> >>> vector. Then it becomes discrete when we ask for the vector.
> >>>
> >>> The problem I think is that there is no way for us to check whether a
> >>> user has overloaded eval() without trying to call it.
> >>
> >> If we didn't require that user-defined functions had a function space
> >> for various operations, this wouldn't be as large a problem.
> >> Then you could do
> >>
> >> class MyFunction(Function)
> >> {
> >>   MyFunction(V): Function(V) {}
> >>   MyFunction(): Function() {}
> >>   void eval(...) { ... }
> >> }
> >>
> >> MyFunction f;
> >> f.vector(); // error, no function space!
> >>
> >> MyFunction f(V);
> >> f.vector(); // ok, should interpolate and make f discrete
> >>
> >> This is already possible, but function spaces get attached
> >> unneccesarily:
> >>
> >>
> >> 1) It's not necessary to attach function spaces to functions for
> >> assembly, since a user-defined function is evaluated through
> >> ufc::finite_element::evaluate_dofs for each cell anyway.
> >> This would remove the need for the side-effect in
> >>
> >> MyFunction f;
> >> MyForm a;
> >> a.f = f; // attaches function space to f

Isn't the function space used to deduct the value rank during assemble now? If 
we go for this approach we probably need to readd the rank function and make 
it virtual?

The geometric dimension is proably deducable from other places during 
assemble.

> >> 2) It's not necessary to require f to have a function space to
> >> interpolate it into another discrete function, as in:
> >>
> >> MyFunction f(V);
> >> Function g(V);
> >> f.interpolate(g.vector(), V)
> >>
> >> could just be:
> >>
> >> MyFunction f;
> >> Function g(V);
> >> f.interpolate(g.vector(), V);
> >>
> >> or even shorter:
> >>
> >> MyFunction f;
> >> Function g(V);
> >> f.interpolate_into(g); // I'm often confused about the direction of
> >> "interpolate"
> >
> > Yes, it's a bit confusing but it can be deduced from the constness or
> > no-constness of the functions.
> >
> > The above example could be
> >
> >   MyFunction f;
> >   Function g(V);
> >   g.interpolate(f); // g interpolates f
> >
> >> Any other places this affects?
> >
> > I think it's worth trying.
>
> Agree.

Just thinking a bit head of things here:
Should it be possible to create a user defined function in python without a 
FunctionSpace too, with the same inherited restrictions of not beeing able to 
create a discrete version of it? 

I would say that we keep the Python interface as it is, i.e., not letting a a 
user create functions without a FunctionSpace. The Functions and 
FunctionSpaces are more connected in python as they (most often) are created 
in connection to each other. 

What you say?

Johan

> Garth
>
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