dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Fri, Oct 31, 2008 at 11:24:23AM +0100, Martin Sandve Alnæs wrote:
> 2008/10/31 Johan Hake <hake@xxxxxxxxx>:
> > On Friday 31 October 2008 11:05:19 Garth N. Wells wrote:
> >> Johan Hake wrote:
> >> > On Friday 31 October 2008 09:38:38 Anders Logg wrote:
> >> >> On Fri, Oct 31, 2008 at 07:57:52AM +0000, Garth N. Wells wrote:
> >> >>> Martin Sandve Alnæs wrote:
> >> >>>> 2008/10/30 Johan Hake <hake@xxxxxxxxx>:
> >> >>>>> On Thursday 30 October 2008 19:20:06 Anders Logg wrote:
> >> >>>>>> Most things are now in place, finally. At least we can run the
> >> >>>>>> Poisson demo again... :-)
> >> >>>>>>
> >> >>>>>> Anyway, take a look at demo/pde/poisson/main.cpp and see what you
> >> >>>>>> think. I think it looks pretty good, but please report if you have
> >> >>>>>> any ideas for further improvements of the interface while we're at
> >> >>>>>> it.
> >> >>>>>
> >> >>>>> It looks nice, but I see some magic which I wish you can shed some
> >> >>>>> light on.
> >> >>>>>
> >> >>>>> The introduction of PoissionFunctionSpace is not clear for me. Why do
> >> >>>>> we have to instantiate the forms with it?
> >> >>>>
> >> >>>> Such that the Forms can use the same FunctionSpaces as are used in the
> >> >>>> rest of the code, while the user stays in control of their allocation.
> >> >>>> In particular, DofMaps needs to be shared between different parts of
> >> >>>> the code.
> >> >>
> >> >> In general the function spaces are named
> >> >>
> >> >>   PoissonBilinearFormArgumentSpace0
> >> >>   PoissonBilinearFormArgumentSpace1
> >> >>
> >> >>   PoissonBilinearFormCoefficientSpace0
> >> >>   PoissonBilinearFormCoefficientSpace1
> >> >>   PoissonBilinearFormCoefficientSpace2
> >> >>   PoissonBilinearFormCoefficientSpace3
> >> >>   ...
> >> >>
> >> >> Then, if all forms appearing in a form file (both a and L) share the
> >> >> same test space (which is natural), then an additional space will be
> >> >> generated:
> >> >>
> >> >>   PoissonTestSpace
> >> >>
> >> >> Same for the trial space:
> >> >>
> >> >>   PoissonTrialSpace
> >> >>
> >> >> If all spaces in the form are the same, then a common space will be
> >> >> generated:
> >> >>
> >> >>   PoissonFunctionSpace
> >> >>
> >> >> As noted by Martin, this allows a user to control the level of reuse
> >> >> of function spaces.
> >> >
> >> > Ok.
> >> >
> >> >>>>> In the form file the two forms L and a are defined. These are then
> >> >>>>> reflected in the main.cpp file as before. This is intuitive. But
> >> >>>>> where does the PoissonFunctionSpace come from? I as a user has not
> >> >>>>> defined it in the form file.
> >> >>>>>
> >> >>>>> It might help if we introduce the notion of FunctionSpace in FFC/UFL.
> >> >>>>> Then some of the magic would disapear I think.
> >> >>>>
> >> >>>> This could reinforce the problem with circular dependencies I
> >> >>>> mentioned earlier. I don't see that anyone has adressed that issue.
> >> >>>>
> >> >>>>> The talk about a Function always knowing its Space was clear, you can
> >> >>>>> always plot it, and so on. But then the introduction of the "magic"
> >> >>>>> dedication of FunctionSpace broke that, and also made the actuall
> >> >>>>> function of FunctionSpace more blurry.
> >> >>>>>
> >> >>>>> Intuitively I would prefer:
> >> >>>>>
> >> >>>>>  ...
> >> >>>>>  PoissonFunctionSpace V(mesh);
> >> >>>>>
> >> >>>>>  Source f(V);
> >> >>>>>  Flux g(V);
> >> >>>
> >> >>> The problem here is that f and g may come from a space other than V.
> >> >>> The difficulty for a user is that it's hard (and error prone) to create
> >> >>> the FunctionSpaces and associate them with the right Functions.
> >> >
> >> > Won't
> >> >
> >> >    PoissonBilinearForm a(V,V);
> >> >    PoissonLinearForm L(V);
> >> >
> >> > be as error prone with different test and trial spaces too?
> >>
> >> No, because there are never more that two, we can use the meaningful
> >> names "Test" and "Trial" and they are always the first arguments to the
> >> Form constructor.
> >>
> >> A possible solution along these lines was discussed last week which
> >> involved giving PoissonBilinearFormCoefficientSpace1, etc, names which
> >> involved the name used in the FFC input.
> >
> > Ok.
> >
> > If we have the same trial and test space, but different functionspaces. Then I
> > suppose one only would like to create one space for the former. How would
> > this be done?
> >
> >  PoissonTestSpace TestV(mesh);
> >  PoissonBilinearForm(TestV,TestV);
> >
> > or do we have to instantiate both?
> >
> >  PoissonTestSpace  TestV(mesh);
> >  PoissonTrialSpace TrialV(mesh);
> >  PoissonBilinearForm(TestV,TrialV);
> >
> > Johan
> 
> Good point.
> 
> The Poisson*Space names should be generated as typedefs
> to unique classes, that should solve part of this problem.

I don't really understand what you mean, but I don't think it would
help.

To get reuse, we need to reuse *instances* of classes. It doesn't
matter if we reuse classes, so typedefs won't help.

-- 
Anders

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