dolfin team mailing list archive

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

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.

--
Martin