dolfin team mailing list archive

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

2008/11/4 Anders Logg <logg@xxxxxxxxx>:
> On Tue, Nov 04, 2008 at 01:59:22PM +0100, Martin Sandve Alnæs wrote:
>> 2008/11/4 Johan Hake <hake@xxxxxxxxx>:
>> > On Tuesday 04 November 2008 13:07:07 Martin Sandve Alnæs wrote:
>> >> 2008/11/4 Johan Hake <hake@xxxxxxxxx>:
>> >> > Hello!
>> >> >
>> >> > I have started the work on the PyDOLFIN. We can now define the forms in
>> >> > the poisson demo, using the syntax previously discussed, see python
>> >> > poisson demo.
>> >> >
>> >> > A FunctionSpace now inherits both dolfin::FunctionSpace and
>> >> > ffc.FiniteElement, and it can be used to instantiate user defined
>> >> > Functions which can be used to define forms.
>> >> >
>> >> > We need to discuss how to implement a discrete function. This is a bit
>> >> > complicated using the metaclass magic that is implemented now. Now we
>> >> > cannot do:
>> >> >
>> >> >  u = Function(V)
>> >> >  x = u.vector()
>> >> >
>> >> > as Function is just a dummy class for creation of userdefined functions.
>>
>> I don't quite understand the problem here.
>> Are you saying that type(u) is not a subclass of cpp_Function or what?
>>
>>
>> >> We don't have to use metaclasses, it would be enough to implement
>> >> Function.__new__(cls, *args). This function can return objects of
>> >> a different type, e.g. a compiled function that doesn't inherit from
>> >> dolfin.Function but directly from dolfin::Function.
>> >
>> > and from ufl.Function too?
>>
>> Yes.
>>
>> >> (I didn't understand this stuff fully until last week...)
>> >
>> > Yes I have thought about that solution, but then the created class wont be a
>> > Function. It can be handy to have a class that all python Function can be
>> > checked if isinstance of. With the __new__ function we create different
>> > classes.
>>
>> This is not a problem. On the contrary, you either wish to
>> check if a function is a cpp_Function or a ufl.Function.
>> I would consider "isinstance(f, dolfin.Function)" a bug in
>> most circumstances. That's one of the things I don't
>> like about this design...
>>
>>
>> > The advantage, as I see it with your suggestion would be that we can use the
>> > present feature of compiling many functions at a time, but we loose the
>> > consistent syntax:
>> >
>> >  class MyFunction(Function):
>> >      def eval(v,x):
>> >           do something
>> >
>> >  class MyCompiledFunction(Function):
>> >      cpp_code = do something
>> >
>> >  f = MyFunction(V)
>> >  g = MyCompiledFunction(V)
>> >
>> > and it could be complicated to pass the right FunctionSpaces to the
>> > compile_function function.
>>
>> I forgot about this syntax. It's nice, but personally I'll be fine
>> without it :-)
>>
>>
>> >> > Is it possible to define a DiscreteFunction class in c++ (or just in
>> >> > swig?) that inherits dolfin::Function, and in its constructor calls
>> >> > vector()?
>> >> >
>> >> > Then we can use this class in python to create discrete functions. We
>> >> > then avoid the director class that is created by swig for all functions
>> >> > that inherits the cpp_Function. The obvious syntax would then be
>> >> >
>> >> >  u = DiscreteFunction(V)
>> >> >
>> >> > in python. I think with some python magic we still can have the syntax
>> >> >
>> >> >  u = Function(V)
>> >> >
>> >> > which would imply that a discrete function is created, but I haven't
>> >> > implemented it.
>> >>
>> >> That would basically be duplicating the design that has been replaced...
>> >> I think dropping the metaclass is a much easier solution.
>> >
>> > No, this is just for the python interface. This could come handy with the
>> > __new__ implementation you want too. By this we circumvent the director class
>> > that swig creates for dolfin::functions. We dont want to call eval on such a
>> > class to often do we? :)
>> >
>> >> > We also have a problem with MixedElements. Now the FunctionSpace inherits
>> >> > ffc.FiniteElement and a MixedElement is not a FiniteElement. I suppose we
>> >> > could overload the __add__ operator for the FunctionSpace together with a
>> >> > new class MixedFunctionSpace, to fix this?
>> >> >
>> >> > Johan
>> >>
>> >> We also have (in UFL at least) the classes VectorElement and TensorElement,
>> >> so this gets complicated. I think we should just make FunctionSpace own an
>> >> element instead.
>> >>
>> >> element = FiniteElement(...)
>> >> V = FunctionSpace(mesh, element)
>> >> f = Function(V) # calls FiniteElement.__init__(self, element)
>> >
>> > Thats looks nice. We could also use the syntax Anders suggested,
>> >
>> >  V = FunctionSpace(mesh, "Lagrange", 1)
>> >
>> > and then instantiate the FiniteElement in the __init__ function.
>>
>> No, we can't, that's exactly the issue you brought up with MixedElement.
>> ("Lagrange", 1) doesn't carry all information about an element.
>> In UFL we have classes FiniteElement, MixedElement, VectorElement,
>> and TensorElement. In addition to (family,domain,degree), a
>> VectorElement can have a dim, and a TensorElement
>> can have a shape and symmetries...
>
> Yes, but all these arguments can easily be added to the FunctionSpace
> constructor.

I strongly disagree. If that was the case we wouldn't have all those classes
to begin with. Show me how to express this using a single constructor:

e1 = TensorElement("Lagrange", "triangle", 2)
e2 = VectorElement("Lagrange", "triangle", 1)
e3 = FiniteElement("DG", "triangle", 0)
element = MixedElement(P3, P2, P1)
V = FunctionSpace(mesh, element)


>> > This wont work for the basis functions though, but we could just add a class
>> > that inherits the ffc.BasisFunction and which can be instantiated with both a
>> > FunctionSpace and a FiniteElement.
>>
>> We'd need to consider both BasisFunction, and BasisFunctions, which is a
>> function and a bit more complicated. Also TestFunction(s) and TrialFunction(s),
>> but those are just syntactic sugar.
>>
>> I'd very much prefer letting BasisFunction be and just passing it an element.
>
> v = TestFunction(V)
>
> looks much nicer than
>
> v = TestFunction(element)

I don't disagree with that statement. But looks nice isn't good enough
if it requires too much code (and code maintenance) to make it work.


> It would be good if we could avoid exposing the FiniteElement class in
> application code since they are mostly concerned with function spaces,
> not placeholders for reference elements.

If you can make it work, fine by me. But it seems tricky to get right.

--
Martin