ffc team mailing list archive

["Garth N. Wells" <gnw20@xxxxxxxxx>]

Anders Logg wrote:
> On Sun, Oct 18, 2009 at 08:37:21PM +0200, Johan Hake wrote:
> > On Sunday 18 October 2009 20:31:41 Garth N. Wells wrote:
> > > Johan Hake wrote:
> > > > On Sunday 18 October 2009 20:07:41 Garth N. Wells wrote:
> > > > > On Oct 18 2009, Johan Hake wrote:
> > > > > > On Sunday 18 October 2009 18:21:23 Garth N. Wells wrote:
> > > > > > > Johan Hake wrote:
> > > > > > > > On Sunday 18 October 2009 16:43:28 Garth N. Wells wrote:
> > > > > > > > > Garth N. Wells wrote:
> > > > > > > > > > Johan Hake wrote:
> > > > > > > > > > > On Saturday 17 October 2009 21:08:14 Garth N. Wells wrote:
> > > > > > > > > > > > Garth N. Wells wrote:
> > > > > > > > > > > > > DOLFIN wrote:
> > > > > > > > > > > > > > One or more new changesets pushed to the primary dolfin
> > > > > > > > > > > > > > repository. A short summary of the last three changesets is
> > > > > > > > > > > > > > included below.
> > > > > > > > > > > > > >
> > > > > > > > > > > > > > changeset:   7378:e5c921e0293a
> > > > > > > > > > > > > > tag:         tip
> > > > > > > > > > > > > > user:        "Johan Hake <hake@xxxxxxxxx>"
> > > > > > > > > > > > > > date:        Sat Oct 17 15:45:36 2009 +0200
> > > > > > > > > > > > > > files:       site-packages/dolfin/function.py
> > > > > > > > > > > > > > site-packages/dolfin/functionspace.py description:
> > > > > > > > > > > > > > Make Mixed FunctionSpace access more consistant.
> > > > > > > > > > > > > >   - All methods are now defined in FunctionSpaceBase.
> > > > > > > > > > > > > >   - We now do not save any spaces in MixedFunctionSpace
> > > > > > > > > > > > > This change broke my code. See below.
> > > > > > > > > > > > Seems that the problem arises with spaces which are restricted,
> > > > > > > > > > > >
> > > > > > > > > > > >      V = FunctionSpace(mesh, "CG", 1, "facet")
> > > > > > > > > > > This is an error in the generated FFC code.
> > > > > > > > > > >    ufc::finite_element::signature()
> > > > > > > > > > > should return a string that can be executed in a ufl namespace and
> > > > > > > > > > > then generate the corresponding ufl.FiniteElement.
> > > > > > > > > > >
> > > > > > > > > > > For a restricted element the signature returns:
> > > > > > > > > > >
> > > > > > > > > > >    "FiniteElement('Lagrange', 'triangle', 1)|_{<interval of degree
> > > > > > > > > > > 1>}"
> > > > > > > > > > >
> > > > > > > > > > > where it should return:
> > > > > > > > > > >
> > > > > > > > > > >    "ElementRestriction(FiniteElement('Lagrange', Cell('triangle',
> > > > > > > > > > > 1, Space(2)), 1), Cell('interval', 1, Space(1)))"
> > > > > > > > > I've had a look, and while I don't yet follow where UFL defines its
> > > > > > > > > signatures,
> > > > > > > >   repr(ulf_object)
> > > > > > > >
> > > > > > > > returns the uniqe signature of an ufl_object.
> > > > > > > >
> > > > > > > > > things are dangerous because FFC formats its own signature
> > > > > > > > > strings, see line 227 of
> > > > > > > > >
> > > > > > > > >      ffc/ffc/fem/finiteelement.py
> > > > > > > > Yes, this is dangerous, at least if we want to use them as we do in
> > > > > > > > PyDOLFIN. However taking repr of the corresponding ufl object is a
> > > > > > > > well defined method that ufl use internally, when for example
> > > > > > > > creating a unique string representation of a form.
> > > > > > > >
> > > > > > > > > We stopped using signature strings in DOLFIN because it gave us all
> > > > > > > > > sorts of problems. Is it desirable to have PyDOLFIN depend on the
> > > > > > > > > generated strings? Can it be avoided?
> > > > > > > > It is a very nice way of constructing an ufl object when we have the
> > > > > > > > compiled version. As the convention of repr(object) is that:
> > > > > > > >
> > > > > > > >   new_object = eval(repr(object))
> > > > > > > >
> > > > > > > > should return a new object of the same kind.
> > > > > > > >
> > > > > > > > So when we have a SubSpace with its compiled FiniteElement, it is
> > > > > > > > easy to just call its signature method of its element to generate the
> > > > > > > > corresponding ufl element, which is used to construct a full fledged
> > > > > > > > dolfin.FunctionSpace.
> > > > > > > >
> > > > > > > > Not sure how this could be done another way.
> > > > > > > Can't we get the sub-element from the original UFL function?
> > > > > > Not when we return a SubSpace which is a compiled C++ structure. To be
> > > > > > able to construct the ufl.FiniteElement (done in the class
> > > > > > FunctionSpaceFromCpp in functionspace.py) we use the signature of the
> > > > > > cpp.FiniteElement.
> > > > > >
> > > > > > > If I do
> > > > > > >
> > > > > > >     (u0, u1) = pde.solve().split()
> > > > > > >
> > > > > > > are u0 and u1 UFL Functions, or just cpp Functions?
> > > > > > They should be both. Their FunctionSpaces (self._V) are constructed
> > > > > > using the the FunctionSpace.sub(i) (operator[]) method, which returns
> > > > > > the compiled SubSpace I am talking about above.
> > > > > OK, but if we have
> > > > >
> > > > >   U = pde.solve()
> > > > >
> > > > > and U is a UFL Function, can't the UFL finiteelement for U be accessed,
> > > > > and then the UFL sub-element(s) accessed and then compiled?
> > > > Yes, this should be possible! (Did not think of getting the sub element
> > > > from a mixed ufl.element :P)
> > > >
> > > > However we do not have to compile them, as we needed to go the other way,
> > > > from compiled to UFL.
> > > OK. Now the situation is clear to me.
> > >
> > > > I still think the signature() -> UFL object is a neat feature!
> > > The problem is that there is nothing that says that a form compiler that
> > > uses UFL and produces UFC-compliant code must return the UFL signature
> > > (repr) in ufc::finite_element::signature().
> > True. However that is something we discussed a year ago when we implemented
> > the transition to FunctionSpace in PyDOLFIN. I thought this went into the ufc
> > documentation, but I now see that this is not the case. For now...
> >
> > Lets get a blueprint at ufc and here what folks says.
> >
> > Johan
>
> How about adding an optional function to the UFC interface for
> returning the UFL string:
>
>   virtual std::string ufl_repr() const { return ""; }
>
> This function can then be implemented optionally by form compilers
> that rely on UFL. We should not tie the UFC interface to UFL.

The really clever way would be to have PyDOLFIN create a sub-class of 
ufc::finite_element which implements ufl_repr().

Garth

> --
> Anders
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> >