dolfin team mailing list archive

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

On 27/04/11 20:50, Johan Hake wrote:
> On Wednesday April 27 2011 12:45:46 Martin Sandve Alnæs wrote:
>> On 27 April 2011 21:34, Anders Logg <logg@xxxxxxxxx> wrote:
>>> On Wed, Apr 27, 2011 at 09:30:08PM +0200, Martin Sandve Alnæs wrote:
>>>> 2011/4/27 Johan Hake <johan.hake@xxxxxxxxx>
>>>>
>>>>     On Wednesday April 27 2011 12:03:56 Martin Sandve Aln s wrote:
>>>>     > On 27 April 2011 19:07, Garth N. Wells <gnw20@xxxxxxxxx> wrote:
>>>>     > > I'm starting here a new thread on how to deal with the recent
>>>
>>> change in
>>>
>>>>     > > UFL that has broken a good number of DOLFIN demos. The previous
>>>
>>> thread
>>>
>>>>     > > meandered and got side-tracked.
>>>>     > > 
>>>>     > > The framework in we need to operate is:
>>>>     > > 
>>>>     > > A. UFL will not allow forms to be modified post-construction.
>>>>     > > 
>>>>     > > B. It should be relatively easy to replace ufl.Coefficients in
>>>>     > > a
>>>
>>> form
>>>
>>>>     > > and return a new form.
>>>>     > > 
>>>>     > > C. The issue with replacing ufl.Coefficients is that we lose
>>>
>>> DOLFIN
>>>
>>>>     data
>>>>     
>>>>     > > (like the eval() functions) associated with the removed
>>>
>>> coefficients.
>>>
>>>>     > > I'll kick off with the obvious solution:
>>>>     > > 
>>>>     > > 1. Require that all DOLFIN Expressions are associated with a
>>>>     > > ufl.FiniteElement.
>>>>     > > 
>>>>     > > Other solutions?
>>>>     > > 
>>>>     > > Garth
>>>>     > 
>>>>     > 2.  At the stage when ffc calls ufl.preprocess, or even in
>>>>     
>>>>     ufl.preprocess,
>>>>     
>>>>     > let the preprocessed form contain ufl Coefficients with new
>>>
>>> elements in
>>>
>>>>     > place of the dolfin.Expressions. This is similar to the
>>>
>>> replacements done
>>>
>>>>     > for renumbering of Coefficients, and could either be done
>>>
>>> simultaneously
>>>
>>>>     or
>>>>     
>>>>     > as an additional step. The original Form and Expression objects
>>>
>>> will be
>>>
>>>>     > untouched, and the preprocessed form will be fine.
>>>>     
>>>>     +
>>>>     
>>>>     However, setting cell and degree is done during analysis and relies
>>>
>>> on
>>>
>>>>     form_data. The form is also preprocessed when the form_data is
>>>
>>> extracted.
>>>
>>>>     This
>>>>     means that for the preprocessed form to get correct signature, cell
>>>
>>> and
>>>
>>>>     degrees being set, we need to break up the logic.
>>>>     
>>>>      1) extract form_data
>>>>      2) set degree and cell
>>>>      3) genererate preprocessed form
>>>>
>>>> Lets figure out the exact algorithm if we need it. It could perhaps be
>>>> integrated better with preprocess. Or it might be better to extract
>>>> just
>>>
>>> the
>>>
>>>> information needed to determine degree and cell first, and pass the
>>>
>>> element
>>>
>>>> replacements to preprocess.
>>>
>>> That's what I suggested in an earlier mail. Preprocess already gets
>>> common_cell. We could also figure out common_degree before calling
>>> preprocess but that requires getting the data stored in
>>> form_data.sub_elements.
>>
>> Extracting all sub elements from a form before preprocessing should be easy
>> and efficient.
>>
>> I assume it's still possible to construct an Expression with a specific
>> FunctionSpace? 
> 
> Yes.
> 

So it seems we've reached a solution that won't require any changes to
DOLFIN, and only minimal changes to FFC. The story is:

1. UFL will permit elements without a cell and without a degree. The
will leads an error for some operations, like grad and div.

2. Add a function to UFL to extract all sub-elements from a form.

3. Add 'unspecified_elements=[]' (perhaps a dict?) to the argument list
of ufl.algorithms.preprocess.

4. For coefficients with incomplete elements, preprocess will replace
these with coefficients based on elements from the list
'unspecified_elements'. The new form will be the 'preprocessed form'.

Is that it? Anything else?

Garth

> Johan
> 
>> It is possible to write forms that will require that even
>> with the above solution.
>>
>> Martin
> 
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