Improve error message when preprocessing coefficient derivative without domain

[Jan Blechta]

from dolfin import *

#parameters.form_compiler.representation = "quadrature"  # Works
#parameters.form_compiler.representation = "tensor"      # Works
#parameters.form_compiler.representation = "uflacs"      # Fails
#parameters.form_compiler.representation = "tsfc"        # Fails

mesh = UnitSquareMesh(10, 10)
V = FunctionSpace(mesh, 'P', 1)
v = TestFunction(V)

element = FiniteElement("P", mesh.ufl_cell(), 1)
p = Expression('2.71 * x[0]', element=element)
assemble(p.dx(0)*v*dx)

fails with

  File "/home/fenics/local/lib/python3.5/site-packages/ffc/analysis.py", line 174, in _analyze_form
    do_apply_restrictions=True,
  File "/home/fenics/local/lib/python3.5/site-packages/ufl/algorithms/compute_form_data.py", line 295, in compute_form_data
    form = apply_derivatives(form)
  File "/home/fenics/local/lib/python3.5/site-packages/ufl/algorithms/apply_derivatives.py", line 1071, in apply_derivatives
    return map_integrand_dags(rules, expression)
  File "/home/fenics/local/lib/python3.5/site-packages/ufl/algorithms/map_integrands.py", line 58, in map_integrand_dags
    form, only_integral_type)
  File "/home/fenics/local/lib/python3.5/site-packages/ufl/algorithms/map_integrands.py", line 39, in map_integrands
    for itg in form.integrals()]
  File "/home/fenics/local/lib/python3.5/site-packages/ufl/algorithms/map_integrands.py", line 39, in <listcomp>
    for itg in form.integrals()]
  File "/home/fenics/local/lib/python3.5/site-packages/ufl/algorithms/map_integrands.py", line 46, in map_integrands
    return itg.reconstruct(function(itg.integrand()))
  File "/home/fenics/local/lib/python3.5/site-packages/ufl/algorithms/map_integrands.py", line 57, in <lambda>
    return map_integrands(lambda expr: map_expr_dag(function, expr, compress),
  File "/home/fenics/local/lib/python3.5/site-packages/ufl/corealg/map_dag.py", line 37, in map_expr_dag
    result, = map_expr_dags(function, [expression], compress=compress)
  File "/home/fenics/local/lib/python3.5/site-packages/ufl/corealg/map_dag.py", line 86, in map_expr_dags
    r = handlers[v._ufl_typecode_](v, *[vcache[u] for u in v.ufl_operands])
  File "/home/fenics/local/lib/python3.5/site-packages/ufl/algorithms/apply_derivatives.py", line 1024, in grad
    return map_expr_dag(rules, f)
  File "/home/fenics/local/lib/python3.5/site-packages/ufl/corealg/map_dag.py", line 37, in map_expr_dag
    result, = map_expr_dags(function, [expression], compress=compress)
  File "/home/fenics/local/lib/python3.5/site-packages/ufl/corealg/map_dag.py", line 84, in map_expr_dags
    r = handlers[v._ufl_typecode_](v)
  File "/home/fenics/local/lib/python3.5/site-packages/ufl/algorithms/apply_derivatives.py", line 538, in reference_value
    K = JacobianInverse(domain)
  File "/home/fenics/local/lib/python3.5/site-packages/ufl/geometry.py", line 104, in __init__
    self._domain = as_domain(domain)
  File "/home/fenics/local/lib/python3.5/site-packages/ufl/domain.py", line 309, in as_domain
    cell = as_cell(domain)
  File "/home/fenics/local/lib/python3.5/site-packages/ufl/cell.py", line 335, in as_cell
    error("Invalid cell (null)." Pell)
  File "/home/fenics/local/lib/python3.5/site-packages/ufl/log.py", line 172, in error
    raise self._exception_type(self._format_raw(*message))
ufl.log.UFLException: Invalid cell None.

It would be better to inform user that a domain should be specified. Indeed p = Expression('2.71 * x[0]', element=element, domain=mesh) does the job.

[Jan Blechta]

Please, reassign to me, @martinal, if busy.

[Jan Blechta]

Add stderr of the MWE.

[Lawrence Mitchell]

I note there is a comment in compute_form_data (line 268 in compute_form_data.py):

    if do_apply_function_pullbacks:
        # Rewrite coefficients and arguments in terms of their
        # reference cell values with Piola transforms and symmetry
        # transforms injected where needed.
        # Decision: Not supporting grad(dolfin.Expression) without a
        #           Domain.  Current dolfin works if Expression has a
        #           cell but this should be changed to a mesh.
        form = apply_function_pullbacks(form)

If you want to support that, you need to reconstruct the expressions with a domain attached I think. You'll always need the element because otherwise UFL doesn't know how to pull back appropriately.

[Jan Blechta]

Thanks, @wence. You're right, specifying domain fixes the problem. Both

p = Expression('2.71 * x[0]', element=element, domain=mesh)
p = Expression('2.71 * x[0]', degree=1, domain=mesh)

in the snippet above work. Nevertheless the error message is a bit unfortunate.