Compute the norm of an expression with multiple fields

[alex-m-h]

When computing the L2-norm of an expression, I am not sure how the discretization should work in this case.
Minimal code example:

domain = Cube(name='Cube')
derham = Derham(domain, ['H1', 'Hcurl', 'Hdiv', 'L2'])
B = element_of(derham.V1, name='B')
u = element_of(derham.V0, name='u')
J = Norm(dot(B - grad(u), B - grad(u)), domain)

domain_h = discretize(domain, ncells=[5,5,5], periodic=[False, False, False])
derham_h = discretize(derham, domain_h, degree=[2,2,2])
J_h = discretize(J, domain_h, derham_h.V0)

which results in the following traceback:

Traceback (most recent call last):
  File "functional_issue.py", line 26, in <module>
    main()
  File "functional_issue.py", line 22, in main
    J_h = discretize(J, domain_h, derham_h.V0)
  File "/home/alex/repos/psydac/psydac/api/discretization.py", line 513, in discretize
    return DiscreteFunctional(a, kernel_expr, *args, **kwargs)
  File "/home/alex/repos/psydac/psydac/api/fem.py", line 1426, in __init__
    BasicDiscrete.__init__(self, expr, kernel_expr, comm=comm, root=0, discrete_space=discrete_space,
  File "/home/alex/repos/psydac/psydac/api/basic.py", line 298, in __init__
    BasicCodeGen.__init__(self, expr, folder=folder, comm=comm, root=root, discrete_space=discrete_space,
  File "/home/alex/repos/psydac/psydac/api/basic.py", line 108, in __init__
    ast = self._create_ast( expr=expr, tag=tag, discrete_space=discrete_space,
  File "/home/alex/repos/psydac/psydac/api/basic.py", line 339, in _create_ast
    return AST(expr, kernel_expr, discrete_space, mapping_space=mapping_space,
  File "/home/alex/repos/psydac/psydac/api/ast/fem.py", line 334, in __init__
    fields_degrees      = get_degrees(fields, spaces)
  File "/home/alex/repos/psydac/psydac/api/ast/fem.py", line 226, in get_degrees
    assert len(funcs) == len(degrees)
AssertionError

Maybe I have misunderstood, how this is supposed to be discretized. In the examples there is only one space used, but we need two spaces for the two fields. It might be good to have an option as for the BilinearForm where the variables are specified and then the spaces, as in

ac = BilinearForm((u, v), integral(domain, gc * u * v))
ach = discretize(ac, domain_h, [Vh, Vh], **kwargs)

(taken from test_assemble_complex_parallel in psydac/api/tests/test_assembly.py)

[yguclu]

I feel that this issue should be solved after #332.

[yguclu]

Probably related to #397