That's too bad... I will think of an alternate route. -- Anders
On Tue, Jun 14, 2011 at 10:01:47PM +0200, Martin Sandve Alnæs wrote: > I agree it is pretty in a way, but here comes the showstopper... Sorry! ;) > > There can be no ufl.Form.__eq__ implementation > that does not conform to the Python conventions > of actually comparing objects and returning > True or False, because that will break usage > of Form objects in built in Python data structures. > > Martin > > On 14 June 2011 21:53, Anders Logg <l...@simula.no> wrote: > > On Tue, Jun 14, 2011 at 09:03:31PM +0200, Marie E. Rognes wrote: > >> On 06/14/2011 08:35 PM, Garth N. Wells wrote: > >> > > >> > > >> >On 14/06/11 19:24, Anders Logg wrote: > >> >>On Tue, Jun 14, 2011 at 10:19:20AM -0700, Johan Hake wrote: > >> >>>On Tuesday June 14 2011 03:33:59 Anders Logg wrote: > >> >>>>On Tue, Jun 14, 2011 at 09:25:17AM +0100, Garth N. Wells wrote: > >> >>>>>On 14/06/11 08:53, Anders Logg wrote: > >> >>>>>>14 jun 2011 kl. 09:18 skrev "Garth N. Wells"<gn...@cam.ac.uk>: > >> >>>>>>>On 14/06/11 08:03, Marie E. Rognes wrote: > >> >>>>>>>>On 06/13/2011 11:16 PM, Anders Logg wrote: > >> >>>>>>>>>>>But while we are heading in that direction, how about > >> >>>>>>>>>>>abolishing the *Problem class(es) altogether, and just use > >> >>>>>>>>>>>LinearVariationalSolver and > >> >>>>>>>>>>>NonlinearVariationalSolver/NewtonSolver taking as input (a, > >> >>>>>>>>>>>L, > >> >>>>>>>>>> > >> >>>>>>>>>>bc) > >> >>>>>>>>>> > >> >>>>>>>>>>>and (F, dF, bcs), respectively. > >> >>>>>>>>> > >> >>>>>>>>>This will be in line with an old blueprint. We noted some time > >> >>>>>>>>>ago that problems/solvers are designed differently for linear > >> >>>>>>>>>systems Ax = b than for variational problems a(u, v) = L(v). > >> >>>>>>>>>For linear systems, we have solvers while for variational > >> >>>>>>>>>problems we have both problem and solver classes. > >> >>>>>>>>> > >> >>>>>>>>>>>I mean, the main difference lies in how to solve the > >> >>>>>>>>>>>problems, right? > >> >>>>>>>>> > >> >>>>>>>>>It looks like the only property a VariationalProblem has in > >> >>>>>>>>>addition to (forms, bc) + solver parameters is the parameter > >> >>>>>>>>>symmetric=true/false. > >> >>>>>>>>> > >> >>>>>>>>>If we go this route, we could mimic the design of the linear > >> >>>>>>>>>algebra solvers and provide two different options, one that > >> >>>>>>>>>offers more control, solver = KrylovSolver() + solver.solve(), > >> >>>>>>>>>and one quick option that just calls solve: > >> >>>>>>>>> > >> >>>>>>>>>1. complex option > >> >>>>>>>>> > >> >>>>>>>>>solver = LinearVariationalSolver() # which arguments to > >> >>>>>>>>>constructor? solver.parameters["foo"] = ... u = solver.solve() > >> >>>>>>> > >> >>>>>>>I favour this option, but I think that the name > >> >>>>>>>'LinearVariationalSolver' is misleading since it's not a > >> >>>>>>>'variational solver', but solves variational problems, nor should > >> >>>>>>>it be confused with a LinearSolver that solves Ax = f. > >> >>>>>>>LinearVariationalProblem is a better name. For total control, we > >> >>>>>>>could have a LinearVariationalProblem constructor that accepts a > >> >>>>>>>GenericLinearSolver as an argument (as the NewtonSolver does). > >> >>>>>>> > >> >>>>>>>>For the eigensolvers, all arguments go in the call to solve. > >> >>>>>>>> > >> >>>>>>>>>2. simple option > >> >>>>>>>>> > >> >>>>>>>>>u = solve(a, L, bc) > >> >>>>>>> > >> >>>>>>>I think that saving one line of code and making the code less > >> >>>>>>>explicit isn't worthwhile. I can foresee users trying to solve > >> >>>>>>>nonlinear problems with this. > >> >>>>>> > >> >>>>>>With the syntax suggested below it would be easy to check for errors. > >> >>>>>> > >> >>>>>>>>Just for linears? > >> >>>>>>>> > >> >>>>>>>>>3. very tempting option (simple to implement in both C++ and > >> >>>>>>>>>Python) > >> >>>>>>>>> > >> >>>>>>>>>u = solve(a == L, bc) # linear u = solve(F == 0, J, bc) # > >> >>>>>>>>>nonlinear > >> >>>>>>> > >> >>>>>>>I don't like this on the same grounds that I don't like the > >> >>>>>>>present design. Also, I don't follow the above syntax > >> >>>>>> > >> >>>>>>I'm not surprised you don't like it. But don't understand why. It's > >> >>>>>>very clear which is linear and which is nonlinear. And it would be > >> >>>>>>easy to check for errors. And it would just be a thin layer on top of > >> >>>>>>the very explicit linear/nonlinear solver classes. And it would > >> >>>>>>follow the exact same design as for la with solver classes plus a > >> >>>>>>quick access solve function. > >> >>>>> > >> >>>>>Is not clear to me - possibly because, as I wrote above, I don't > >> >>>>>understand the syntax. What does the '==' mean? > >> >>>> > >> >>>>Here's how I see it: > >> >>>> > >> >>>>1. Linear problems > >> >>>> > >> >>>> solve(a == L, bc) > >> >>>> > >> >>>> solve the linear variational problem a = L subject to bc > >> >>>> > >> >>>>2. Nonlinear problems > >> >>>> > >> >>>> solve(F == 0, bc) > >> >>>> > >> >>>> solve the nonlinear variational problem F = 0 subject to bc > >> >>>> > >> >>>>It would be easy to in the first case check that the first operand (a) > >> >>>>is a bilinear form and the second (L) is a linear form. > >> >>>> > >> >>>>And it would be easy to check in the second case that the first > >> >>>>operand (F) is a linear form and the second is an integer that must be > >> >>>>zero. > >> >>>> > >> >>>>In both cases one can print an informative error message and catch any > >> >>>>pitfalls. > >> >>>> > >> >>>>The nonlinear case would in C++ accept an additional argument J for > >> >>>>the Jacobian (and in Python an optional additional argument): > >> >>>> > >> >>>> solve(F == 0, J, bc); > >> >>>> > >> >>>>The comparison operator == would for a == L return an object of class > >> >>>>LinearVariationalProblem and in the second case > >> >>>>NonlinearVariationalProblem. These two would just be simple classes > >> >>>>holding shared pointers to the forms. Then we can overload solve() to > >> >>>>take either of the two and pass the call on to either > >> >>>>LinearVariationalSolver or NonlinearVariationalSolver. > >> >>>> > >> >>>>I'm starting to think this would be an ideal solution. It's compact, > >> >>>>fairly intuitive, and it's possible to catch errors. > >> >>>> > >> >>>>The only problem I see is overloading operator== in Python if that > >> >>>>has implications for UFL that Martin objects to... :-) > >> >>> > >> >>>Wow, you really like magical syntaxes ;) > >> >> > >> >>Yes, a pretty syntax has been a priority for me ever since we > >> >>started. I think it is worth a lot. > >> >> > >> > > >> >Magic and pretty are not the same thing. > > > > That's true, but some magic is usually required to make pretty. > > > > Being able to write dot(grad(u), grad(v))*dx is also a bit magic. > > The step from there to solve(a == L) is short. > > > >> >>>The problem with this syntax is that who on earth would expect a > >> >>>VariationalProblem to be the result of an == operator... > >> >> > >> >>I don't think that's an issue. Figuring out how to solve variational > >> >>problems is not something one picks up by reading the Programmer's > >> >>Reference. It's something that will be stated on the first page of any > >> >>FEniCS tutorial or user manual. > >> >> > >> >>I think the solve(a == L) is the one missing piece to make the form > >> >>language complete. We have all the nice syntax for expressing forms in > >> >>a declarative way, but then it ends with > >> >> > >> >>problem = VariationalProblem(a, L) > >> >>problem.solve() > >> >> > >> >>which I think looks ugly. It's not as extreme as this example taken > >> >>from cppunit, but it follows the same "create object, call method on > >> >>object" paradigm which I think is ugly: > >> >> > >> >> TestResult result; > >> >> TestResultCollector collected_results; > >> >> result.addListener(&collected_results); > >> >> TestRunner runner; > >> >> > >> >> runner.addTest(CppUnit::TestFactoryRegistry::getRegistry().makeTest()); > >> >> runner.run(result); > >> >> CompilerOutputter outputter(&collected_results, std::cerr); > >> >> outputter.write (); > >> >> > >> >>>I see the distinction between FEniCS developers who have programming > >> >>>versus > >> >>>math in mind when designing the api ;) > >> >> > >> >>It's always been one of the top priorities in our work on FEniCS to > >> >>build an API with the highest possible level of mathematical > >> >>expressiveness to the API. That sometimes leads to challenges, like > >> >>needing to develop a special form language, form compilers, JIT > >> >>compilation, the Expression class etc, but that's the sort of thing > >> >>we're pretty good at and one of the main selling points of FEniCS. > >> >> > >> > > >> >This is an exaggeration to me. The code > >> > > >> > problem = [Linear]VariationalProblem(a, L) > >> > u = problem.solve() > >> > > >> >is compact and explicit. It's a stretch to call it ugly. > > > > Yes, of course it's a stretch. It's not very ugly, but enough to > > bother me. > > > >> >>>Also __eq__ is already used in ufl.Form to compare two forms. > >> >> > >> >>I think it would be worth replacing the use of form0 == form1 by > >> >>repr(form0) == repr(form1) in UFL to be able to use __eq__ for this: > >> >> > >> >>class Equation: > >> >> def __init__(self, lhs, rhs): > >> >> self.lhs = lhs > >> >> self.rhs = rhs > >> >> > >> >>class Form: > >> >> > >> >> def __eq__(self, other): > >> >> return Equation(self, other) > >> >> > >> >>I understand there are other priorities, and others don't care as much > >> >>as I do about how fancy we can make the DOLFIN Python and C++ interface, > >> >>but I think this would make a nice final touch to the interface. > >> >> > >> > > >> >I don't see value in it. In fact the opposite - it introduces complexity > >> >and a degree of ambiguity. > > > > Complexity yes (but not much, it would require say around 50-100 > > additional lines of code that I will gladly contribute), but I don't > > think it's ambiguous. We could perform very rigorous and helpful > > checks on the input arguments. > > > >> Evidently, we all see things differently. I fully support Anders in > >> that mathematical expressiveness is one of the key features of > >> FEniCS, and I think that without pushing these types of boundaries > >> with regard to the language, it will end up as yet another finite > >> element library. > >> > >> Could we compromise on having the two versions, one explicit (based > >> on LinearVariational[Problem|Solver] or something of the kind) and > >> one terse (based on solve(x == y)) ? > > > > That's what I'm suggesting. The solve(x == y) would just rely on the > > more "explicit" version and do > > > > <lots of checks> > > LinearVariationalSolver solver(x, y, ...); > > solver.solve(u); > > > > So in essence what I'm asking for is please let me add that tiny layer > > on top of what we already have + remove the Problem classes (replaced > > by the Solver classes). > > > > > > _______________________________________________ > > Mailing list: https://launchpad.net/~dolfin > > Post to : dolfin@lists.launchpad.net > > Unsubscribe : https://launchpad.net/~dolfin > > More help : https://help.launchpad.net/ListHelp > > _______________________________________________ Mailing list: https://launchpad.net/~dolfin Post to : dolfin@lists.launchpad.net Unsubscribe : https://launchpad.net/~dolfin More help : https://help.launchpad.net/ListHelp
