Setting options on matrices obtained from a DM

[Jed Brown]

Matthew Knepley wrote:
> It would be impossible for me to disagree more strongly. A user cannot
> get information that is not setup in the current model. I really do not
> understand your objection, and the Haskell discussion is bizarre. All I am
> suggestion is that we use a better model for remembering what to do.

If I call MatGetXXX(), I expect to either get a meaningful and usable
result, or get an error.  As I understand your suggestion, I wouldn't
get that unless SetUp had been called.  But I wanted to do something
with that result which means that I will either get a request object (I
keep control of when I do that thing with the result, this will be a
PITA) or I give you a continuation for whatever I wanted to do with it
(but then I don't know when it will be executed which is confusing
because the continuation will have side-effects, and it's a PITA to do
continuations in C/C++/etc).  In Haskell, it's possible to maintain
control of side-effects and have sane syntax for all these
continuations, but we don't have (or want to be using) Haskell for this.

To be concrete, someone is going to do:

  TSGetSNES
  SNESGetKSP
  KSPGetPC
  PCGetOperators
  MatGetVecs
  /* do something with the matrices and vectors */

If the objects are not sufficiently set up when this is called, the
current model will give an error which is easy to understand.  With your
model, if we haven't called SetUp(), none of these objects will be
available, so instead we'll pass a continuation for everything to do
with them.  In pseudo-Haskell and skipping the obvious sugar, we'd have
something like

  TSGetSNES ts (\snes ->
    SNESGetKSP snes (\ksp ->
      KSPGetPC ksp (\pc ->
        PCGetOperators pc (\(a,p) ->
          MatGetVecs p (\(x,y) ->
            -- do something with a,x,y
          )))))

The monad that this runs under would be able to control side-effects.
That is, we have a type signature like

  XXXGetYYY :: x -> (y -> M a) -> M a

where the monad M controls the side-effects.  Doing this in C would be
insane which means that I must be misunderstanding what you want to do.

Jed

-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 260 bytes
Desc: OpenPGP digital signature
URL: <http://lists.mcs.anl.gov/pipermail/petsc-dev/attachments/20090723/cdab5073/attachment.sig>