Setting options on matrices obtained from a DM

[Jed Brown]

Matthew Knepley wrote:

> There is no way we would be doing conintuations. Why even bring this up?
> This just complicates the discussion and distracts from the main issues.
> 
> The answer is incredibly easy and straightforward. Just like MANY
> functions in PETSc, you will not get the correct answer until things
> have been initialized.  This just sounds like arguing for the sake of
> argument.

I should of course get an error, not a wrong answer, but that's not the
issue here.

It is useful to be able to call XGetY() once the X has enough
information to provide Y, but before it has enough information for
XSetUp() to be called.  This is fine with the current model.  With your
model, it looks like I would always have to wait until *after* SetUp has
been called.  I believe there are several circumstances where it is not
possible to get access to the object after setup and before whatever
else happens.  This apparent "locking" causes (at least in other codes)
the introduction of multiple SetUp-like calls which are more of a pain.

If you are confident that you can restructure things so that it is
always possible for XSetUp() to called before you need to call XGetY(),
then I have no objections.


Jed


PS: Continuations are a way to make whatever you want happen at the
appropriate time with respect to SetUp, but they are obviously insane
for this purpose.  Request objects are probably worse.

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