Setting options on matrices obtained from a DM

[Jed Brown]

Matthew Knepley wrote:
>> MatExecute()?  What about MatGetXXX() before MatExecute()?
> 
> It will reregister that function, so you would need another SetUp()
> call, exactly the same interface we have now.

But the user wants to call MatGetXXX() and use the result right away
(instead of getting a request object or passing a continuation to be
executed when the result is available).  The whole point of the
continuation is to perform a side-effect which is going to cause
confusing behavior.

Currently they get an error if the result isn't available.  With the
"register attributes, then execute" model, the result will almost never
be available so everything they might do would go into a continuation.

We could probably make this work (even with reasonable syntax) in a
language like Haskell, but in an impure strict language I think it will
snowball into a mess of side-effecting continuations being called at
peculiar times.

Jed

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