Adding PetscMap getters

[Barry Smith]

    Dave,

      This has been a controversial issue over the years. We didn't  
have a map concept, it was introduced, it was then
"hidden".  The fact that there exist public PetscMap functions at all  
is my compromise for a persistent user.

      Two key "rules" I use in making decisions in PETSc code are:

1) try really really hard to minimize the number of abstract objects  
in the PETSc interface (this is because I believe
      the difficulty of using a "class" library grows exponentially  
with the number of abstract objects in the interface)
     (obviously not everyone believes this, it seems most class  
libraries add new classes at the drop of a hat)
2) there should be ONE way to do each operation in the package (this  
is because I believe having multiple ways to
    do the same thing is confusing to most users). For example, to set  
the size of a vector you can only use VecSetSizes(),
    to solve a nonlinear system you call SNESSolve()*.

   If we bring PetscMap up as "first class user object" this violates  
both 1 and 2. It is a new basic object and manipulating/accessing
Vec/Mat size information would have TWO ways of doing everything:  
VecGetSizes(), VecGetPetscMap() -> PetscMapGetSizes() etc.
I actually dream about the opposite; shrinking PetscMap down so it is  
so small it can be strangled in a bathtub. I tried once
to remove the VecGetSizes() etc and have access only through PetscMap  
(to satisfy rule 2) but ended up removing the public
PetscMap because I didn't see having PetscMap as the basic "layout  
definer" as a good PETSc interface.

    So my answer is, I really do not want to add these getters.  you  
really have to convince me that the PetscMap object
is so fundamentally powerful it is worth violating 1 and 2.

   Barry


Note that the PetscMap() methods are prototyped in vecimpl.h,  
indicating they are not public methods for PETSc.





*sadly there is also DMMGSolve(), this I view as an interface bug and  
dream about someday merging this functionality
back to the SNES.


On Aug 21, 2008, at 5:52 PM, Dave May wrote:

> Hello,
>    Would it be possible to have the following small getters added to  
> petsc-dev?
>         VecGetOwnershipRanges()
> Since we are just manipulating the maps why not include the  
> following as well
>        VecGetMap()
>        MatGetRowMap()
>        MatGetColumnMap()
>
> It seems reasonable to add these (to me at least) given there a few
> interfaces for
> PetscMap already made public.
>
> Cheers,
>    Dave.
>