Export AbstractSparseMatrixCSC + interface

[j-fu]

See discussion in #265

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 84.24%. Comparing base (81d49e9) to head (4fc922a).

Files	Patch %	Lines
src/sparsematrix.jl	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #395      +/-   ##
==========================================
+ Coverage   84.11%   84.24%   +0.13%     
==========================================
  Files          12       13       +1     
  Lines        9102     9129      +27     
==========================================
+ Hits         7656     7691      +35     
+ Misses       1446     1438       -8     

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[j-fu]

Similar question for sparse(m::AbstractSparseMatrixCSC). In 1.10 it returns a copy(m). Which may be reasonable. One could argue as well (I tend to prefer this) that the return value should be a SparseMatrixCSC like with the other methods of sparse.

[ViralBShah]

Generally, the thought with sparse has been that it shouldn't be seen as SparseMatrixCSC specific.

[ViralBShah]

My concern with exporting was that wouldn't we better off with something more general? AbstractSparseMatrixCSC feels too specialized a thing as an abstract type. But of course, it isn't a huge change per se.

@rayegun Any thoughts?

[ViralBShah]

Pinging @KristofferC and @fredrikekre on any thoughts here. Another set of eyes would be good here. While I don't see a major issue, a public API would be a big commitment going forward, and AbstractSparseMatrixCSC feels like a bit of a specialized abstract class.

[j-fu]

... I'd like to come back to this - Something more general would of course be appreciated, as e.g. discussed in #538 . OTOH, as a matter of fact the interface is used in LinearSolve.jl, see e.g. here and here.
So IMHO it would be good to have this fixed as public API. May be instead of exporting it could be made public now.

[j-fu]

@ChrisRackauckas, @oscardssmith, what do you think about this ?

[ChrisRackauckas]

I think we need to write down what that expected interface is if you're going to consider it public. Like https://docs.julialang.org/en/v1/manual/interfaces/ . Can you try doing a first draft of it?

Maybe @mbauman and @timholy would be good to have on-board as well since they drafted the original array interface and I know they have comments on what a sparse array interface would need to have.

[j-fu]

I started a discussion in #559. Not sure if this is the right place. See also #538.

[timholy]

Do Diagonal, Bidiagonal, Tridiagonal, and SymTridiagonal all support this interface? They're examples of sparse matrices even if they are not subtypes of AbstractSparseMatrix. Would a CSR matrix be expected to support getcolptr? What would that even mean?

The deepest thinking I've done on a real interface for sparse arrays of all kinds is in https://github.com/timholy/ArrayIteration.jl (unfinished and effectively abandoned for lack of time). That interface is not so tightly coupled to a particular internal representation. A good start is to ask yourself the question, "what is needed to write algorithms on sparse matrices even if I don't know how they are stored?"

[j-fu]

... Certainly, as CSR matrix should have getrowptr and not getcolptr...
Very interesting discussion in ArrayIterators.jl !

I see two strands of discussion here (as I tried to outline in #559):

• "what is needed to write algorithms on sparse matrices even if I don't know how they are stored?" -- which needs time (which I might not have as well...) and dedication
• Low level interfaces for coupling to all kinds of (often binary) packages which rely upon the concrete CSC or CSR structure. In this context, the AbstractSparseMatrixCSC interface (getcolptr etc. ) is currently used as a matter of fact though it is not exported or documented as such.

IMHO, both levels of interfaces should (and can) coexist.