Proposal for new operation: Diagonal Matrix Multiplication

[jim22k]

A very useful pattern has emerged to broadcast a vector across a matrix.

1. Convert the vector into a diagonal matrix
2. Perform matrix multiplication using an any_XXX semiring

If the diagonal matrix is on the left, it broadcasts the vector as if it were a column vector.
If the diagonal matrix is on the right, it broadcasts the vector as if it were a row vector.

While this works, it has downsides.

• While the storage of a diagonal matrix isn't large, it does require creating another object besides the original vector
• The monoid part of the semiring is pointless, but must be specified

I think it might make sense to create a graphblas.diag_matmul operation which takes a vector and matrix and a binary operator and does the broadcasting. The order of the vector determines how it is broadcast.