[BUG] `TensorAlgebra.qr(::SparseArrays.SparseMatrixCSC, ...)` broken

[mtfishman]

For example:

julia> using TensorAlgebra

julia> using SparseArrays

julia> TensorAlgebra.qr(sprandn(2, 2, 0.5), ("i", "j"), ("i",), ("j",))
ERROR: MethodError: no method matching iterate(::SparseArrays.SPQR.QRSparse{Float64, Int64})
The function `iterate` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  iterate(::SimpleTraits.GenerateTypeVars{:upcase})
   @ SimpleTraits ~/.julia/packages/SimpleTraits/l1ZsK/src/SimpleTraits.jl:445
  iterate(::SimpleTraits.GenerateTypeVars{:lcase})
   @ SimpleTraits ~/.julia/packages/SimpleTraits/l1ZsK/src/SimpleTraits.jl:446
  iterate(::SimpleTraits.GenerateTypeVars{:upcase}, ::Any)
   @ SimpleTraits ~/.julia/packages/SimpleTraits/l1ZsK/src/SimpleTraits.jl:445
  ...

Stacktrace:
 [1] indexed_iterate(I::SparseArrays.SPQR.QRSparse{Float64, Int64}, i::Int64)
   @ Base ./tuple.jl:162
 [2] qr(a::SparseMatrixCSC{Float64, Int64}, biperm::TensorAlgebra.BlockedPermutation{2, (1, 1), Tuple{Int64, Int64}})
   @ TensorAlgebra ~/.julia/packages/TensorAlgebra/p4Ouf/src/factorizations.jl:8
 [3] qr(a::SparseMatrixCSC{Float64, Int64}, labels_a::Tuple{String, String}, labels_codomain::Tuple{String}, labels_domain::Tuple{String})
   @ TensorAlgebra ~/.julia/packages/TensorAlgebra/p4Ouf/src/factorizations.jl:20
 [4] top-level scope
   @ REPL[10]:1

julia> pkgversion(TensorAlgebra)
v"0.1.7"

The reason for the error is that LinearAlgebra.qr(::SparseArrays.SparseMatrixCSC) outputs a factorization object that can't be unpacked as a tuple:

julia> using LinearAlgebra

julia> using SparseArrays

julia> q, r = qr(sprandn(2, 2, 0.5))
ERROR: MethodError: no method matching iterate(::SparseArrays.SPQR.QRSparse{Float64, Int64})
The function `iterate` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  iterate(::SimpleTraits.GenerateTypeVars{:upcase})
   @ SimpleTraits ~/.julia/packages/SimpleTraits/l1ZsK/src/SimpleTraits.jl:445
  iterate(::SimpleTraits.GenerateTypeVars{:lcase})
   @ SimpleTraits ~/.julia/packages/SimpleTraits/l1ZsK/src/SimpleTraits.jl:446
  iterate(::SimpleTraits.GenerateTypeVars{:upcase}, ::Any)
   @ SimpleTraits ~/.julia/packages/SimpleTraits/l1ZsK/src/SimpleTraits.jl:445
  ...

Stacktrace:
 [1] indexed_iterate(I::SparseArrays.SPQR.QRSparse{Float64, Int64}, i::Int64)
   @ Base ./tuple.jl:162
 [2] top-level scope
   @ REPL[16]:1

which is being used inside TensorAlgebra.qr.

[mtfishman]

@lkdvos maybe this could be a good use case for MatrixAlgebraKit.jl, i.e. defining a uniform interface for sparse and dense QR.

[lkdvos]

I'm trying to get that pushed along indeed, since that would also be a good place to have the truncated SVD. I am hoping to get that registered by the end of next week, but if not I can do a temporary hack and just add it to the ITensor registry.

[mtfishman]

I'm trying to get that pushed along indeed, since that would also be a good place to have the truncated SVD. I am hoping to get that registered by the end of next week, but if not I can do a temporary hack and just add it to the ITensor registry.

Sounds good! Probably it would be helpful for us to meet up for you to go over that package with me since I haven't been following along with the development.