Why is `BandedMatrix` mutable?

[jishnub]

BandedMatrices.jl/src/banded/BandedMatrix.jl

Line 16 in 7315691

mutable struct BandedMatrix{T, CONTAINER, RAXIS} <: AbstractBandedMatrix{T}

Tests for this package seem to pass even without mutability, so I wonder if this is used downstream. Immutability would presumably permit compiler optimizations.

[dlfivefifty]

I suspect it’s used in ApproxFun or InfiniteLinearAlgebra when ∞ matrices are cached and dynamically grown in inf QR. This could (and arguably should) be redesigned, eg using

https://github.com/JuliaArrays/ElasticArrays.jl

[dlfivefifty]

@jishnub Do you feel like trying to make this change?

[jishnub]

Yes, I'll check if tests for these packages pass with this change locally

[dlfivefifty]

Actually I double checked and InfiniteLinearAlgebra.jl should be fine if BandedMatrix is immutable: it just uses a CachedMatrix for the back-end. (CachedMatrix could also be made immutable using ElasticArrays.jl but we can leave that for now.)

ApproxFunBase.jl has issues however: when a banded CachedOperator is resized it calls pad!:

https://github.com/JuliaApproximation/ApproxFunBase.jl/blob/defba13423935cec39c9c2a98748802c43cb85f3/src/Caching/banded.jl#L18

Which then swaps out the data:

https://github.com/JuliaApproximation/ApproxFunBase.jl/blob/9f8af07f7cbb23bcb13d0099eade91f605cb8be4/src/hacks.jl#L4

I think a quick fix would be to replace the pad! line with:

B.data = pad(B.data,min(N,2n),m)

since CachedOperator is mutable.