a Diagonal of a SVector doesn't keep its type

[yakir12]

First seen here:

julia> m = SVector(1,2,3)
3-element SVector{3,Int64}:
 1
 2
 3

julia> x = Diagonal(m)
3×3 Diagonal{Int64}:
 1  ⋅  ⋅
 ⋅  2  ⋅
 ⋅  ⋅  3

julia> x.diag
3-element Array{Int64,1}:
 1
 2
 3

[fredrikekre]

Thats by definition of Diagonal in Base, it is always a Vector.

[yakir12]

I see.

[fredrikekre]

Could perhaps define an unexported Diagonal in StaticArrays. Something like:

struct Diagonal{T, N}
    diag::SVector{N, T}
end
Base.Diagonal(v::SVector) = Diagonal(v)

julia> m = SVector(1,2,3)
3-element SVector{3,Int64}:
 1
 2
 3

julia> Diagonal(m)
StaticArrays.Diagonal{Int64,3}([1, 2, 3])

Perhaps too many additional methods are needed to make this usable though.

[c42f]

Oh, thanks for pointing this out. It's another of many places in Base.LinAlg where things don't work quite right if you're using StaticArrays.

Obviously it's best to fix Base for 0.7, though I guess as a rather nasty hack, we could overload Diagonal(::StaticArray) to return our own special StaticArrays.Diagonal. Hmm.

[c42f]

Or we could just define SDiagonal for now.

[yakir12]

The only objective I'd care about, as a user, is the performance and ease of use. So aesthetics aside, SDiagonal sounds great.

[c42f]

Yes, SDiagonal is probably the best option for clarity, until Base.Diagonal can support a static vector. Hijacking the Diagonal constructor might still be worth it: it's a bit evil, but probably less of a surprise to users than silently allocating data. If they really want a normal Diagonal, they can get it explicitly with Diagonal(collect(sa)) for some static array sa. It will require reimplementing a bunch of stuff from Base, as usual, but it's probably not that hard if we just go through and port the required pices from base/linalg/diagonal.jl (PRs welcome ;-))

[mschauer]

@getzdan is working on a port of https://github.com/mschauer/Bridge.jl to use StaticArrays and added relevant code, see
https://github.com/getzdan/Bridge.jl/blob/390c6cd9f3adc9abb16b4b8e3b3abb0ec3fd35f7/src/diagonal.jl

[andyferris]

SDiagonal now exists.