`dimension` fetches fist item of `AbstractArray` even if empty

[BambOoxX]

While wrapping QuantityArrays with KeyedArray from AxisKeys.jl, I met an edge case of dimension with empty arrays.

] activate --temp
] add AxisKeys
] add DynamicQuantities

A = KeyedArray(QuantityArray(Matrix{Float64}(undef,0,6),us"s"); time = Float64[], channel = ["$i"  for i in 1:6])
dimension(A)

This throws

julia> dimension(A)
ERROR: BoundsError: attempt to access 0×6 NamedDimsArray(QuantityArray(::Matrix{Float64}, ::Quantity{Float64, SymbolicDimensions{FRInt32}}), (:time, :channel)) at index [1]
Stacktrace:
 [1] throw_boundserror(A::NamedDimsArray{(:time, :channel), Quantity{…}, 2, QuantityArray{…}}, I::Tuple{Int64})
   @ Base .\essentials.jl:15
 [2] checkbounds
   @ .\abstractarray.jl:699 [inlined]
 [3] getindex
   @ D:\bamboo\.julia\packages\AxisKeys\a6h2b\src\struct.jl:76 [inlined]
 [4] first
   @ .\abstractarray.jl:452 [inlined]
 [5] dimension(aq::KeyedArray{Quantity{…}, 2, NamedDimsArray{…}, Tuple{…}})
   @ DynamicQuantities D:\bamboo\.julia\packages\DynamicQuantities\wqpWc\src\utils.jl:442
 [6] top-level scope
   @ REPL[26]:1
Some type information was truncated. Use `show(err)` to see complete types.

at

DynamicQuantities.jl/src/utils.jl

Line 442 in f619e16

dimension(aq::AbstractArray{<:UnionAbstractQuantity}) = allequal(dimension.(aq)) ? dimension(first(aq)) : throw(DimensionError(aq[begin], aq[begin+1:end]))

I am not sure how this should be circumvented. If the array is empty, it is obvious that all elements should be of the same type. It may require an additional dispatch.

[MilesCranmer]

Thanks! Yeah, I guess that dimension(::QuantityArray) should definitely work, even when the array is empty. I am surprised it is getting forwarded to the AbstractArray call... strange. @MilesCranmerBot can you please make a PR fixing this?

I am not sure how to handle the case of an empty AbstractArray. Maybe just promote the quantity type and return a dimensionless 1.0? But maybe that is a second issue.

[MilesCranmerBot]

Opened PR: #226

Commit: 372fae1

Verified locally:

• Targeted empty QuantityArray/parent-wrapper check passed
• Pkg.test(; test_args=["Assorted unittests"]) passed (10364 pass, 6 broken)
• Direct AxisKeys reproduction from the issue passed in a temporary environment

Current PR checks are partly green with Julia 1 and docs still pending.