stack should accept multiple dims

[LilithHafner]

From the docstring, "With keyword dims::Integer [...] stack reverses the action of eachslice with the same dims." Those semantics should be extended to dims::Integer....

julia> x = rand(3,3,3);

julia> all(stack(eachslice(x, dims=i), dims=i) == x for i in 1:3)
true

julia> stack(eachslice(x, dims=(1,2)), dims=(1,2)) == x
ERROR: MethodError: no method matching _stack(::Tuple{Int64, Int64}, ::Slices{Array{Float64, 3}, Tuple{Int64, Int64, Colon}, Tuple{Base.OneTo{Int64}, Base.OneTo{Int64}}, SubArray{Float64, 1, Array{Float64, 3}, Tuple{Int64, Int64, Base.Slice{Base.OneTo{Int64}}}, true}, 2})

Closest candidates are:
  _stack(::Union{Colon, Integer}, ::Any)
   @ Base abstractarray.jl:2769
  _stack(::Any, ::Union{Base.HasLength, Base.HasShape}, ::Any)
   @ Base abstractarray.jl:2773
  _stack(::Any, ::Base.IteratorSize, ::Any)
   @ Base abstractarray.jl:2771

Stacktrace:
 [1] stack(iter::Slices{Array{Float64, 3}, Tuple{Int64, Int64, Colon}, Tuple{Base.OneTo{Int64}, Base.OneTo{Int64}}, SubArray{Float64, 1, Array{Float64, 3}, Tuple{Int64, Int64, Base.Slice{Base.OneTo{Int64}}}, true}, 2}; dims::Tuple{Int64, Int64})
   @ Base ./abstractarray.jl:2737
 [2] kwcall(::NamedTuple{(:dims,), Tuple{Tuple{Int64, Int64}}}, ::typeof(stack), iter::Slices{Array{Float64, 3}, Tuple{Int64, Int64, Colon}, Tuple{Base.OneTo{Int64}, Base.OneTo{Int64}}, SubArray{Float64, 1, Array{Float64, 3}, Tuple{Int64, Int64, Base.Slice{Base.OneTo{Int64}}}, true}, 2})
   @ Base ./abstractarray.jl:2737
 [3] top-level scope
   @ REPL[68]:1

cc @mcabbott, author of stack. (#43334)

[mcabbott]

Yes I think this has an obvious meaning, although with some edge case where ndims(outer) != length(dims). Going by eachslice's move on from dims::Integer, the ETA is Julia 1.17... What does work at the moment is all the trailing dimensions:

julia> y = rand(3,3,3,3);

julia> y == stack(eachslice(y, dims=(3,4))) == stack(eachslice(y, dims=(2,3,4)))
true

Perhaps related questions, but easier to implement:

• Should there be a stack!?
• Python has hstack and vstack, is that too many verbs & too close to hcat etc?
• [Edit] At present stack((i,2i) for i in 3:5; dims=4) is forbidden in the name of type-stability. It could allow dims=Val(4) like cat does.

[jariji]

Python has hstack and vstack, is that too many verbs & too close to hcat etc?

It's not too many, imo. It could be a eachrow => stackrows, eachcol => stackcols?