Add showarg(io, ::Slices) printing for eachslice

[mcabbott]

This proposes to print the Slices objects from #32310 in a similar way to SubArray, ReshapedArray, etc. These compose nicely:

julia> view(rand(10)', 2:3)
2-element view(reshape(adjoint(::Vector{Float64}), 10), 2:3) with eltype Float64:
 0.09114060604932506
 0.6108851444289344

julia> eachcol(ans)
1-element eachcol(reshape(view(reshape(adjoint(::Vector{Float64}), 10), 2:3), 2, 1)) of 2-element slices with eltype Float64:
 [0.09114060604932506, 0.6108851444289344]

julia> eltype(ans)
SubArray{Float64, 1, Base.ReshapedArray{Float64, 2, SubArray{Float64, 1, Base.ReshapedArray{Float64, 1, Adjoint{Float64, Vector{Float64}}, Tuple{}}, Tuple{UnitRange{Int64}}, true}, Tuple{}}, Tuple{Base.Slice{Base.OneTo{Int64}}, Int64}, true}

Most showarg methods have the outermost one print the eltype of the object. Here this is a long and not especially illuminating SubArray type, so instead it prints the size of the elements, and eltype(eltype(s)). Examples (edit: focusing on the empty case, as initially this needed special treatment):

julia> eachrow(Int[])
0-element eachrow(::Matrix{Int64}) of 1-element slices with eltype Int64

julia> eachcol(Int[])
1-element eachcol(::Matrix{Int64}) of 0-element slices with eltype Int64:
 0-element view(::Matrix{Int64}, :, 1) with eltype Int64

Finally, it also hides this SubArray type in compact show. Not completely sure that's a good idea: (removed for now)

julia> eachcol([1 2 3; 44 55 66]) |> repr
"[[1, 44], [2, 55], [3, 66]]"

julia> (0, eachcol(Float32[1 2; 3 4]), 5)  # array within a tuple uses 2-arg show
(0, [Float32[1.0, 3.0], Float32[2.0, 4.0]], 5)

[nalimilan]

Most showarg methods have the outermost one print the eltype of the object. Here this is a long and not especially illuminating SubArray type, so instead it prints the size of the elements, and eltype(eltype(s)):

julia> eachrow(Int[])
0-element eachrow(::Matrix{Int64}) of 1-element slices with eltype Int64

julia> eachcol(Int[])
1-element eachcol(::Matrix{Int64}) of 0-element slices with eltype Int64:
 0-element view(::Matrix{Int64}, :, 1) with eltype Int64

Is there a copy/paste bug here? I don't understand where the "0-element view(::Matrix{Int64}, :, 1) with eltype Int64" part comes from, and it doesn't appear in the examples in the diff.

[nalimilan]

I don't really understand this comment. Maybe develop a bit more?

[mcabbott]

To be clear what this is doing, this is the effect:

julia> (0, eachcol(Float32[1 2; 3 4]), 5)  # master
(0, SubArray{Float32, 1, Matrix{Float32}, Tuple{Base.Slice{Base.OneTo{Int64}}, Int64}, true}[[1.0, 3.0], [2.0, 4.0]], 5)

julia> (0, eachcol(Float32[1 2; 3 4]), 5)  # this PR
(0, [Float32[1.0, 3.0], Float32[2.0, 4.0]], 5)

The default 2-arg show for an array of arrays prints the whole type of the inner arrays as the eltype of the outer. This line removes that. But also notifies show that the eltype of the inner arrays has not been printed anywhere, hence they become Float32[...].

Will expand the comment a bit.

[mcabbott]

Expanded in f37584d

[nalimilan]

OK. The new printing is certainly clearer, but I wonder it's consistent with what we do elsewhere. Are there any equivalent constructs that could benefit from the same simplification? My concern is that if it's the only case where we omit the actual eltype then people could be misled into thinking they really have a Vector{Vector{Float32}}.

[mcabbott]

Yes, I'm not sure. Compact printing is sometimes lossy, e.g. Transpose is not marked here (but matters):

julia> ([1 2], transpose([3,4]), [[5,6], [6,7]]')
([1 2], [3 4], Adjoint{Int64, Vector{Int64}}[[5 6] [6 7]])

julia> Adjoint{Int64, Vector{Int64}}[[5 6] [6 7]]
ERROR: MethodError: Cannot `convert` an object of type Int64 to an object of type Adjoint{Int64, Vector{Int64}}

Also, the Adjoint printing is not copy-pastable, it does not re-create such elements. That's also true of the SubArray types which will be shown for eachcol. Perhaps both should be abbreviated somehow?

[mcabbott]

f3869c9 reverts this for now.

[mcabbott]

Is there a copy/paste bug here?

I don't think so. eachcol reshapes a vector, thus size (0,) becomes size (0,1) which has 1 column.

But it's a confusing example now, sorry. Initially I treated empty cases differently, to use size(first(s)) when non-empty. But not anymore.

[mcabbott]

Last commit removes the changes to 2-arg show, keeping only showarg which is called by 3-arg show.

[mcabbott]

Changing these examples from 2x2 to 2x3 will, I hope, make it more obvious which dimension is which.

[mcabbott]

Added because the type alias RowSlices is no longer printed.