Support an empty table with Union{} rows

[andyferris]

Fixes #55.

@tkf I was wondering if this is the kind of thing you would like to push!! to? I guess it would be nice to see a prototype working.

[codecov-io]

Codecov Report

Merging #56 into master will decrease coverage by 0.86%.
The diff coverage is 12.5%.

@@            Coverage Diff            @@
##           master     #56      +/-   ##
=========================================
- Coverage   67.06%   66.2%   -0.87%     
=========================================
  Files           6       6              
  Lines         498     506       +8     
=========================================
+ Hits          334     335       +1     
- Misses        164     171       +7

Impacted Files	Coverage Δ
src/Table.jl	71.69% <0%> (-4.31%)	⬇️
src/show.jl	50% <50%> (ø)	⬆️

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[tkf]

What I had in mind was something like

Suggested change

	Base.empty(::Table{<:Any, 1}, ::Type{Union{}}) = EmptyTable{1}(nothing)
	Base.empty(::Type{<:Table{<:Any, 1}}) = EmptyTable{1}(nothing)

i.e., I want an empty table given its type. I also would like to suggest omitting the eltype since this is the only eltype that is guaranteed to be able to widen to anything.

[andyferris]

Oh you want to use types not instances? I’ve been heavily trending the other direction lately.

And the last part - you mean omit the eltype in empty’s function signature and default to Union{}, not the eltype of the input?

[tkf]

For the first part, please see my comment in #55 (comment)

you mean omit the eltype in empty’s function signature and default to Union{}, not the eltype of the input?

I was suggesting to make the second argument optional. So, e.g., returning Union{}[] from empty(::Type{Vector}). Though empty(::Type{Vector{Float64}}) probably still should return Float64[].