collect_structarray does not work with empty iterator

[tkf]

julia> StructVector(x for x in [] if false)
ERROR: UndefVarError: T not defined
Stacktrace:
 [1] buildfromschema(::Function, ::Core.TypeofBottom) at /home/takafumi/.julia/dev/StructArrays/src/lazy.jl:25
 [2] (::StructArrays.StructArrayInitializer{StructArrays.var"#41#43",typeof(StructArrays.default_array)})(::Type, ::Tuple{Int64}) at /home/takafumi/.julia/dev/StructArrays/src/collect.jl:13
 [3] #collect_empty_structarray#104 at /home/takafumi/.julia/dev/StructArrays/src/collect.jl:52 [inlined]
 [4] #collect_structarray#103 at /home/takafumi/.julia/dev/StructArrays/src/collect.jl:47 [inlined]
 [5] #collect_structarray#101 at /home/takafumi/.julia/dev/StructArrays/src/collect.jl:41 [inlined]
 [6] #StructArray#40 at /home/takafumi/.julia/dev/StructArrays/src/structarray.jl:81 [inlined]
 [7] StructArray at /home/takafumi/.julia/dev/StructArrays/src/structarray.jl:81 [inlined]
 [8] #StructVector#8 at /home/takafumi/.julia/dev/StructArrays/src/structarray.jl:49 [inlined]
 [9] StructArray{T,1,C,I} where I where C<:Union{Tuple, NamedTuple} where T(::Base.Generator{Base.Iterators.Filter{var"#128#130",Array{Any,1}},var"#127#129"}) at /home/takafumi/.julia/dev/StructArrays/src/structarray.jl:49

I think it makes sense to at least make the error message less cryptic.

If you want to make it return something, my preference is to return an array of Union{} element type. Related discussion in: JuliaData/TypedTables.jl#55

[piever]

This does create some headaches, esp. in IndexedTables, which uses this machinery. In particular, I need to allow the awkward collect_structarray(itr, state) method for technical reasons (I got rid of it at the moment, but can't release without breaking IndexedTables). The current approach is to use inference, which of course is not ideal, but is what Base does to collect am empty collection.

The principled approach is to use Union{} I guess, and suggest that the user always goes with append!!, but it does create some type instabilities. I'm giving it a go here #103 but some @inferred fail. There is also the extra annoyance that I need to keep passing initializer to append!! (I allow a custom initialization for collecting the StructArray), because if I'm appending to the neutral element I still need to initialize the columns of the StructArray.

[piever]

The easiest for now would be to fix the error you see. The problem is that it tries to build a StructArray of eltype Union{} which is not possible. I could probably just support StructArray{Union{}} with no columns, and get buildfromschema to do the correct thing in that case.

[tkf]

some @inferred fail

Maybe something like this would work?

ifnotbottom(x) = eltype(x) <: Union{} ? error() : x
@inferred ifnotbottom(expr_to_be_tested)

Since Julia compiler is good at union splitting now, I guess it's fine to return small union?

I could probably just support StructArray{Union{}} with no columns

If you do this, you can maybe also specialize isempty etc. for this type so that users can do

structarray = ...
isempty(structarray) && return ...
# here the compiler can prove `structarray` is not of type `StructArray{Union{}}`

when the performance is important.