[WIP] Merge ComponentVectors

[Qfl3x]

Issue #186

Implemented merge on 2 (or more) ComponentVectors, similar to merge on NamedTuples or Dicts. With docs.

[Qfl3x]

This PR will break precompilation. Working on using Macros instead of the Eval (bad idea)

[Qfl3x]

I think it's resolved now.

[jonniediegelman]

I'm going to go ahead and reopen this because I think it will be useful to have. If you get a chance to change it and write some tests for it, I'd appreciate it. If not, I can do it at some point hopefully soon.

[jonniediegelman]

It might be good to have this in as

Base.merge(ca::ComponentVector) = ca
Base.merge(ca1::ComponentVector, ca2::ComponentVector) = ComponentVector(ca2; ca1...)
Base.merge(ca1::ComponentVector, ca2::ComponentVector, others...) = merge(merge(ca1, ca2), others...)

Because the ComponentVector(ca2; ca1...) syntax can already perform the merge via destructuring and splatting ca1.

[codecov-commenter]

Codecov Report

Merging #217 (6326604) into main (59031c1) will decrease coverage by 1.70%.
Report is 1 commits behind head on main.
The diff coverage is 0.00%.

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@@            Coverage Diff             @@
##             main     #217      +/-   ##
==========================================
- Coverage   72.75%   71.06%   -1.70%     
==========================================
  Files          20       21       +1     
  Lines         690      698       +8     
==========================================
- Hits          502      496       -6     
- Misses        188      202      +14     

Files Changed	Coverage Δ
src/ComponentArrays.jl	100.00% <ø> (ø)
src/componentarray.jl	71.12% <0.00%> (-7.78%)	⬇️

... and 7 files with indirect coverage changes

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[Qfl3x]

The whole point was to make it differentiable via ChainRules, though this isn't. A solution exists for making it work with ChainRules, however, I think focusing on #207 is best.

[Qfl3x]

Last two commits added type promotion.

[jonniedie]

Ah, I see. Yeah, we might want to make this work with merge directly then because ChainRules doesn't support differentiating through keyword arguments. We'll likely need to do something similar to your first approach, but with @generated functions for type-stability.

[Qfl3x]

I think something similar to what @vboussange did could work:

import Base
function Base.merge(ca::ComponentArray{T}, ca2::ComponentArray{T}) where T
    ax = getaxes(ca)
    ax2 = getaxes(ca2)
    vks = valkeys(ax[1])
    vks2 = valkeys(ax2[1])
    _p = Vector{T}()
    for vk in vks
        if vk in vks2
            _p = vcat(_p, ca2[vk])
        else
            _p = vcat(_p, ca[vk])
        end
    end
    ComponentArray(_p, ax)
end

Though this one implies that ca2 is a subset of ca. So I'm working on "generalizing" it a bit further. It all comes down to a "smart" Axis merge.

[Qfl3x]

The last commit has an example that might be expanded on. Though it's a bit convoluted, it does work.