Define functional setters using Setfield

[tkf]

With this PR, you can do:

julia> using VegaLite

julia> using Setfield  # to import `@set`

julia> plt = NamedTuple{(:x, :y)}.(tuple.(randn(1000), randn(1000))) |>
       @vlplot(
           :point,
           x=:x,
           y=:y,
       )

julia> plt2 = @set plt.mark = :line

This is built on top of #190 and will close #168.

[codecov-io]

Codecov Report

Merging #191 into master will decrease coverage by 0.24%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master     #191      +/-   ##
==========================================
- Coverage   84.87%   84.63%   -0.25%     
==========================================
  Files          12       12              
  Lines         410      423      +13     
==========================================
+ Hits          348      358      +10     
- Misses         62       65       +3

Impacted Files	Coverage Δ
src/VegaLite.jl	100% <ø> (ø)	⬆️
src/spec_utils.jl	84% <88.88%> (-7.67%)	⬇️

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[davidanthoff]

Cool, so this now is purely functional, with no mutation, right?

[tkf]

Yep, it's purely functional. Internally, Setfield uses something called lens which is invented by a Haskeller: https://github.com/ekmett/lens

[davidanthoff]

Alright, yes, then lets do this. Do you have to rebase now that #190 is merged, or do we merge this right away?

[tkf]

It looks like it has no conflict so it can be merged as-is? But let me know if you want me to rebase.

(I just restarted the CI for now.)

[davidanthoff]

The diff here on the PR looks weird: it looks like it will add ObjectProxy, but that is already on master, right? Slightly confused...

[tkf]

Oh, I wasn't looking at the diff. I rebased the PR.

[tkf]

This could be written as @set getparams(spec)[keytype(params)(name)] = value if Base had setindex for Dict...: JuliaLang/julia#33495

[davidanthoff]

Thanks, I really like this now! If you have time, maybe add some docs as another PR?

[tkf]

Will do. Nice thing about Setfield is that it works with other constructs. Things like

obj = (
    plt1 = @vlplot(...),
    plots = (
        @vlplot(...),
        @vlplot(...),
        @vlplot(...),
    ),
)

@set obj.plt1.mark = :point
@set obj.plots[2].mark = :line
@set last(obj.plots).mark = :line

should work now.

[davidanthoff]

Ok, that is pretty amazing!