Fix restack for struct with undef fields

[maartenvd]

this is the kind of 'fix' I had in mind, but it's not very pretty

close #497

[tkf]

Thanks for giving a shot at it! Yeah, we may need something like this. I'm a bit worried that it stresses the compiler too much (esp. for a large struct), though.

We can also check if all fields are isdefined and just return the input as-is if not. It may miss some optimization opportunities but it could be a better solution overall.

[codecov]

Codecov Report

Merging #498 (8014fcd) into master (1e5e530) will decrease coverage by 0.89%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #498      +/-   ##
==========================================
- Coverage   93.40%   92.50%   -0.90%     
==========================================
  Files          32       32              
  Lines        2214     2215       +1     
==========================================
- Hits         2068     2049      -19     
- Misses        146      166      +20     

Flag	Coverage Δ
Pkg.test	92.66% <100.00%> (+0.01%)	⬆️
Run.test	92.37% <100.00%> (-0.90%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/AutoObjectsReStacker.jl	95.65% <100.00%> (+0.65%)	⬆️
src/reduce.jl	72.56% <0.00%> (-9.89%)	⬇️
src/basics.jl	88.88% <0.00%> (-5.56%)	⬇️
src/partitionby.jl	90.90% <0.00%> (-3.90%)	⬇️
src/core.jl	89.62% <0.00%> (-0.95%)	⬇️
src/progress.jl	91.58% <0.00%> (-0.94%)	⬇️
src/library.jl	95.81% <0.00%> (+0.25%)	⬆️

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[maartenvd]

I think I only have to check the last field, which is why I made that the outer loop in the original pull request. This version is now just the outer if, where I return x if it contains uninitialized fields

[maartenvd]

@tkf - should I change it to check every field, or are you fine with just checking the last field - which should also tell us if it's a fully or partially initialized struct?

[tkf]

just checking the last field

Ah, good point! I think this works.

Can you add a test in https://github.com/JuliaFolds/Transducers.jl/blob/master/test/test_restack.jl ? I think it's good to go then.

[maartenvd]

something like this? I want to use transducers in my own package, I'm getting promising results so far :)

[tkf]

Thanks! LGTM!

Good to know that it works in your package :)