Add across etc. for easier working with multiple columns

[pdeffebach]

As referenced on discourse, lots of users have trouble with complicated source => fun => dest pairs. Maybe something like R's across might be useful.

It could live here. It could also live in a 3rd package that we re-export, like Chain.jl.

[bkamins]

The question is if the problem is that we should add across or instead of using broadcasting (which is nice, clean, but maybe too magical) start promoting using a comprehension instead?

The only challenge is what while Not(:x) .=> fun will soon work [n => fun for n in Not(:x)] is impossible to be made working, so maybe indeed @across within DataFramesMeta.jl that would be data-frame context sensitive (rewriting Not(:x) etc. as appropriate) would make sense?

[nalimilan]

As I said at JuliaData/DataFrames.jl#2870 I'd rather wait a bit after Between, All, Cols and Not support .=> before deciding whether adding across would really make people's life easier. I'm concerned that people find across natural simply because they are used to it.

[pdeffebach]

I agree. I think we can make a lot of progress helping people ease into src => fun => dest. If that approach remains too hard after tutorials / community knowledge etc, then we can consider across.

[xiaodaigh]

using DataFrames, DataFrameMacros


df = DataFrame(a = 1:3)

function return2(a)
  (a = a/2, b = a/3)
end
transform(df, :a =>  return2 => [:c, :d])

The above return2 function returns 2 columns which I'd like to assign to two new columns. I don't it's possible currently with this package.

Wish we can do something there.