Add operator kwarg to Cols

[bkamins]

x-ref https://github.com/JuliaData/DataFrames.jl/pull/3224

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[bkamins]

CC @jar @pdeffebach @yjunechoe

[adienes]

I think we discussed this in the previous issue but I forgot---could you remind me why we use keywords here rather than a function? Then it would have Cols(...; operation=any) and Cols(...; operation=all) using built in functions any and all, and could use user-defined predicates.

[bkamins]

We have not discussed it previously. Actually what you propose is a good idea. But any and all are not correct functions to be passed. We can say that union function is the default and intersect is a function to pass if one wants intersection. However, potentially user could pass any function operating on sets and returning a set as a result.

Then the definition in DataFrames.jl would be:

@inline Base.getindex(x::AbstractIndex, idx::Cols) =
    isempty(idx.cols) ? Int[] : idx.operation(getindex.(Ref(x), idx.cols)...)

It would be more error prone than the current implementation (potentially leading to crazy error stack traces) but more flexible indeed. @nalimilan - what do you think?

[adienes]

Makes sense. Then we'd also get setdiff and symdiff for free

[jariji]

Yep, I agree with adienes.

[nalimilan]

Yeah why not. Maybe we need a more specific name than "operation" then for the argument. Again the same discussion as the one about "combine" in unstack? :-)

[bkamins]

AFAICT this fits well. In mathematics union and intersect etc. are typically called "operations", c.f. e.g. https://en.wikipedia.org/wiki/Set_(mathematics)#Basic_operations

[bkamins]

I have updated the PR.

[adienes]

Perhaps operator is very slightly a better choice than operation for some very tenuous reasons:

1. often the arguments in Cols will be indicator/characteristic functions of sets, and a function that acts on other functions is often called an operator
2. to me it scans slightly more clearly that what follows operator= should be a verb (i.e. a function)
3. it is 1 character shorter :)

The wikipedia page for the Algebra of Sets uses both terms interchangeably, so I'm not sure which is more standard.

[bkamins]

I checked several references, and the border between operation and operator is thin. Operation (mathematics) gives a most clear distinction:

An operator is similar to an operation in that it refers to the symbol or the process used to denote the operation, hence their point of view is different. For instance, one often speaks of "the operation of addition" or "the addition operation", when focusing on the operands and result, but one switches to "addition operator" (rarely "operator of addition"), when focusing on the process

Given this explanation indeed operator seems better, but I am not sure.

[adienes]

especially when the symbols ∩ and ∪ are used instead of intersect and union then operator definitely seems like the better word there

[bkamins]

changed

[bkamins]

Thank you!