Advanced transformation examples

[bkamins]

Fixes #3430

[bkamins]

@nalimilan - what do you think (CI errors are unrelated)?

[nalimilan]

Suggested change

	The reason why this is needed is that instead `combine` iterates the contents of the value returned
	by the operation specification function and tries to expand it, which in our case is a tuple of numbers,
	so one gets an error:
	Without `Ref`, `combine` iterates the contents of the value returned by the operation specification function, which in our case is a tuple of numbers, and tries to expand it, so one gets an error:

[nalimilan]

Can you remind me why this isn't allowed? AFAICT it would be possible to destructure the tuple and use its values to fill the columns? BTW, is "expand" the right word here?

[bkamins]

The reason is related to the issue we recently discussed with the Tables.jl conflict against Julia 1.11. DataFrames.jl does not recognize Tuple as a valid input so it sends it to Tables.jl columntable function. So essentially - the error is because we fall-back to Tables.jl in cases that we do not explicitly handle as special cases (like vectors, matrices, DataFrames etc.).

[bkamins]

Your suggestion is applied in the commit I pushed (I just had to re-word it a bit).

[nalimilan]

OK, thanks. But then, when combine iterates, it expects each value to represent a column, not a row, and my suggestion is incorrect, right?

[bkamins]

I just have written something more precise as your suggestion was not precise enough (I think). What is done is I think best visible here:

julia> df = DataFrame(x=1:2)
2×1 DataFrame
 Row │ x
     │ Int64
─────┼───────
   1 │     1
   2 │     2

julia> combine(df, :x => (x -> ((a=x,), (a=2x,), (a=3x,))) => AsTable)
3×1 DataFrame
 Row │ a
     │ Array…
─────┼────────
   1 │ [1, 2]
   2 │ [2, 4]
   3 │ [3, 6]

The returned tuple ((a=x,), (a=2x,), (a=3x,)) has three elements so it produces three rows. Things work, because each element is a NamedTuple which provides column names that can be used to generate column names with AsTable.

Is it clearer now?

[bkamins]

@nalimilan - I applied all your suggestions and expanded the examples even more to be more explicit.

[nalimilan]

OK, thanks. But then, when combine iterates, it expects each value to represent a column, not a row, and my suggestion is incorrect, right?

[bkamins]

@nalimilan - can you please have a look at it. If it is OK we could merge it. Thank you!

[bkamins]

Thank you!