refactor: remove RPolarsProtoExprArray and rewrite <Expr>$over()

[etiennebacher]

It's called only 3 times on the R side. I think we now have other mechanisms to replace it.

This PR also introduces mapping_strategy for $over() but it's mostly because it was needed to compile. I'll add tests, examples, and a bullet point in NEWS in a followup PR.

[etiennebacher]

Not very happy about this but I couldn't find an alternative that works with all our tests.

[eitsupi]

I rewrote this in 88c2f46
How about that?

R side:

  list_of_exprs = list2(...) |>
    lapply(\(x) {
      if (is.character(x)) {
        as.list(x)
      } else {
        x
      }
    }) |>
    unlist(recursive = FALSE) |>
    lapply(\(x) {
      if (is.character(x)) {
        pl$col(x)
      } else {
        x
      }
    })

Rust side:

robj_to!(Vec, PLExpr, list_of_exprs)?

It is similar to pl$col(), but differs in that it converts vectors of length 2 or greater to [col("a"), col("b")], whereas pl$col() converts them as cols(["a", "b"]).

This would be the equivalent of parse_as_list_of_expressions in Python, so it would be worth rewriting the following sections and others using this.

r-polars/R/functions__lazy.R

Lines 592 to 595 in 0c3ce3c

	pl_concat_list = function(exprs) {
	concat_list(as.list(exprs)) |>
	unwrap(" in pl$concat_list():")
	}

[etiennebacher]

How about that?

This seems good since it passes tests but it departs a bit from the python version which has expr + .... This was the reason why I had implemented it like that:

  expr = c(
    wrap_elist_result(expr, str_to_lit = FALSE) |>
      unwrap("in $over():"),
    list2(...)
  )

Do you think that's a problem?

[eitsupi]

It is similar to pl$col() before rewriting it in #923.
In R, I think it is more natural to handle it in variable length arguments ....

[etiennebacher]

It is similar to pl$col() before rewriting it in #923.

Indeed.

Thanks for completing this PR!

[eitsupi]

Good catch, thanks!