map_elements cause R session crach for struct object

[Yunuuuu]

map_elements crash for struct object, Here is the map_elements document for struct in Python:
https://docs.pola.rs/user-guide/expressions/user-defined-functions/#combining-multiple-column-values

library(polars)
pl$DataFrame(mtcars)$
    select(pl$col("mpg", "cyl")$to_struct()$map_elements(\(x) x$mpg))

pl$DataFrame(mtcars)$
    select(pl$col("mpg", "cyl")$to_struct()$map_elements(\(x) x[["mpg"]]))

pl$DataFrame(mtcars)$
    select(
    pl$col("mpg", "cyl")$to_struct()$
        map_elements(\(x) x$struct$field("mpg"))
)

[eitsupi]

Struct type is not supported yet.

r-polars/src/rust/src/series.rs

Lines 363 to 390 in acc29eb

	let r_iter: Box<dyn Iterator<Item = Option<Robj>>> = match inp_type {
	Float64 => apply_input!(self.0, f64, rfun, na_fun),
	Float32 => apply_input!(self.0, f32, rfun, na_fun),
	Int64 => apply_input!(self.0, i64, rfun, na_fun),
	Int32 => apply_input!(self.0, i32, rfun, na_fun),
	Int16 => apply_input!(self.0, i16, rfun, na_fun),
	Int8 => apply_input!(self.0, i8, rfun, na_fun),
	String => apply_input!(self.0, str, rfun, na_fun),
	Boolean => apply_input!(self.0, bool, rfun, na_fun),
	//List(..) => apply_input!(self.0, list, rfun, na_fun),
	List(..) => {
	let ca_list = self.0.list().unwrap();
	let y = ca_list.into_iter().map(|opt_ser| {
	if let Some(ser) = opt_ser {
	rfun.call(pairlist!(RPolarsSeries(ser))).ok()
	} else {
	unreachable!("internal error: oh it was possible to get a None Series");
	}
	});
	Box::new(y)
	}
	x => {
	dbg!(x);
	todo!("this input type is not implemented")
	}
	};

PRs are welcome!

[Yunuuuu]

I'll try to implement it.

[etiennebacher]

Shorter reprex:

s = pl$Series(values = 1)$to_struct()
s$map_elements(\(x) x)

Error in .pr$Series$map_elements(self, fun, datatype, strict_return_type,  : 
  user function panicked: map_elements

[etiennebacher]

Turns out to_struct() shouldn't be available for all Expr, only for those in the list and arr subnamespaces: https://docs.pola.rs/py-polars/html/search.html?q=to_struct

---

Doesn't solve the issue though:

library(polars)

pl$DataFrame(mtcars)$
  select(pl$struct(c("mpg", "cyl"))$map_elements(\(x) x[["mpg"]]))
#>    1: user function raised an error: EvalError(lang!(function (s) {    s$map_elements(f, return_type, strict_return_type, allow_fail_eval)}, ExternalPtr.set_class(["RPolarsSeries"]))