Function element

Alias for an element in evaluated in an eval expression.

Example

*
 *  A horizontal rank computation by taking the elements of a list
 *
 *  >df = pl.DataFrame({"a": [1, 8, 3], "b": [4, 5, 2]})
 *  >df.withColumn(
 *  ...     pl.concatList(["a", "b"]).arr.eval(pl.element().rank()).alias("rank")
 *  ... )
 *  shape: (3, 3)
 *  ┌─────┬─────┬────────────┐
 *  │ a   ┆ b   ┆ rank       │
 *  │ --- ┆ --- ┆ ---        │
 *  │ i64 ┆ i64 ┆ list[f32]  │
 *  ╞═════╪═════╪════════════╡
 *  │ 1   ┆ 4   ┆ [1.0, 2.0] │
 *  ├╌╌╌╌╌┼╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌┤
 *  │ 8   ┆ 5   ┆ [2.0, 1.0] │
 *  ├╌╌╌╌╌┼╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌┤
 *  │ 3   ┆ 2   ┆ [2.0, 1.0] │
 *  └─────┴─────┴────────────┘
 *
 *  A mathematical operation on array elements
 *
 *  >df = pl.DataFrame({"a": [1, 8, 3], "b": [4, 5, 2]})
 *  >df.withColumn(
 *  ...     pl.concatList(["a", "b"]).arr.eval(pl.element().multiplyBy(2)).alias("a_b_doubled")
 *  ... )
 *  shape: (3, 3)
 *  ┌─────┬─────┬─────────────┐
 *  │ a   ┆ b   ┆ a_b_doubled │
 *  │ --- ┆ --- ┆ ---         │
 *  │ i64 ┆ i64 ┆ list[i64]   │
 *  ╞═════╪═════╪═════════════╡
 *  │ 1   ┆ 4   ┆ [2, 8]      │
 *  ├╌╌╌╌╌┼╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌┤
 *  │ 8   ┆ 5   ┆ [16, 10]    │
 *  ├╌╌╌╌╌┼╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌┤
 *  │ 3   ┆ 2   ┆ [6, 4]      │
 *  └─────┴─────┴─────────────┘
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