Map and apply function to perform custom operations

[tcpessoa]

Ability to perform custom operations by using the map and apply functions

Feature demonstration with expected result

I would like to have the ability to perform a custom operation on a Series/Expr as such:

import pl from 'nodejs-polars';

const data = [
	{ a: 'A', b: 10},
	{ a: 'B', b: 20},
	{ a: 'B', b: 13},
	{ a: 'C', b: 40},
];

const mapping = { D: 'OtherD'};

const df = pl.DataFrame(data);
const dfWithOtherA = df.withColumns([pl.col("a").map((val) => mapping[val]).alias('otherA')]);

/* expected result:
  ┌─────┬──────┬────────┐
  │ a   ┆ b    ┆ otherA │
  │ --- ┆ ---  ┆ ---    │
  │ str ┆ f64  ┆ str    │
  ╞═════╪══════╪════════╡
  │ A   ┆ 10.0 ┆ A      │
  ├╌╌╌╌╌┼╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌┤
  │ B   ┆ 20.0 ┆ OtherB │
  ├╌╌╌╌╌┼╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌┤
  │ B   ┆ 13.0 ┆ OtherB │
  ├╌╌╌╌╌┼╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌┤
  │ C   ┆ 40.0 ┆ C      │
  └─────┴──────┴────────┘
*/

The expected behavior should follow what is described in the polars docs.

Other comments

I would be happy to help with some guidance, but I still am not aware of polars internals to go for a PR.
Thank you for the awesome work in all other APIs and functions, keep the great work! :)

[ChakshuGautam]

I would love to help as well!!

[universalmind303]

so this is actually one of the more challenging ones to implement. It requires extensive knowledge of the n-api framework, as well as the rust polars internals. If anyone is up for a challenge, you can likely take a look at the python implementation to get a general idea of what they are doing.

Keep in mind, Pyo3 is a bit more flexible in what you can do than n-api is, so it will be a bit more challenging than the python implementation.

[ritchie46]

You could go anyvalue only. That's a bit slower, but much easier to implement.