Description

Currently, trying to use repeat_by with a list dtype results in the following error:

InvalidOperationError: `repeat_by` operation not supported for dtype `list[i64]`

MWE:

import polars as pl

df = pl.DataFrame({
    "a": [[1], [2, 3], [4, 5, 6]],
    "n": [0, 3, 1],
})
df.select(pl.col("a").repeat_by("n"))

Conceptually, I see no issue that this cannot be supported, lists can be repeated similarly to e.g. strings.

Happy to open a PR for this but I might need some guidance where to do this...

• I found crates/polars-ops/src/chunked_array/repeat_by.rs where List needs to be allowed as another dtype
• It seems like one should extend ListFromIter in crates/polars-arrow/src/legacy/array/mod.rs (e.g. with a from_iter_list_trusted_len method)?
• Once I'm there, I can't really figure out how to turn an iterator into a list again; MutableListArray seems to require concrete types and this is impossible as lists can be arbitrarily nested

That being said, the functionality to do this should already be in polars. group_by.agg also allows to chain arbitrary lists.