bug: The polarsArray Join operation does not behave like documented

[GLeurquin]

What happened?

The polars ArrayJoin operation does not behave like documented

When the array is empty, the expected value is NULL, but the result is an empty string. (ID 3 below).

ibis.set_backend("polars")
table = ibis.memtable(
    [
        {"id": 1, "arr": ["a", "b", "c"], "expected": "a|b|c"},
        {"id": 2, "arr": None, "expected": None},
        {"id": 3, "arr": [], "expected": None},
        {"id": 4, "arr": ["b", None], "expected": "b"},
    ]
)

table.mutate(
    result=_["arr"].join("|"),
).preview()

┏━━━━━━━┳━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━┳━━━━━━━━┓
┃ id    ┃ arr                ┃ expected ┃ result ┃
┡━━━━━━━╇━━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━╇━━━━━━━━┩
│ int64 │ array<string>      │ string   │ string │
├───────┼────────────────────┼──────────┼────────┤
│     1 │ ['a', 'b', ... +1] │ a|b|c    │ a|b|c  │
│     2 │ NULL               │ NULL     │ NULL   │
│     3 │ []                 │ NULL     │ ~      │ <-- issue here
│     4 │ ['b', None]        │ b        │ b      │
└───────┴────────────────────┴──────────┴────────┘

Proposed change for code:

@translate.register(ops.ArrayStringJoin)
def array_string_join(op, **kw):
    arg = translate(op.arg, **kw)
    sep = _literal_value(op.sep)
    return (
        pl.when(arg.list.len() > 0)
        .then(arg.list.join(sep))
        .otherwise(None)
    )

What version of ibis are you using?

10.1.0

What backend(s) are you using, if any?

polars

Relevant log output

Code of Conduct

• I agree to follow this project's Code of Conduct

[cpcloud]

Thanks for the issue, seems like a reasonable change in implementation!

[cpcloud]

@GLeurquin Are you interested in making a PR with that change?

[GLeurquin]

Here is a PR: #10913

However I don't quite understand how to add the test example to the tests to avoid regression in the future, if you can guide me to where I should add it I can modify the PR