I have a hard time with what should be a very common use case on polars dataframes.
I simply want to create a new column on an existing dataframe based on some other column.
Here is the code I try, but doesn't work:

```
import polars as pl
df.with_columns([
                (pl.col('old_col').apply(lambda x: func(x)).alias("new_col"),
            ])

```