Can any one provide example related to polars as_struct().apply()

Question

I try to add array column to the existing data-frame. input is something like this

| 1 | 3 |
| 2 | 4 |

and output is

| 1 | 3 | [?, ?, ?, ?] |
| 2 | 4 | [?, ?, ?, ?] |

value of the array will be populated by some custom function.

I try to implement something like this.

let df = df![
  "a" => [1, 2],
  "b" => [3, 4]
]?;
        
let lf: LazyFrame = df.lazy().select(
  as_struct(&[col("a"), col("b")]).apply(
    somefn,
    GetOutput::from_type(DataType::List(Box::new(DataType::Float32))),
));

I don't know how to implement this somefn.