用户指南中有一些为apply函数编写自定义函数的示例，如下所示：https://pola-rs.github.io/polars/user-guide/expressions/user-defined-functions/
忽略标题暗示它只适用于Python的事实，也有Rust示例。
从该页面：

let out = df
    .lazy()
    .select([
        // pack to struct to get access to multiple fields in a custom `apply/map`
        as_struct(vec![col("keys"), col("values")])
            // we will compute the len(a) + b
            .apply(
                |s| {
                    // downcast to struct
                    let ca = s.struct_()?;

                    // get the fields as Series
                    let s_a = &ca.fields()[0];
                    let s_b = &ca.fields()[1];

                    // downcast the `Series` to their known type
                    let ca_a = s_a.utf8()?;
                    let ca_b = s_b.i32()?;

                    // iterate both `ChunkedArrays`
                    let out: Int32Chunked = ca_a
                        .into_iter()
                        .zip(ca_b)
                        .map(|(opt_a, opt_b)| match (opt_a, opt_b) {
                            (Some(a), Some(b)) => Some(a.len() as i32 + b),
                            _ => None,
                        })
                        .collect();

                    Ok(Some(out.into_series()))
                },
                GetOutput::from_type(DataType::Int32),
            )
            .alias("solution_apply"),
        (col("keys").str().count_matches(lit("."), true) + col("values"))
            .alias("solution_expr"),
    ])
    .collect()?;
println!("{}", out);

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