Function polars_lazy::dsl::apply_mul

pub fn apply_mul<F, E>(function: F, expr: E, output_type: GetOutput) -> Expr where
    F: Fn(&mut [Series]) -> Result<Series> + 'static + Send + Sync,
    E: AsRef<[Expr]>, 

This is supported on crate feature compile only.

Apply a function/closure over the groups of multiple columns. This should only be used in a groupby aggregation.

It is the responsibility of the caller that the schema is correct by giving the correct output_type. If None given the output type of the input expr is used.

This difference with [map_mul] is that [apply_mul] will create a separate [Series] per group.

• [map_mul] should be used for operations that are independent of groups, e.g. multiply * 2, or raise to the power
• [apply_mul] should be used for operations that work on a group of data. e.g. sum, count, etc.