polars: add new command polars over (#15551)

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# Description
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Introducing a basic implementation of the polars expression for window
functions: `over`
(https://docs.pola.rs/api/python/stable/reference/expressions/api/polars.Expr.over.html).
Note that this PR only implements the default values for the sorting and
`mapping_strategy` parameters. Implementations for other values for
these parameters may be added in a future PR, as the demand arises.

```nushell
 # Compute expression over an aggregation window
  > [[a b]; [x 2] [x 4] [y 6] [y 4]]
        | polars into-lazy
        | polars select a (polars col b | polars cumulative sum | polars over a | polars as cum_b)
        | polars collect
  ╭───┬───┬───────╮
  │ # │ a │ cum_b │
  ├───┼───┼───────┤
  │ 0 │ x │     2 │
  │ 1 │ x │     6 │
  │ 2 │ y │     6 │
  │ 3 │ y │    10 │
  ╰───┴───┴───────╯

# Compute expression over an aggregation window where partitions are defined by expressions
  > [[a b]; [x 2] [X 4] [Y 6] [y 4]]
        | polars into-lazy
        | polars select a (polars col b | polars cumulative sum | polars over (polars col a | polars lowercase) | polars as cum_b)
        | polars collect
  ╭───┬───┬───────╮
  │ # │ a │ cum_b │
  ├───┼───┼───────┤
  │ 0 │ x │     2 │
  │ 1 │ X │     6 │
  │ 2 │ Y │     6 │
  │ 3 │ y │    10 │
  ╰───┴───┴───────╯
```
 
# User-Facing Changes
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helps us keep track of breaking changes. -->
No breaking changes. This PR seeks to add a new command only.

# Tests + Formatting
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Example tests are included.

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crates/nu_plugin_polars/src/dataframe/command/aggregation/mod.rs

@@ -10,6 +10,7 @@ mod median;
 mod min;
 mod n_null;
 mod n_unique;
+mod over;
 mod quantile;
 mod rolling;
 mod std;
@@ -30,6 +31,7 @@ use mean::ExprMean;
 use min::ExprMin;
 pub use n_null::NNull;
 pub use n_unique::NUnique;
+pub use over::Over;
 pub use rolling::Rolling;
 use std::ExprStd;
 pub use sum::ExprSum;
@@ -52,6 +54,7 @@ pub(crate) fn aggregation_commands() -> Vec<Box<dyn PluginCommand<Plugin = Polar
         Box::new(median::LazyMedian),
         Box::new(quantile::LazyQuantile),
         Box::new(groupby::ToLazyGroupBy),
+        Box::new(Over),
         Box::new(Rolling),
         Box::new(ValueCount),
         Box::new(NNull),

crates/nu_plugin_polars/src/dataframe/command/aggregation/over.rs

@@ -0,0 +1,119 @@
+use crate::{
+    dataframe::values::{NuDataFrame, NuExpression},
+    values::{cant_convert_err, CustomValueSupport, PolarsPluginObject, PolarsPluginType},
+    PolarsPlugin,
+};
+use nu_plugin::{EngineInterface, EvaluatedCall, PluginCommand};
+use nu_protocol::{
+    Category, Example, LabeledError, PipelineData, Signature, Span, SyntaxShape, Type, Value,
+};
+use polars::df;
+
+#[derive(Clone)]
+pub struct Over;
+
+impl PluginCommand for Over {
+    type Plugin = PolarsPlugin;
+
+    fn name(&self) -> &str {
+        "polars over"
+    }
+
+    fn description(&self) -> &str {
+        "Compute expressions over a window group defined by partition expressions."
+    }
+
+    fn signature(&self) -> Signature {
+        Signature::build(self.name())
+            .rest(
+                "partition by expressions",
+                SyntaxShape::Any,
+                "Expression(s) that define the partition window",
+            )
+            .input_output_type(
+                Type::Custom("expression".into()),
+                Type::Custom("expression".into()),
+            )
+            .category(Category::Custom("lazyframe".into()))
+    }
+
+    fn examples(&self) -> Vec<Example> {
+        vec![
+            Example {
+                description: "Compute expression over an aggregation window",
+                example: r#"[[a b]; [x 2] [x 4] [y 6] [y 4]]
+        | polars into-lazy
+        | polars select a (polars col b | polars cumulative sum | polars over a | polars as cum_b)
+        | polars collect"#,
+                result: Some(
+                    NuDataFrame::from(
+                        df!(
+                            "a" => &["x", "x", "y", "y"],
+                            "cum_b" => &[2, 6, 6, 10]
+                        )
+                        .expect("should not fail"),
+                    )
+                    .into_value(Span::test_data()),
+                ),
+            },
+            Example {
+                description: "Compute expression over an aggregation window where partitions are defined by expressions",
+                example: r#"[[a b]; [x 2] [X 4] [Y 6] [y 4]]
+        | polars into-lazy
+        | polars select a (polars col b | polars cumulative sum | polars over (polars col a | polars lowercase) | polars as cum_b)
+        | polars collect"#,
+                result: Some(
+                    NuDataFrame::from(
+                        df!(
+                            "a" => &["x", "X", "Y", "y"],
+                            "cum_b" => &[2, 6, 6, 10]
+                        )
+                        .expect("should not fail"),
+                    )
+                    .into_value(Span::test_data()),
+                ),
+            },
+        ]
+    }
+
+    fn run(
+        &self,
+        plugin: &Self::Plugin,
+        engine: &EngineInterface,
+        call: &EvaluatedCall,
+        input: PipelineData,
+    ) -> Result<PipelineData, LabeledError> {
+        let vals: Vec<Value> = call.rest(0)?;
+        let expr_value = Value::list(vals, call.head);
+        let expressions = NuExpression::extract_exprs(plugin, expr_value)?;
+
+        let input_value = input.into_value(call.head)?;
+
+        match PolarsPluginObject::try_from_value(plugin, &input_value)? {
+            PolarsPluginObject::NuExpression(expr) => {
+                let expr: NuExpression = expr
+                    .into_polars()
+                    .over_with_options(expressions, None, Default::default())
+                    .into();
+                expr.to_pipeline_data(plugin, engine, call.head)
+            }
+            _ => Err(cant_convert_err(
+                &input_value,
+                &[PolarsPluginType::NuExpression],
+            )),
+        }
+        .map_err(LabeledError::from)
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+    use crate::test::test_polars_plugin_command;
+    use nu_protocol::ShellError;
+
+    #[test]
+    fn test_examples() -> Result<(), ShellError> {
+        test_polars_plugin_command(&Over)
+    }
+}