Lazy dataframes (#5687)

* change between lazy and eager

* when expressions

* examples for aggregations

* more examples for agg

* examples for dataframes

* checked examples

* cargo fmt

crates/nu-command/src/dataframe/expressions/as_nu.rs

@@ -0,0 +1,70 @@
+use super::super::values::NuExpression;
+
+use nu_protocol::{
+    ast::Call,
+    engine::{Command, EngineState, Stack},
+    Category, Example, PipelineData, ShellError, Signature, Span, Value,
+};
+
+#[derive(Clone)]
+pub struct ExprAsNu;
+
+impl Command for ExprAsNu {
+    fn name(&self) -> &str {
+        "dfr as-nu"
+    }
+
+    fn usage(&self) -> &str {
+        "Convert expression to a nu value for access and exploration"
+    }
+
+    fn signature(&self) -> Signature {
+        Signature::build(self.name()).category(Category::Custom("expression".into()))
+    }
+
+    fn examples(&self) -> Vec<Example> {
+        vec![Example {
+            description: "Convert a col expression into a nushell value",
+            example: "dfr col col_a | dfr as-nu",
+            result: Some(Value::Record {
+                cols: vec!["expr".into(), "value".into()],
+                vals: vec![
+                    Value::String {
+                        val: "column".into(),
+                        span: Span::test_data(),
+                    },
+                    Value::String {
+                        val: "col_a".into(),
+                        span: Span::test_data(),
+                    },
+                ],
+                span: Span::test_data(),
+            }),
+        }]
+    }
+
+    fn run(
+        &self,
+        _engine_state: &EngineState,
+        _stack: &mut Stack,
+        call: &Call,
+        input: PipelineData,
+    ) -> Result<PipelineData, ShellError> {
+        let expr = NuExpression::try_from_pipeline(input, call.head)?;
+        let value = expr.to_value(call.head);
+
+        Ok(PipelineData::Value(value, None))
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::super::super::test_dataframe::test_dataframe;
+    use super::super::ExprCol;
+    use super::*;
+
+    #[test]
+    fn test_examples() {
+        test_dataframe(vec![Box::new(ExprAsNu {}), Box::new(ExprCol {})])
+    }
+}