polars first and polars last will now handle lazy frames natively (#13555)

# Description
Prior this pull request `polars first` and `polars last` would collect a
lazy frame into an eager frame before performing operations. Now `polars
first` will to a `LazyFrame::limit` and `polars last` will perform a
`LazyFrame::tail`. This is really useful in working with very large
datasets.

crates/nu_plugin_polars/src/dataframe/eager/first.rs

@@ -1,5 +1,5 @@
 use crate::{
-    values::{Column, CustomValueSupport, NuLazyFrame},
+    values::{Column, CustomValueSupport, NuLazyFrame, PolarsPluginObject},
     PolarsPlugin,
 };
 
@@ -98,10 +98,14 @@ impl PluginCommand for FirstDF {
         input: PipelineData,
     ) -> Result<PipelineData, LabeledError> {
         let value = input.into_value(call.head)?;
-        if NuDataFrame::can_downcast(&value) || NuLazyFrame::can_downcast(&value) {
+        match PolarsPluginObject::try_from_value(plugin, &value)? {
-            let df = NuDataFrame::try_from_value_coerce(plugin, &value, call.head)?;
+            PolarsPluginObject::NuDataFrame(df) => {
-            command(plugin, engine, call, df).map_err(|e| e.into())
+                command_eager(plugin, engine, call, df).map_err(|e| e.into())
-        } else {
+            }
+            PolarsPluginObject::NuLazyFrame(lazy) => {
+                command_lazy(plugin, engine, call, lazy).map_err(|e| e.into())
+            }
+            _ => {
                 let expr = NuExpression::try_from_value(plugin, &value)?;
                 let expr: NuExpression = expr.into_polars().first().into();
 
@@ -109,9 +113,10 @@ impl PluginCommand for FirstDF {
                     .map_err(LabeledError::from)
             }
         }
+    }
 }
 
-fn command(
+fn command_eager(
     plugin: &PolarsPlugin,
     engine: &EngineInterface,
     call: &EvaluatedCall,
@@ -126,6 +131,19 @@ fn command(
     res.to_pipeline_data(plugin, engine, call.head)
 }
 
+fn command_lazy(
+    plugin: &PolarsPlugin,
+    engine: &EngineInterface,
+    call: &EvaluatedCall,
+    lazy: NuLazyFrame,
+) -> Result<PipelineData, ShellError> {
+    let rows: Option<u32> = call.opt(0)?;
+    let rows = rows.unwrap_or(1);
+
+    let res: NuLazyFrame = lazy.to_polars().limit(rows).into();
+    res.to_pipeline_data(plugin, engine, call.head)
+}
+
 #[cfg(test)]
 mod test {
     use super::*;

crates/nu_plugin_polars/src/dataframe/eager/last.rs

@@ -1,5 +1,5 @@
 use crate::{
-    values::{Column, CustomValueSupport, NuLazyFrame},
+    values::{Column, CustomValueSupport, NuLazyFrame, PolarsPluginObject},
     PolarsPlugin,
 };
 
@@ -73,10 +73,14 @@ impl PluginCommand for LastDF {
         input: PipelineData,
     ) -> Result<PipelineData, LabeledError> {
         let value = input.into_value(call.head)?;
-        if NuDataFrame::can_downcast(&value) || NuLazyFrame::can_downcast(&value) {
+        match PolarsPluginObject::try_from_value(plugin, &value)? {
-            let df = NuDataFrame::try_from_value_coerce(plugin, &value, call.head)?;
+            PolarsPluginObject::NuDataFrame(df) => {
-            command(plugin, engine, call, df).map_err(|e| e.into())
+                command_eager(plugin, engine, call, df).map_err(|e| e.into())
-        } else {
+            }
+            PolarsPluginObject::NuLazyFrame(lazy) => {
+                command_lazy(plugin, engine, call, lazy).map_err(|e| e.into())
+            }
+            _ => {
                 let expr = NuExpression::try_from_value(plugin, &value)?;
                 let expr: NuExpression = expr.into_polars().last().into();
 
@@ -84,9 +88,10 @@ impl PluginCommand for LastDF {
                     .map_err(LabeledError::from)
             }
         }
+    }
 }
 
-fn command(
+fn command_eager(
     plugin: &PolarsPlugin,
     engine: &EngineInterface,
     call: &EvaluatedCall,
@@ -100,6 +105,20 @@ fn command(
     res.to_pipeline_data(plugin, engine, call.head)
 }
 
+fn command_lazy(
+    plugin: &PolarsPlugin,
+    engine: &EngineInterface,
+    call: &EvaluatedCall,
+    lazy: NuLazyFrame,
+) -> Result<PipelineData, ShellError> {
+    let rows: Option<u32> = call.opt(0)?;
+    let rows = rows.unwrap_or(DEFAULT_ROWS as u32);
+
+    let res: NuLazyFrame = lazy.to_polars().tail(rows).into();
+
+    res.to_pipeline_data(plugin, engine, call.head)
+}
+
 #[cfg(test)]
 mod test {
     use crate::test::test_polars_plugin_command;