polars: expand polars col to handle multiple columns and by types (#15570)

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# Description
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This PR seeks to expand `polars col` functionality to allow selecting
multiple columns and columns by type, which is particularly useful when
piping to subsequent expressions that should be applied to each column
selected (e.g., `polars col int --type | polars sum` as a shorthand for
`[(polars col a | polars sum), (polars col b | polars sum)]`). See
examples below.

```nushell
#  Select multiple columns (cannot be used with asterisk wildcard)
  > [[a b c]; [x 1 1.1] [y 2 2.2] [z 3 3.3]] | polars into-df 
          | polars select (polars col b c | polars sum) | polars collect
  ╭───┬───┬──────╮
  │ # │ b │  c   │
  ├───┼───┼──────┤
  │ 0 │ 6 │ 6.60 │
  ╰───┴───┴──────╯

#  Select multiple columns by types (cannot be used with asterisk wildcard)
  > [[a b c]; [x o 1.1] [y p 2.2] [z q 3.3]] | polars into-df 
           | polars select (polars col str f64 --type | polars max) | polars collect
  ╭───┬───┬───┬──────╮
  │ # │ a │ b │  c   │
  ├───┼───┼───┼──────┤
  │ 0 │ z │ q │ 3.30 │
  ╰───┴───┴───┴──────╯
```

# User-Facing Changes
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helps us keep track of breaking changes. -->
No breaking changes. Users have the additional capability to select
multiple columns in `polars col`.

# Tests + Formatting
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Examples have been added to `polars col`.

# After Submitting
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crates/nu_plugin_polars/src/dataframe/command/data/col.rs

@@ -1,14 +1,14 @@
 use crate::{
     dataframe::values::NuExpression,
-    values::{Column, CustomValueSupport, NuDataFrame},
+    values::{str_to_dtype, Column, CustomValueSupport, NuDataFrame},
     PolarsPlugin,
 };
 use nu_plugin::{EngineInterface, EvaluatedCall, PluginCommand};
 use nu_protocol::{
-    record, Category, Example, LabeledError, PipelineData, Signature, Span, SyntaxShape, Type,
+    record, Category, Example, LabeledError, PipelineData, ShellError, Signature, Span,
-    Value,
+    SyntaxShape, Type, Value,
 };
-use polars::prelude::col;
+use polars::prelude::DataType;
 
 #[derive(Clone)]
 pub struct ExprCol;
@@ -31,6 +31,12 @@ impl PluginCommand for ExprCol {
                 SyntaxShape::String,
                 "Name of column to be used. '*' can be used for all columns.",
             )
+            .rest(
+                "more columns",
+                SyntaxShape::String,
+                "Additional columns to be used. Cannot be '*'",
+            )
+            .switch("type", "Treat column names as type names", Some('t'))
             .input_output_type(Type::Any, Type::Custom("expression".into()))
             .category(Category::Custom("expression".into()))
     }
@@ -57,6 +63,31 @@ impl PluginCommand for ExprCol {
                     .into_value(Span::test_data()),
                 ),
             },
+            Example {
+                description: "Select multiple columns (cannot be used with asterisk wildcard)",
+                example: "[[a b c]; [x 1 1.1] [y 2 2.2] [z 3 3.3]] | polars into-df | polars select (polars col b c | polars sum) | polars collect",
+                result: Some(
+                    NuDataFrame::try_from_columns(vec![
+                        Column::new("b".to_string(), vec![Value::test_int(6)]),
+                        Column::new("c".to_string(), vec![Value::test_float(6.6)]),
+                    ],None)
+                    .expect("should not fail")
+                    .into_value(Span::test_data()),
+                ),
+            },
+            Example {
+                description: "Select multiple columns by types (cannot be used with asterisk wildcard)",
+                example: "[[a b c]; [x o 1.1] [y p 2.2] [z q 3.3]] | polars into-df | polars select (polars col str f64 --type | polars max) | polars collect",
+                result: Some(
+                    NuDataFrame::try_from_columns(vec![
+                        Column::new("a".to_string(), vec![Value::test_string("z")]),
+                        Column::new("b".to_string(), vec![Value::test_string("q")]),
+                        Column::new("c".to_string(), vec![Value::test_float(3.3)]),
+                    ],None)
+                    .expect("should not fail")
+                    .into_value(Span::test_data()),
+                ),
+            },
         ]
     }
 
@@ -71,8 +102,27 @@ impl PluginCommand for ExprCol {
         call: &EvaluatedCall,
         _input: PipelineData,
     ) -> Result<PipelineData, LabeledError> {
-        let name: String = call.req(0)?;
+        let mut names: Vec<String> = vec![call.req(0)?];
-        let expr: NuExpression = col(name.as_str()).into();
+        names.extend(call.rest(1)?);
+
+        let as_type = call.has_flag("type")?;
+
+        let expr: NuExpression = match as_type {
+            false => match names.as_slice() {
+                [single] => polars::prelude::col(single).into(),
+                _ => polars::prelude::cols(&names).into(),
+            },
+            true => {
+                let dtypes = names
+                    .iter()
+                    .map(|n| str_to_dtype(n, call.head))
+                    .collect::<Result<Vec<DataType>, ShellError>>()
+                    .map_err(LabeledError::from)?;
+
+                polars::prelude::dtype_cols(dtypes).into()
+            }
+        };
+
         expr.to_pipeline_data(plugin, engine, call.head)
             .map_err(LabeledError::from)
     }