Ability to cast a dataframe's column to a different dtype (#11803)

Provides the ability to cast columns in dataframes, lazy dataframes, and
expressions.

<img width="587" alt="Screenshot 2024-02-14 at 13 53 01"
src="https://github.com/nushell/nushell/assets/56345/b894f746-0e37-472e-9fb0-eb6f71f2bf27">

<img width="616" alt="Screenshot 2024-02-14 at 13 52 37"
src="https://github.com/nushell/nushell/assets/56345/cf10efa7-d89c-4189-ab71-d368b2354d19">

<img width="626" alt="Screenshot 2024-02-14 at 13 54 58"
src="https://github.com/nushell/nushell/assets/56345/cd57cdf0-5096-41dd-8ab5-46e3d1e061b8">

---------

Co-authored-by: Jack Wright <jack.wright@disqo.com>

crates/nu-cmd-dataframe/src/dataframe/eager/cast.rs

@@ -0,0 +1,207 @@
+use crate::dataframe::values::{str_to_dtype, NuExpression, NuLazyFrame};
+
+use super::super::values::NuDataFrame;
+use nu_engine::CallExt;
+use nu_protocol::{
+    ast::Call,
+    engine::{Command, EngineState, Stack},
+    Category, Example, PipelineData, Record, ShellError, Signature, Span, SyntaxShape, Type, Value,
+};
+use polars::prelude::*;
+
+#[derive(Clone)]
+pub struct CastDF;
+
+impl Command for CastDF {
+    fn name(&self) -> &str {
+        "dfr cast"
+    }
+
+    fn usage(&self) -> &str {
+        "Cast a column to a different dtype."
+    }
+
+    fn signature(&self) -> Signature {
+        Signature::build(self.name())
+            .input_output_types(vec![
+                (
+                    Type::Custom("expression".into()),
+                    Type::Custom("expression".into()),
+                ),
+                (
+                    Type::Custom("dataframe".into()),
+                    Type::Custom("dataframe".into()),
+                ),
+            ])
+            .required(
+                "dtype",
+                SyntaxShape::String,
+                "The dtype to cast the column to",
+            )
+            .optional(
+                "column",
+                SyntaxShape::String,
+                "The column to cast. Required when used with a dataframe.",
+            )
+            .category(Category::Custom("dataframe".into()))
+    }
+
+    fn examples(&self) -> Vec<Example> {
+        vec![
+            Example {
+                description: "Cast a column in a dataframe to a different dtype",
+                example: "[[a b]; [1 2] [3 4]] | dfr into-df | dfr cast u8 a | dfr schema",
+                result: Some(Value::record(
+                    Record::from_raw_cols_vals_unchecked(
+                        vec!["a".to_string(), "b".to_string()],
+                        vec![
+                            Value::string("u8", Span::test_data()),
+                            Value::string("i64", Span::test_data()),
+                        ],
+                    ),
+                    Span::test_data(),
+                )),
+            },
+            Example {
+                description: "Cast a column in a lazy dataframe to a different dtype",
+                example: "[[a b]; [1 2] [3 4]] | dfr into-df | dfr into-lazy | dfr cast u8 a | dfr schema",
+                result: Some(Value::record(
+                    Record::from_raw_cols_vals_unchecked(
+                        vec!["a".to_string(), "b".to_string()],
+                        vec![
+                            Value::string("u8", Span::test_data()),
+                            Value::string("i64", Span::test_data()),
+                        ],
+                    ),
+                    Span::test_data(),
+                )),
+            },
+            Example {
+                description: "Cast a column in a expression to a different dtype",
+                example: r#"[[a b]; [1 2] [1 4]] | dfr into-df | dfr group-by a | dfr agg [ (dfr col b | dfr cast u8 | dfr min | dfr as "b_min") ] | dfr schema"#,
+                result: None
+            }
+        ]
+    }
+
+    fn run(
+        &self,
+        engine_state: &EngineState,
+        stack: &mut Stack,
+        call: &Call,
+        input: PipelineData,
+    ) -> Result<PipelineData, ShellError> {
+        let value = input.into_value(call.head);
+        if NuLazyFrame::can_downcast(&value) {
+            let (dtype, column_nm) = df_args(engine_state, stack, call)?;
+            let df = NuLazyFrame::try_from_value(value)?;
+            command_lazy(call, column_nm, dtype, df)
+        } else if NuDataFrame::can_downcast(&value) {
+            let (dtype, column_nm) = df_args(engine_state, stack, call)?;
+            let df = NuDataFrame::try_from_value(value)?;
+            command_eager(call, column_nm, dtype, df)
+        } else {
+            let dtype: String = call.req(engine_state, stack, 0)?;
+            let dtype = str_to_dtype(&dtype, call.head)?;
+
+            let expr = NuExpression::try_from_value(value)?;
+            let expr: NuExpression = expr.into_polars().cast(dtype).into();
+
+            Ok(PipelineData::Value(
+                NuExpression::into_value(expr, call.head),
+                None,
+            ))
+        }
+    }
+}
+
+fn df_args(
+    engine_state: &EngineState,
+    stack: &mut Stack,
+    call: &Call,
+) -> Result<(DataType, String), ShellError> {
+    let dtype = dtype_arg(engine_state, stack, call)?;
+    let column_nm: String =
+        call.opt(engine_state, stack, 1)?
+            .ok_or(ShellError::MissingParameter {
+                param_name: "column_name".into(),
+                span: call.head,
+            })?;
+    Ok((dtype, column_nm))
+}
+
+fn dtype_arg(
+    engine_state: &EngineState,
+    stack: &mut Stack,
+    call: &Call,
+) -> Result<DataType, ShellError> {
+    let dtype: String = call.req(engine_state, stack, 0)?;
+    str_to_dtype(&dtype, call.head)
+}
+
+fn command_lazy(
+    call: &Call,
+    column_nm: String,
+    dtype: DataType,
+    lazy: NuLazyFrame,
+) -> Result<PipelineData, ShellError> {
+    let column = col(&column_nm).cast(dtype);
+    let lazy = lazy.into_polars().with_columns(&[column]);
+    let lazy = NuLazyFrame::new(false, lazy);
+
+    Ok(PipelineData::Value(
+        NuLazyFrame::into_value(lazy, call.head)?,
+        None,
+    ))
+}
+
+fn command_eager(
+    call: &Call,
+    column_nm: String,
+    dtype: DataType,
+    nu_df: NuDataFrame,
+) -> Result<PipelineData, ShellError> {
+    let mut df = nu_df.df;
+    let column = df
+        .column(&column_nm)
+        .map_err(|e| ShellError::GenericError {
+            error: format!("{e}"),
+            msg: "".into(),
+            span: Some(call.head),
+            help: None,
+            inner: vec![],
+        })?;
+
+    let casted = column.cast(&dtype).map_err(|e| ShellError::GenericError {
+        error: format!("{e}"),
+        msg: "".into(),
+        span: Some(call.head),
+        help: None,
+        inner: vec![],
+    })?;
+
+    let _ = df
+        .with_column(casted)
+        .map_err(|e| ShellError::GenericError {
+            error: format!("{e}"),
+            msg: "".into(),
+            span: Some(call.head),
+            help: None,
+            inner: vec![],
+        })?;
+
+    let df = NuDataFrame::new(false, df);
+    Ok(PipelineData::Value(df.into_value(call.head), None))
+}
+
+#[cfg(test)]
+mod test {
+
+    use super::super::super::test_dataframe::test_dataframe;
+    use super::*;
+
+    #[test]
+    fn test_examples() {
+        test_dataframe(vec![Box::new(CastDF {})])
+    }
+}

crates/nu-cmd-dataframe/src/dataframe/eager/mod.rs

@@ -1,4 +1,5 @@
 mod append;
+mod cast;
 mod columns;
 mod drop;
 mod drop_duplicates;
@@ -35,6 +36,7 @@ use nu_protocol::engine::StateWorkingSet;
 
 pub use self::open::OpenDataFrame;
 pub use append::AppendDF;
+pub use cast::CastDF;
 pub use columns::ColumnsDF;
 pub use drop::DropDF;
 pub use drop_duplicates::DropDuplicates;
@@ -78,6 +80,7 @@ pub fn add_eager_decls(working_set: &mut StateWorkingSet) {
     // Dataframe commands
     bind_command!(
         AppendDF,
+        CastDF,
         ColumnsDF,
         DataTypes,
         Summary,

crates/nu-cmd-dataframe/src/dataframe/test_dataframe.rs

@@ -5,7 +5,7 @@ use nu_protocol::{
     Example, PipelineData, Span,
 };
 
-use super::eager::ToDataFrame;
+use super::eager::{SchemaDF, ToDataFrame};
 use super::expressions::ExprCol;
 use super::lazy::{LazyCollect, ToLazyFrame};
 use nu_cmd_lang::Let;
@@ -36,6 +36,7 @@ pub fn build_test_engine_state(cmds: Vec<Box<dyn Command + 'static>>) -> Box<Eng
         working_set.add_decl(Box::new(ToLazyFrame));
         working_set.add_decl(Box::new(LazyCollect));
         working_set.add_decl(Box::new(ExprCol));
+        working_set.add_decl(Box::new(SchemaDF));
 
         // Adding the command that is being tested to the working set
         for cmd in cmds.clone() {

crates/nu-cmd-dataframe/src/dataframe/values/mod.rs

@@ -10,5 +10,5 @@ pub use nu_dataframe::{Axis, Column, NuDataFrame};
 pub use nu_expression::NuExpression;
 pub use nu_lazyframe::NuLazyFrame;
 pub use nu_lazygroupby::NuLazyGroupBy;
-pub use nu_schema::NuSchema;
+pub use nu_schema::{str_to_dtype, NuSchema};
 pub use nu_when::NuWhen;