nushell/crates/nu-command/src/dataframe/eager/first.rs

use super::super::values::{utils::DEFAULT_ROWS, Column, NuDataFrame};
use crate::dataframe::values::NuExpression;
use nu_engine::CallExt;
use nu_protocol::{
    ast::Call,
    engine::{Command, EngineState, Stack},
    Category, Example, PipelineData, ShellError, Signature, Span, SyntaxShape, Value,
};

#[derive(Clone)]
pub struct FirstDF;

impl Command for FirstDF {
    fn name(&self) -> &str {
        "dfr first"
    }

    fn usage(&self) -> &str {
        "Creates new dataframe with first rows or creates a first expression"
    }

    fn signature(&self) -> Signature {
        Signature::build(self.name())
            .optional("rows", SyntaxShape::Int, "Number of rows for head")
            .category(Category::Custom("dataframe".into()))
    }

    fn examples(&self) -> Vec<Example> {
        vec![
            Example {
                description: "Create new dataframe with head rows",
                example: "[[a b]; [1 2] [3 4]] | dfr to-df | dfr first 1",
                result: Some(
                    NuDataFrame::try_from_columns(vec![
                        Column::new("a".to_string(), vec![Value::test_int(1)]),
                        Column::new("b".to_string(), vec![Value::test_int(2)]),
                    ])
                    .expect("simple df for test should not fail")
                    .into_value(Span::test_data()),
                ),
            },
            Example {
                description: "Creates a first expression from a column",
                example: "dfr col a | dfr first",
                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 NuExpression::can_downcast(&value) {
            let expr = NuExpression::try_from_value(value)?;
            let expr: NuExpression = expr.into_polars().is_null().into();

            Ok(PipelineData::Value(
                NuExpression::into_value(expr, call.head),
                None,
            ))
        } else if NuDataFrame::can_downcast(&value) {
            let df = NuDataFrame::try_from_value(value)?;
            command(engine_state, stack, call, df)
        } else {
            Err(ShellError::CantConvert(
                "expression or query".into(),
                value.get_type().to_string(),
                value.span()?,
                None,
            ))
        }
    }
}

fn command(
    engine_state: &EngineState,
    stack: &mut Stack,
    call: &Call,
    df: NuDataFrame,
) -> Result<PipelineData, ShellError> {
    let rows: Option<usize> = call.opt(engine_state, stack, 0)?;
    let rows = rows.unwrap_or(DEFAULT_ROWS);

    let res = df.as_ref().head(Some(rows));
    Ok(PipelineData::Value(
        NuDataFrame::dataframe_into_value(res, 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(FirstDF {})])
    }
}