nushell/crates/nu-command/src/commands/dataframe/head.rs

use crate::prelude::*;
use nu_engine::WholeStreamCommand;
use nu_errors::ShellError;
use nu_protocol::{dataframe::NuDataFrame, Signature, SyntaxShape};

use nu_source::Tagged;

pub struct DataFrame;

impl WholeStreamCommand for DataFrame {
    fn name(&self) -> &str {
        "dataframe head"
    }

    fn usage(&self) -> &str {
        "[DataFrame] Creates new dataframe with head rows"
    }

    fn signature(&self) -> Signature {
        Signature::build("dataframe select").optional(
            "rows",
            SyntaxShape::Number,
            "Number of rows for head",
        )
    }

    fn run(&self, args: CommandArgs) -> Result<OutputStream, ShellError> {
        command(args)
    }

    fn examples(&self) -> Vec<Example> {
        vec![Example {
            description: "Create new dataframe with head rows",
            example: "[[a b]; [1 2] [3 4]] | dataframe to-df | dataframe head",
            result: None,
        }]
    }
}

fn command(mut args: CommandArgs) -> Result<OutputStream, ShellError> {
    let tag = args.call_info.name_tag.clone();
    let rows: Option<Tagged<usize>> = args.opt(0)?;

    let rows = match rows {
        Some(val) => val.item,
        None => 5,
    };

    let df = NuDataFrame::try_from_stream(&mut args.input, &tag.span)?;
    let res = df.as_ref().head(Some(rows));

    Ok(OutputStream::one(NuDataFrame::dataframe_to_value(res, tag)))
}