nushell/src/commands/first.rs

use crate::commands::WholeStreamCommand;
use crate::context::CommandRegistry;
use crate::prelude::*;
use nu_errors::ShellError;
use nu_protocol::{Signature, SyntaxShape};
use nu_source::Tagged;

pub struct First;

#[derive(Deserialize)]
pub struct FirstArgs {
    rows: Option<Tagged<u64>>,
}

impl WholeStreamCommand for First {
    fn name(&self) -> &str {
        "first"
    }

    fn signature(&self) -> Signature {
        Signature::build("first").optional(
            "rows",
            SyntaxShape::Int,
            "starting from the front, the number of rows to return",
        )
    }

    fn usage(&self) -> &str {
        "Show only the first number of rows."
    }

    fn run(
        &self,
        args: CommandArgs,
        registry: &CommandRegistry,
    ) -> Result<OutputStream, ShellError> {
        args.process(registry, first)?.run()
    }
}

fn first(
    FirstArgs { rows }: FirstArgs,
    context: RunnableContext,
) -> Result<OutputStream, ShellError> {
    let rows_desired = if let Some(quantity) = rows {
        *quantity
    } else {
        1
    };

    Ok(OutputStream::from_input(
        context.input.values.take(rows_desired),
    ))
}