nushell/crates/nu-cli/src/commands/where_.rs

use crate::commands::WholeStreamCommand;
use crate::context::CommandRegistry;
use crate::evaluate::evaluate_baseline_expr;
use crate::prelude::*;
use nu_errors::ShellError;
use nu_protocol::{
    hir::Block, hir::ClassifiedCommand, ReturnSuccess, Scope, Signature, SyntaxShape,
};

pub struct Where;

#[derive(Deserialize)]
pub struct WhereArgs {
    block: Block,
}

impl WholeStreamCommand for Where {
    fn name(&self) -> &str {
        "where"
    }

    fn signature(&self) -> Signature {
        Signature::build("where").required(
            "condition",
            SyntaxShape::Math,
            "the condition that must match",
        )
    }

    fn usage(&self) -> &str {
        "Filter table to match the condition."
    }

    fn run(
        &self,
        args: CommandArgs,
        registry: &CommandRegistry,
    ) -> Result<OutputStream, ShellError> {
        args.process(registry, where_command)?.run()
    }
}
fn where_command(
    WhereArgs { block }: WhereArgs,
    RunnableContext {
        name: tag,
        registry,
        input,
        ..
    }: RunnableContext,
) -> Result<OutputStream, ShellError> {
    let condition = {
        if block.block.len() != 1 {
            return Err(ShellError::labeled_error(
                "Expected a condition",
                "expected a condition",
                tag,
            ));
        }
        match block.block[0].list.get(0) {
            Some(item) => match item {
                ClassifiedCommand::Expr(expr) => expr.clone(),
                _ => {
                    return Err(ShellError::labeled_error(
                        "Expected a condition",
                        "expected a condition",
                        tag,
                    ))
                }
            },
            None => {
                return Err(ShellError::labeled_error(
                    "Expected a condition",
                    "expected a condition",
                    tag,
                ));
            }
        }
    };

    let mut input = input;

    let stream = async_stream! {
        while let Some(input) = input.next().await {

            //FIXME: should we use the scope that's brought in as well?
            let condition = evaluate_baseline_expr(&condition, &registry, &Scope::new(input.clone()))?;

            match condition.as_bool() {
                Ok(b) => {
                    if b {
                        yield Ok(ReturnSuccess::Value(input));
                    }
                }
                Err(e) => yield Err(e),
            };
        }
    };

    Ok(stream.to_output_stream())
}