nushell/crates/nu-engine/src/eval.rs

use crate::{current_dir_str, get_full_help};
use nu_path::expand_path_with;
use nu_protocol::{
    ast::{
        eval_operator, Argument, Assignment, Bits, Block, Boolean, Call, Comparison, Expr,
        Expression, Math, Operator, PathMember, PipelineElement, Redirection,
    },
    engine::{EngineState, Stack},
    IntoInterruptiblePipelineData, IntoPipelineData, PipelineData, Range, Record, ShellError, Span,
    Spanned, Unit, Value, VarId, ENV_VARIABLE_ID,
};
use std::collections::HashMap;

pub fn eval_call(
    engine_state: &EngineState,
    caller_stack: &mut Stack,
    call: &Call,
    input: PipelineData,
) -> Result<PipelineData, ShellError> {
    if nu_utils::ctrl_c::was_pressed(&engine_state.ctrlc) {
        return Ok(Value::nothing(call.head).into_pipeline_data());
    }
    let decl = engine_state.get_decl(call.decl_id);

    if !decl.is_known_external() && call.named_iter().any(|(flag, _, _)| flag.item == "help") {
        let mut signature = engine_state.get_signature(decl);
        signature.usage = decl.usage().to_string();
        signature.extra_usage = decl.extra_usage().to_string();

        let full_help = get_full_help(
            &signature,
            &decl.examples(),
            engine_state,
            caller_stack,
            decl.is_parser_keyword(),
        );
        Ok(Value::string(full_help, call.head).into_pipeline_data())
    } else if let Some(block_id) = decl.get_block_id() {
        let block = engine_state.get_block(block_id);

        let mut callee_stack = caller_stack.gather_captures(engine_state, &block.captures);

        for (param_idx, param) in decl
            .signature()
            .required_positional
            .iter()
            .chain(decl.signature().optional_positional.iter())
            .enumerate()
        {
            let var_id = param
                .var_id
                .expect("internal error: all custom parameters must have var_ids");

            if let Some(arg) = call.positional_nth(param_idx) {
                let result = eval_expression(engine_state, caller_stack, arg)?;
                callee_stack.add_var(var_id, result);
            } else if let Some(value) = &param.default_value {
                callee_stack.add_var(var_id, value.to_owned());
            } else {
                callee_stack.add_var(var_id, Value::nothing(call.head));
            }
        }

        if let Some(rest_positional) = decl.signature().rest_positional {
            let mut rest_items = vec![];

            for arg in call.positional_iter().skip(
                decl.signature().required_positional.len()
                    + decl.signature().optional_positional.len(),
            ) {
                let result = eval_expression(engine_state, caller_stack, arg)?;
                rest_items.push(result);
            }

            let span = if let Some(rest_item) = rest_items.first() {
                rest_item.span()
            } else {
                call.head
            };

            callee_stack.add_var(
                rest_positional
                    .var_id
                    .expect("Internal error: rest positional parameter lacks var_id"),
                Value::list(rest_items, span),
            )
        }

        for named in decl.signature().named {
            if let Some(var_id) = named.var_id {
                let mut found = false;
                for call_named in call.named_iter() {
                    if let (Some(spanned), Some(short)) = (&call_named.1, named.short) {
                        if spanned.item == short.to_string() {
                            if let Some(arg) = &call_named.2 {
                                let result = eval_expression(engine_state, caller_stack, arg)?;

                                callee_stack.add_var(var_id, result);
                            } else if let Some(value) = &named.default_value {
                                callee_stack.add_var(var_id, value.to_owned());
                            } else {
                                callee_stack.add_var(var_id, Value::bool(true, call.head))
                            }
                            found = true;
                        }
                    } else if call_named.0.item == named.long {
                        if let Some(arg) = &call_named.2 {
                            let result = eval_expression(engine_state, caller_stack, arg)?;

                            callee_stack.add_var(var_id, result);
                        } else if let Some(value) = &named.default_value {
                            callee_stack.add_var(var_id, value.to_owned());
                        } else {
                            callee_stack.add_var(var_id, Value::bool(true, call.head))
                        }
                        found = true;
                    }
                }

                if !found {
                    if named.arg.is_none() {
                        callee_stack.add_var(var_id, Value::bool(false, call.head))
                    } else if let Some(value) = named.default_value {
                        callee_stack.add_var(var_id, value);
                    } else {
                        callee_stack.add_var(var_id, Value::nothing(call.head))
                    }
                }
            }
        }

        let result = eval_block_with_early_return(
            engine_state,
            &mut callee_stack,
            block,
            input,
            call.redirect_stdout,
            call.redirect_stderr,
        );

        if block.redirect_env {
            redirect_env(engine_state, caller_stack, &callee_stack);
        }

        result
    } else {
        // We pass caller_stack here with the knowledge that internal commands
        // are going to be specifically looking for global state in the stack
        // rather than any local state.
        decl.run(engine_state, caller_stack, call, input)
    }
}

/// Redirect the environment from callee to the caller.
pub fn redirect_env(engine_state: &EngineState, caller_stack: &mut Stack, callee_stack: &Stack) {
    // Grab all environment variables from the callee
    let caller_env_vars = caller_stack.get_env_var_names(engine_state);

    // remove env vars that are present in the caller but not in the callee
    // (the callee hid them)
    for var in caller_env_vars.iter() {
        if !callee_stack.has_env_var(engine_state, var) {
            caller_stack.remove_env_var(engine_state, var);
        }
    }

    // add new env vars from callee to caller
    for (var, value) in callee_stack.get_stack_env_vars() {
        caller_stack.add_env_var(var, value);
    }
}

enum RedirectTarget {
    Piped(bool, bool),
    CombinedPipe,
}

#[allow(clippy::too_many_arguments)]
fn eval_external(
    engine_state: &EngineState,
    stack: &mut Stack,
    head: &Expression,
    args: &[Expression],
    input: PipelineData,
    redirect_target: RedirectTarget,
    is_subexpression: bool,
) -> Result<PipelineData, ShellError> {
    let decl_id = engine_state
        .find_decl("run-external".as_bytes(), &[])
        .ok_or(ShellError::ExternalNotSupported { span: head.span })?;

    let command = engine_state.get_decl(decl_id);

    let mut call = Call::new(head.span);

    call.add_positional(head.clone());

    for arg in args {
        call.add_positional(arg.clone())
    }

    match redirect_target {
        RedirectTarget::Piped(redirect_stdout, redirect_stderr) => {
            if redirect_stdout {
                call.add_named((
                    Spanned {
                        item: "redirect-stdout".into(),
                        span: head.span,
                    },
                    None,
                    None,
                ))
            }

            if redirect_stderr {
                call.add_named((
                    Spanned {
                        item: "redirect-stderr".into(),
                        span: head.span,
                    },
                    None,
                    None,
                ))
            }
        }
        RedirectTarget::CombinedPipe => call.add_named((
            Spanned {
                item: "redirect-combine".into(),
                span: head.span,
            },
            None,
            None,
        )),
    }

    if is_subexpression {
        call.add_named((
            Spanned {
                item: "trim-end-newline".into(),
                span: head.span,
            },
            None,
            None,
        ))
    }

    command.run(engine_state, stack, &call, input)
}

pub fn eval_expression(
    engine_state: &EngineState,
    stack: &mut Stack,
    expr: &Expression,
) -> Result<Value, ShellError> {
    match &expr.expr {
        Expr::Bool(b) => Ok(Value::bool(*b, expr.span)),
        Expr::Int(i) => Ok(Value::int(*i, expr.span)),
        Expr::Float(f) => Ok(Value::float(*f, expr.span)),
        Expr::Binary(b) => Ok(Value::binary(b.clone(), expr.span)),
        Expr::ValueWithUnit(e, unit) => match eval_expression(engine_state, stack, e)? {
            Value::Int { val, .. } => compute(val, unit.item, unit.span),
            x => Err(ShellError::CantConvert {
                to_type: "unit value".into(),
                from_type: x.get_type().to_string(),
                span: e.span,
                help: None,
            }),
        },
        Expr::Range(from, next, to, operator) => {
            let from = if let Some(f) = from {
                eval_expression(engine_state, stack, f)?
            } else {
                Value::nothing(expr.span)
            };

            let next = if let Some(s) = next {
                eval_expression(engine_state, stack, s)?
            } else {
                Value::nothing(expr.span)
            };

            let to = if let Some(t) = to {
                eval_expression(engine_state, stack, t)?
            } else {
                Value::nothing(expr.span)
            };

            Ok(Value::range(
                Range::new(expr.span, from, next, to, operator)?,
                expr.span,
            ))
        }
        Expr::Var(var_id) => eval_variable(engine_state, stack, *var_id, expr.span),
        Expr::VarDecl(_) => Ok(Value::nothing(expr.span)),
        Expr::CellPath(cell_path) => Ok(Value::cell_path(cell_path.clone(), expr.span)),
        Expr::FullCellPath(cell_path) => {
            let value = eval_expression(engine_state, stack, &cell_path.head)?;

            value.follow_cell_path(&cell_path.tail, false)
        }
        Expr::ImportPattern(_) => Ok(Value::nothing(expr.span)),
        Expr::Overlay(_) => {
            let name =
                String::from_utf8_lossy(engine_state.get_span_contents(expr.span)).to_string();

            Ok(Value::string(name, expr.span))
        }
        Expr::Call(call) => {
            // FIXME: protect this collect with ctrl-c
            Ok(eval_call(engine_state, stack, call, PipelineData::empty())?.into_value(call.head))
        }
        Expr::ExternalCall(head, args, is_subexpression) => {
            let span = head.span;
            // FIXME: protect this collect with ctrl-c
            Ok(eval_external(
                engine_state,
                stack,
                head,
                args,
                PipelineData::empty(),
                RedirectTarget::Piped(false, false),
                *is_subexpression,
            )?
            .into_value(span))
        }
        Expr::DateTime(dt) => Ok(Value::date(*dt, expr.span)),
        Expr::Operator(_) => Ok(Value::nothing(expr.span)),
        Expr::MatchPattern(pattern) => Ok(Value::match_pattern(*pattern.clone(), expr.span)),
        Expr::MatchBlock(_) => Ok(Value::nothing(expr.span)), // match blocks are handled by `match`
        Expr::UnaryNot(expr) => {
            let lhs = eval_expression(engine_state, stack, expr)?;
            match lhs {
                Value::Bool { val, .. } => Ok(Value::bool(!val, expr.span)),
                other => Err(ShellError::TypeMismatch {
                    err_message: format!("expected bool, found {}", other.get_type()),
                    span: expr.span,
                }),
            }
        }
        Expr::BinaryOp(lhs, op, rhs) => {
            let op_span = op.span;
            let op = eval_operator(op)?;

            match op {
                Operator::Boolean(boolean) => {
                    let lhs = eval_expression(engine_state, stack, lhs)?;
                    match boolean {
                        Boolean::And => {
                            if lhs.is_false() {
                                Ok(Value::bool(false, expr.span))
                            } else {
                                let rhs = eval_expression(engine_state, stack, rhs)?;
                                lhs.and(op_span, &rhs, expr.span)
                            }
                        }
                        Boolean::Or => {
                            if lhs.is_true() {
                                Ok(Value::bool(true, expr.span))
                            } else {
                                let rhs = eval_expression(engine_state, stack, rhs)?;
                                lhs.or(op_span, &rhs, expr.span)
                            }
                        }
                        Boolean::Xor => {
                            let rhs = eval_expression(engine_state, stack, rhs)?;
                            lhs.xor(op_span, &rhs, expr.span)
                        }
                    }
                }
                Operator::Math(math) => {
                    let lhs = eval_expression(engine_state, stack, lhs)?;
                    let rhs = eval_expression(engine_state, stack, rhs)?;

                    match math {
                        Math::Plus => lhs.add(op_span, &rhs, expr.span),
                        Math::Minus => lhs.sub(op_span, &rhs, expr.span),
                        Math::Multiply => lhs.mul(op_span, &rhs, expr.span),
                        Math::Divide => lhs.div(op_span, &rhs, expr.span),
                        Math::Append => lhs.append(op_span, &rhs, expr.span),
                        Math::Modulo => lhs.modulo(op_span, &rhs, expr.span),
                        Math::FloorDivision => lhs.floor_div(op_span, &rhs, expr.span),
                        Math::Pow => lhs.pow(op_span, &rhs, expr.span),
                    }
                }
                Operator::Comparison(comparison) => {
                    let lhs = eval_expression(engine_state, stack, lhs)?;
                    let rhs = eval_expression(engine_state, stack, rhs)?;
                    match comparison {
                        Comparison::LessThan => lhs.lt(op_span, &rhs, expr.span),
                        Comparison::LessThanOrEqual => lhs.lte(op_span, &rhs, expr.span),
                        Comparison::GreaterThan => lhs.gt(op_span, &rhs, expr.span),
                        Comparison::GreaterThanOrEqual => lhs.gte(op_span, &rhs, expr.span),
                        Comparison::Equal => lhs.eq(op_span, &rhs, expr.span),
                        Comparison::NotEqual => lhs.ne(op_span, &rhs, expr.span),
                        Comparison::In => lhs.r#in(op_span, &rhs, expr.span),
                        Comparison::NotIn => lhs.not_in(op_span, &rhs, expr.span),
                        Comparison::RegexMatch => {
                            lhs.regex_match(engine_state, op_span, &rhs, false, expr.span)
                        }
                        Comparison::NotRegexMatch => {
                            lhs.regex_match(engine_state, op_span, &rhs, true, expr.span)
                        }
                        Comparison::StartsWith => lhs.starts_with(op_span, &rhs, expr.span),
                        Comparison::EndsWith => lhs.ends_with(op_span, &rhs, expr.span),
                    }
                }
                Operator::Bits(bits) => {
                    let lhs = eval_expression(engine_state, stack, lhs)?;
                    let rhs = eval_expression(engine_state, stack, rhs)?;
                    match bits {
                        Bits::BitAnd => lhs.bit_and(op_span, &rhs, expr.span),
                        Bits::BitOr => lhs.bit_or(op_span, &rhs, expr.span),
                        Bits::BitXor => lhs.bit_xor(op_span, &rhs, expr.span),
                        Bits::ShiftLeft => lhs.bit_shl(op_span, &rhs, expr.span),
                        Bits::ShiftRight => lhs.bit_shr(op_span, &rhs, expr.span),
                    }
                }
                Operator::Assignment(assignment) => {
                    let rhs = eval_expression(engine_state, stack, rhs)?;

                    let rhs = match assignment {
                        Assignment::Assign => rhs,
                        Assignment::PlusAssign => {
                            let lhs = eval_expression(engine_state, stack, lhs)?;
                            lhs.add(op_span, &rhs, op_span)?
                        }
                        Assignment::MinusAssign => {
                            let lhs = eval_expression(engine_state, stack, lhs)?;
                            lhs.sub(op_span, &rhs, op_span)?
                        }
                        Assignment::MultiplyAssign => {
                            let lhs = eval_expression(engine_state, stack, lhs)?;
                            lhs.mul(op_span, &rhs, op_span)?
                        }
                        Assignment::DivideAssign => {
                            let lhs = eval_expression(engine_state, stack, lhs)?;
                            lhs.div(op_span, &rhs, op_span)?
                        }
                        Assignment::AppendAssign => {
                            let lhs = eval_expression(engine_state, stack, lhs)?;
                            lhs.append(op_span, &rhs, op_span)?
                        }
                    };

                    match &lhs.expr {
                        Expr::Var(var_id) | Expr::VarDecl(var_id) => {
                            let var_info = engine_state.get_var(*var_id);
                            if var_info.mutable {
                                stack.add_var(*var_id, rhs);
                                Ok(Value::nothing(lhs.span))
                            } else {
                                Err(ShellError::AssignmentRequiresMutableVar { lhs_span: lhs.span })
                            }
                        }
                        Expr::FullCellPath(cell_path) => {
                            match &cell_path.head.expr {
                                Expr::Var(var_id) | Expr::VarDecl(var_id) => {
                                    // The $env variable is considered "mutable" in Nushell.
                                    // As such, give it special treatment here.
                                    let is_env = var_id == &ENV_VARIABLE_ID;
                                    if is_env || engine_state.get_var(*var_id).mutable {
                                        let mut lhs =
                                            eval_expression(engine_state, stack, &cell_path.head)?;

                                        lhs.upsert_data_at_cell_path(&cell_path.tail, rhs)?;
                                        if is_env {
                                            if cell_path.tail.is_empty() {
                                                return Err(ShellError::CannotReplaceEnv {
                                                    span: cell_path.head.span,
                                                });
                                            }

                                            // The special $env treatment: for something like $env.config.history.max_size = 2000,
                                            // get $env.config (or whichever one it is) AFTER the above mutation, and set it
                                            // as the "config" environment variable.
                                            let vardata = lhs.follow_cell_path(
                                                &[cell_path.tail[0].clone()],
                                                false,
                                            )?;
                                            match &cell_path.tail[0] {
                                                PathMember::String { val, span, .. } => {
                                                    if val == "FILE_PWD"
                                                        || val == "CURRENT_FILE"
                                                        || val == "PWD"
                                                    {
                                                        return Err(ShellError::AutomaticEnvVarSetManually {
                                                    envvar_name: val.to_string(),
                                                    span: *span,
                                                });
                                                    } else {
                                                        stack.add_env_var(val.to_string(), vardata);
                                                    }
                                                }
                                                // In case someone really wants an integer env-var
                                                PathMember::Int { val, .. } => {
                                                    stack.add_env_var(val.to_string(), vardata);
                                                }
                                            }
                                        } else {
                                            stack.add_var(*var_id, lhs);
                                        }
                                        Ok(Value::nothing(cell_path.head.span))
                                    } else {
                                        Err(ShellError::AssignmentRequiresMutableVar {
                                            lhs_span: lhs.span,
                                        })
                                    }
                                }
                                _ => Err(ShellError::AssignmentRequiresVar { lhs_span: lhs.span }),
                            }
                        }
                        _ => Err(ShellError::AssignmentRequiresVar { lhs_span: lhs.span }),
                    }
                }
            }
        }
        Expr::Subexpression(block_id) => {
            let block = engine_state.get_block(*block_id);

            // FIXME: protect this collect with ctrl-c
            Ok(
                eval_subexpression(engine_state, stack, block, PipelineData::empty())?
                    .into_value(expr.span),
            )
        }
        Expr::RowCondition(block_id) | Expr::Closure(block_id) => {
            let mut captures = HashMap::new();
            let block = engine_state.get_block(*block_id);

            for var_id in &block.captures {
                captures.insert(*var_id, stack.get_var(*var_id, expr.span)?);
            }
            Ok(Value::closure(*block_id, captures, expr.span))
        }
        Expr::Block(block_id) => Ok(Value::block(*block_id, expr.span)),
        Expr::List(x) => {
            let mut output = vec![];
            for expr in x {
                output.push(eval_expression(engine_state, stack, expr)?);
            }
            Ok(Value::list(output, expr.span))
        }
        Expr::Record(fields) => {
            let mut record = Record::new();

            for (col, val) in fields {
                // avoid duplicate cols.
                let col_name = eval_expression(engine_state, stack, col)?.as_string()?;
                let pos = record.cols.iter().position(|c| c == &col_name);
                match pos {
                    Some(index) => {
                        return Err(ShellError::ColumnDefinedTwice {
                            second_use: col.span,
                            first_use: fields[index].0.span,
                        })
                    }
                    None => {
                        record.push(col_name, eval_expression(engine_state, stack, val)?);
                    }
                }
            }

            Ok(Value::record(record, expr.span))
        }
        Expr::Table(headers, vals) => {
            let mut output_headers = vec![];
            for expr in headers {
                output_headers.push(eval_expression(engine_state, stack, expr)?.as_string()?);
            }

            let mut output_rows = vec![];
            for val in vals {
                let mut row = vec![];
                for expr in val {
                    row.push(eval_expression(engine_state, stack, expr)?);
                }
                output_rows.push(Value::record(
                    Record {
                        cols: output_headers.clone(),
                        vals: row,
                    },
                    expr.span,
                ));
            }
            Ok(Value::list(output_rows, expr.span))
        }
        Expr::Keyword(_, _, expr) => eval_expression(engine_state, stack, expr),
        Expr::StringInterpolation(exprs) => {
            let mut parts = vec![];
            for expr in exprs {
                parts.push(eval_expression(engine_state, stack, expr)?);
            }

            let config = engine_state.get_config();

            parts
                .into_iter()
                .into_pipeline_data(None)
                .collect_string("", config)
                .map(|x| Value::string(x, expr.span))
        }
        Expr::String(s) => Ok(Value::string(s.clone(), expr.span)),
        Expr::Filepath(s) => {
            let cwd = current_dir_str(engine_state, stack)?;
            let path = expand_path_with(s, cwd);

            Ok(Value::string(path.to_string_lossy(), expr.span))
        }
        Expr::Directory(s) => {
            if s == "-" {
                Ok(Value::string("-", expr.span))
            } else {
                let cwd = current_dir_str(engine_state, stack)?;
                let path = expand_path_with(s, cwd);

                Ok(Value::string(path.to_string_lossy(), expr.span))
            }
        }
        Expr::GlobPattern(s) => {
            let cwd = current_dir_str(engine_state, stack)?;
            let path = expand_path_with(s, cwd);

            Ok(Value::string(path.to_string_lossy(), expr.span))
        }
        Expr::Signature(_) => Ok(Value::nothing(expr.span)),
        Expr::Garbage => Ok(Value::nothing(expr.span)),
        Expr::Nothing => Ok(Value::nothing(expr.span)),
    }
}

/// Checks the expression to see if it's a internal or external call. If so, passes the input
/// into the call and gets out the result
/// Otherwise, invokes the expression
///
/// It returns PipelineData with a boolean flag, indicating if the external failed to run.
/// The boolean flag **may only be true** for external calls, for internal calls, it always to be false.
pub fn eval_expression_with_input(
    engine_state: &EngineState,
    stack: &mut Stack,
    expr: &Expression,
    mut input: PipelineData,
    redirect_stdout: bool,
    redirect_stderr: bool,
) -> Result<(PipelineData, bool), ShellError> {
    match expr {
        Expression {
            expr: Expr::Call(call),
            ..
        } => {
            if !redirect_stdout || redirect_stderr {
                // we're doing something different than the defaults
                let mut call = call.clone();
                call.redirect_stdout = redirect_stdout;
                call.redirect_stderr = redirect_stderr;
                input = eval_call(engine_state, stack, &call, input)?;
            } else {
                input = eval_call(engine_state, stack, call, input)?;
            }
        }
        Expression {
            expr: Expr::ExternalCall(head, args, is_subexpression),
            ..
        } => {
            input = eval_external(
                engine_state,
                stack,
                head,
                args,
                input,
                RedirectTarget::Piped(redirect_stdout, redirect_stderr),
                *is_subexpression,
            )?;
        }

        Expression {
            expr: Expr::Subexpression(block_id),
            ..
        } => {
            let block = engine_state.get_block(*block_id);

            // FIXME: protect this collect with ctrl-c
            input = eval_subexpression(engine_state, stack, block, input)?;
        }

        elem @ Expression {
            expr: Expr::FullCellPath(full_cell_path),
            ..
        } => match &full_cell_path.head {
            Expression {
                expr: Expr::Subexpression(block_id),
                span,
                ..
            } => {
                let block = engine_state.get_block(*block_id);

                // FIXME: protect this collect with ctrl-c
                input = eval_subexpression(engine_state, stack, block, input)?;
                let value = input.into_value(*span);
                input = value
                    .follow_cell_path(&full_cell_path.tail, false)?
                    .into_pipeline_data()
            }
            _ => {
                input = eval_expression(engine_state, stack, elem)?.into_pipeline_data();
            }
        },

        elem => {
            input = eval_expression(engine_state, stack, elem)?.into_pipeline_data();
        }
    };

    Ok(might_consume_external_result(input))
}

// Try to catch and detect if external command runs to failed.
fn might_consume_external_result(input: PipelineData) -> (PipelineData, bool) {
    input.is_external_failed()
}

fn eval_element_with_input(
    engine_state: &EngineState,
    stack: &mut Stack,
    element: &PipelineElement,
    mut input: PipelineData,
    redirect_stdout: bool,
    redirect_stderr: bool,
) -> Result<(PipelineData, bool), ShellError> {
    match element {
        PipelineElement::Expression(_, expr) => eval_expression_with_input(
            engine_state,
            stack,
            expr,
            input,
            redirect_stdout,
            redirect_stderr,
        ),
        PipelineElement::Redirection(span, redirection, expr) => match &expr.expr {
            Expr::String(_)
            | Expr::FullCellPath(_)
            | Expr::StringInterpolation(_)
            | Expr::Filepath(_) => {
                let exit_code = match &mut input {
                    PipelineData::ExternalStream { exit_code, .. } => exit_code.take(),
                    _ => None,
                };
                let input = match (redirection, input) {
                    (
                        Redirection::Stderr,
                        PipelineData::ExternalStream {
                            stderr,
                            exit_code,
                            span,
                            metadata,
                            trim_end_newline,
                            ..
                        },
                    ) => PipelineData::ExternalStream {
                        stdout: stderr,
                        stderr: None,
                        exit_code,
                        span,
                        metadata,
                        trim_end_newline,
                    },
                    (_, input) => input,
                };

                if let Some(save_command) = engine_state.find_decl(b"save", &[]) {
                    eval_call(
                        engine_state,
                        stack,
                        &Call {
                            decl_id: save_command,
                            head: *span,
                            arguments: vec![
                                Argument::Positional(expr.clone()),
                                Argument::Named((
                                    Spanned {
                                        item: "raw".into(),
                                        span: *span,
                                    },
                                    None,
                                    None,
                                )),
                                Argument::Named((
                                    Spanned {
                                        item: "force".into(),
                                        span: *span,
                                    },
                                    None,
                                    None,
                                )),
                            ],
                            redirect_stdout: false,
                            redirect_stderr: false,
                            parser_info: HashMap::new(),
                        },
                        input,
                    )
                    .map(|_| {
                        // save is internal command, normally it exists with non-ExternalStream
                        // but here in redirection context, we make it returns ExternalStream
                        // So nu handles exit_code correctly
                        (
                            PipelineData::ExternalStream {
                                stdout: None,
                                stderr: None,
                                exit_code,
                                span: *span,
                                metadata: None,
                                trim_end_newline: false,
                            },
                            false,
                        )
                    })
                } else {
                    Err(ShellError::CommandNotFound(*span))
                }
            }
            _ => Err(ShellError::CommandNotFound(*span)),
        },
        PipelineElement::SeparateRedirection {
            out: (out_span, out_expr),
            err: (err_span, err_expr),
        } => match (&out_expr.expr, &err_expr.expr) {
            (
                Expr::String(_)
                | Expr::FullCellPath(_)
                | Expr::StringInterpolation(_)
                | Expr::Filepath(_),
                Expr::String(_)
                | Expr::FullCellPath(_)
                | Expr::StringInterpolation(_)
                | Expr::Filepath(_),
            ) => {
                if let Some(save_command) = engine_state.find_decl(b"save", &[]) {
                    let exit_code = match &mut input {
                        PipelineData::ExternalStream { exit_code, .. } => exit_code.take(),
                        _ => None,
                    };
                    eval_call(
                        engine_state,
                        stack,
                        &Call {
                            decl_id: save_command,
                            head: *out_span,
                            arguments: vec![
                                Argument::Positional(out_expr.clone()),
                                Argument::Named((
                                    Spanned {
                                        item: "stderr".into(),
                                        span: *err_span,
                                    },
                                    None,
                                    Some(err_expr.clone()),
                                )),
                                Argument::Named((
                                    Spanned {
                                        item: "raw".into(),
                                        span: *out_span,
                                    },
                                    None,
                                    None,
                                )),
                                Argument::Named((
                                    Spanned {
                                        item: "force".into(),
                                        span: *out_span,
                                    },
                                    None,
                                    None,
                                )),
                            ],
                            redirect_stdout: false,
                            redirect_stderr: false,
                            parser_info: HashMap::new(),
                        },
                        input,
                    )
                    .map(|_| {
                        // save is internal command, normally it exists with non-ExternalStream
                        // but here in redirection context, we make it returns ExternalStream
                        // So nu handles exit_code correctly
                        (
                            PipelineData::ExternalStream {
                                stdout: None,
                                stderr: None,
                                exit_code,
                                span: *out_span,
                                metadata: None,
                                trim_end_newline: false,
                            },
                            false,
                        )
                    })
                } else {
                    Err(ShellError::CommandNotFound(*out_span))
                }
            }
            (_out_other, err_other) => {
                if let Expr::String(_) = err_other {
                    Err(ShellError::CommandNotFound(*out_span))
                } else {
                    Err(ShellError::CommandNotFound(*err_span))
                }
            }
        },
        PipelineElement::SameTargetRedirection {
            cmd: (cmd_span, cmd_exp),
            redirection: (redirect_span, redirect_exp),
        } => {
            // general idea: eval cmd and call save command to redirect stdout to result.
            input = match &cmd_exp.expr {
                Expr::ExternalCall(head, args, is_subexpression) => {
                    // if cmd's expression is ExternalStream, then invoke run-external with
                    // special --redirect-combine flag.
                    eval_external(
                        engine_state,
                        stack,
                        head,
                        args,
                        input,
                        RedirectTarget::CombinedPipe,
                        *is_subexpression,
                    )?
                }
                _ => {
                    // we need to redirect output, so the result can be saved and pass to `save` command.
                    eval_element_with_input(
                        engine_state,
                        stack,
                        &PipelineElement::Expression(*cmd_span, cmd_exp.clone()),
                        input,
                        true,
                        redirect_stderr,
                    )
                    .map(|x| x.0)?
                }
            };
            eval_element_with_input(
                engine_state,
                stack,
                &PipelineElement::Redirection(
                    *redirect_span,
                    Redirection::Stdout,
                    redirect_exp.clone(),
                ),
                input,
                redirect_stdout,
                redirect_stderr,
            )
        }
        PipelineElement::And(_, expr) => eval_expression_with_input(
            engine_state,
            stack,
            expr,
            input,
            redirect_stdout,
            redirect_stderr,
        ),
        PipelineElement::Or(_, expr) => eval_expression_with_input(
            engine_state,
            stack,
            expr,
            input,
            redirect_stdout,
            redirect_stderr,
        ),
    }
}

pub fn eval_block_with_early_return(
    engine_state: &EngineState,
    stack: &mut Stack,
    block: &Block,
    input: PipelineData,
    redirect_stdout: bool,
    redirect_stderr: bool,
) -> Result<PipelineData, ShellError> {
    match eval_block(
        engine_state,
        stack,
        block,
        input,
        redirect_stdout,
        redirect_stderr,
    ) {
        Err(ShellError::Return(_, value)) => Ok(PipelineData::Value(*value, None)),
        x => x,
    }
}

pub fn eval_block(
    engine_state: &EngineState,
    stack: &mut Stack,
    block: &Block,
    mut input: PipelineData,
    redirect_stdout: bool,
    redirect_stderr: bool,
) -> Result<PipelineData, ShellError> {
    // if Block contains recursion, make sure we don't recurse too deeply (to avoid stack overflow)
    if let Some(recursive) = block.recursive {
        // picked 50 arbitrarily, should work on all architectures
        const RECURSION_LIMIT: u64 = 50;
        if recursive {
            if *stack.recursion_count >= RECURSION_LIMIT {
                stack.recursion_count = Box::new(0);
                return Err(ShellError::RecursionLimitReached {
                    recursion_limit: RECURSION_LIMIT,
                    span: block.span,
                });
            }
            *stack.recursion_count += 1;
        }
    }

    let num_pipelines = block.len();

    for (pipeline_idx, pipeline) in block.pipelines.iter().enumerate() {
        let mut elements_iter = pipeline.elements.iter().peekable();
        while let Some(element) = elements_iter.next() {
            let redirect_stderr = redirect_stderr
                || (elements_iter.peek().map_or(false, |next_element| {
                    matches!(
                        next_element,
                        PipelineElement::Redirection(_, Redirection::Stderr, _)
                            | PipelineElement::Redirection(_, Redirection::StdoutAndStderr, _)
                            | PipelineElement::SeparateRedirection { .. }
                    )
                }));

            let redirect_stdout = redirect_stdout
                || (elements_iter.peek().map_or(false, |next_element| {
                    matches!(
                        next_element,
                        PipelineElement::Redirection(_, Redirection::Stdout, _)
                            | PipelineElement::Redirection(_, Redirection::StdoutAndStderr, _)
                            | PipelineElement::Expression(..)
                            | PipelineElement::SeparateRedirection { .. }
                    )
                }));

            // if eval internal command failed, it can just make early return with `Err(ShellError)`.
            let eval_result = eval_element_with_input(
                engine_state,
                stack,
                element,
                input,
                redirect_stdout,
                redirect_stderr,
            );

            match (eval_result, redirect_stderr) {
                (Ok((pipeline_data, _)), true) => {
                    input = pipeline_data;

                    // external command may runs to failed
                    // make early return so remaining commands will not be executed.
                    // don't return `Err(ShellError)`, so nushell wouldn't show extra error message.
                }
                (Err(error), true) => {
                    input = PipelineData::Value(
                        Value::error(
                            error,
                            Span::unknown(), // FIXME: where does this span come from?
                        ),
                        None,
                    )
                }
                (output, false) => {
                    let output = output?;
                    input = output.0;
                    // external command may runs to failed
                    // make early return so remaining commands will not be executed.
                    // don't return `Err(ShellError)`, so nushell wouldn't show extra error message.
                    if output.1 {
                        return Ok(input);
                    }
                }
            }
        }

        if pipeline_idx < (num_pipelines) - 1 {
            match input {
                PipelineData::Value(Value::Nothing { .. }, ..) => {}
                PipelineData::ExternalStream {
                    ref mut exit_code, ..
                } => {
                    let exit_code = exit_code.take();

                    input.drain()?;

                    if let Some(exit_code) = exit_code {
                        let mut v: Vec<_> = exit_code.collect();

                        if let Some(v) = v.pop() {
                            stack.add_env_var("LAST_EXIT_CODE".into(), v);
                        }
                    }
                }
                _ => input.drain()?,
            }

            input = PipelineData::empty()
        }
    }

    Ok(input)
}

pub fn eval_subexpression(
    engine_state: &EngineState,
    stack: &mut Stack,
    block: &Block,
    mut input: PipelineData,
) -> Result<PipelineData, ShellError> {
    for pipeline in block.pipelines.iter() {
        for expr in pipeline.elements.iter() {
            input = eval_element_with_input(engine_state, stack, expr, input, true, false)?.0
        }
    }

    Ok(input)
}

pub fn eval_variable(
    engine_state: &EngineState,
    stack: &Stack,
    var_id: VarId,
    span: Span,
) -> Result<Value, ShellError> {
    match var_id {
        // $nu
        nu_protocol::NU_VARIABLE_ID => {
            if let Some(val) = engine_state.get_constant(var_id) {
                Ok(val.clone())
            } else {
                Err(ShellError::VariableNotFoundAtRuntime { span })
            }
        }
        // $env
        ENV_VARIABLE_ID => {
            let env_vars = stack.get_env_vars(engine_state);
            let env_columns = env_vars.keys();
            let env_values = env_vars.values();

            let mut pairs = env_columns
                .map(|x| x.to_string())
                .zip(env_values.cloned())
                .collect::<Vec<(String, Value)>>();

            pairs.sort_by(|a, b| a.0.cmp(&b.0));

            Ok(Value::record(pairs.into_iter().collect(), span))
        }
        var_id => stack.get_var(var_id, span),
    }
}

fn compute(size: i64, unit: Unit, span: Span) -> Result<Value, ShellError> {
    unit.to_value(size, span)
}