datafusion 4.0.0

// Licensed to the Apache Software Foundation (ASF) under one
// or more contributor license agreements.  See the NOTICE file
// distributed with this work for additional information
// regarding copyright ownership.  The ASF licenses this file
// to you under the Apache License, Version 2.0 (the
// "License"); you may not use this file except in compliance
// with the License.  You may obtain a copy of the License at
//
//   http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing,
// software distributed under the License is distributed on an
// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
// KIND, either express or implied.  See the License for the
// specific language governing permissions and limitations
// under the License.

//! This module provides an `Expr` enum for representing expressions
//! such as `col = 5` or `SUM(col)`. See examples on the [`Expr`] struct.

pub use super::Operator;

use std::fmt;
use std::sync::Arc;

use aggregates::{AccumulatorFunctionImplementation, StateTypeFunction};
use arrow::{compute::can_cast_types, datatypes::DataType};

use crate::error::{DataFusionError, Result};
use crate::logical_plan::{DFField, DFSchema};
use crate::physical_plan::{
    aggregates, expressions::binary_operator_data_type, functions, udf::ScalarUDF,
};
use crate::{physical_plan::udaf::AggregateUDF, scalar::ScalarValue};
use functions::{ReturnTypeFunction, ScalarFunctionImplementation, Signature};
use std::collections::HashSet;

/// `Expr` is a central struct of DataFusion's query API, and
/// represent logical expressions such as `A + 1`, or `CAST(c1 AS
/// int)`.
///
/// An `Expr` can compute its [DataType](arrow::datatypes::DataType)
/// and nullability, and has functions for building up complex
/// expressions.
///
/// # Examples
///
/// ## Create an expression `c1` referring to column named "c1"
/// ```
/// # use datafusion::logical_plan::*;
/// let expr = col("c1");
/// assert_eq!(expr, Expr::Column("c1".to_string()));
/// ```
///
/// ## Create the expression `c1 + c2` to add columns "c1" and "c2" together
/// ```
/// # use datafusion::logical_plan::*;
/// let expr = col("c1") + col("c2");
///
/// assert!(matches!(expr, Expr::BinaryExpr { ..} ));
/// if let Expr::BinaryExpr { left, right, op } = expr {
///   assert_eq!(*left, col("c1"));
///   assert_eq!(*right, col("c2"));
///   assert_eq!(op, Operator::Plus);
/// }
/// ```
///
/// ## Create expression `c1 = 42` to compare the value in coumn "c1" to the literal value `42`
/// ```
/// # use datafusion::logical_plan::*;
/// # use datafusion::scalar::*;
/// let expr = col("c1").eq(lit(42));
///
/// assert!(matches!(expr, Expr::BinaryExpr { ..} ));
/// if let Expr::BinaryExpr { left, right, op } = expr {
///   assert_eq!(*left, col("c1"));
///   let scalar = ScalarValue::Int32(Some(42));
///   assert_eq!(*right, Expr::Literal(scalar));
///   assert_eq!(op, Operator::Eq);
/// }
/// ```
#[derive(Clone, PartialEq)]
pub enum Expr {
    /// An expression with a specific name.
    Alias(Box<Expr>, String),
    /// A named reference to a field in a schema.
    Column(String),
    /// A named reference to a variable in a registry.
    ScalarVariable(Vec<String>),
    /// A constant value.
    Literal(ScalarValue),
    /// A binary expression such as "age > 21"
    BinaryExpr {
        /// Left-hand side of the expression
        left: Box<Expr>,
        /// The comparison operator
        op: Operator,
        /// Right-hand side of the expression
        right: Box<Expr>,
    },
    /// Negation of an expression. The expression's type must be a boolean to make sense.
    Not(Box<Expr>),
    /// Whether an expression is not Null. This expression is never null.
    IsNotNull(Box<Expr>),
    /// Whether an expression is Null. This expression is never null.
    IsNull(Box<Expr>),
    /// arithmetic negation of an expression, the operand must be of a signed numeric data type
    Negative(Box<Expr>),
    /// Whether an expression is between a given range.
    Between {
        /// The value to compare
        expr: Box<Expr>,
        /// Whether the expression is negated
        negated: bool,
        /// The low end of the range
        low: Box<Expr>,
        /// The high end of the range
        high: Box<Expr>,
    },
    /// The CASE expression is similar to a series of nested if/else and there are two forms that
    /// can be used. The first form consists of a series of boolean "when" expressions with
    /// corresponding "then" expressions, and an optional "else" expression.
    ///
    /// CASE WHEN condition THEN result
    ///      [WHEN ...]
    ///      [ELSE result]
    /// END
    ///
    /// The second form uses a base expression and then a series of "when" clauses that match on a
    /// literal value.
    ///
    /// CASE expression
    ///     WHEN value THEN result
    ///     [WHEN ...]
    ///     [ELSE result]
    /// END
    Case {
        /// Optional base expression that can be compared to literal values in the "when" expressions
        expr: Option<Box<Expr>>,
        /// One or more when/then expressions
        when_then_expr: Vec<(Box<Expr>, Box<Expr>)>,
        /// Optional "else" expression
        else_expr: Option<Box<Expr>>,
    },
    /// Casts the expression to a given type and will return a runtime error if the expression cannot be cast.
    /// This expression is guaranteed to have a fixed type.
    Cast {
        /// The expression being cast
        expr: Box<Expr>,
        /// The `DataType` the expression will yield
        data_type: DataType,
    },
    /// Casts the expression to a given type and will return a null value if the expression cannot be cast.
    /// This expression is guaranteed to have a fixed type.
    TryCast {
        /// The expression being cast
        expr: Box<Expr>,
        /// The `DataType` the expression will yield
        data_type: DataType,
    },
    /// A sort expression, that can be used to sort values.
    Sort {
        /// The expression to sort on
        expr: Box<Expr>,
        /// The direction of the sort
        asc: bool,
        /// Whether to put Nulls before all other data values
        nulls_first: bool,
    },
    /// Represents the call of a built-in scalar function with a set of arguments.
    ScalarFunction {
        /// The function
        fun: functions::BuiltinScalarFunction,
        /// List of expressions to feed to the functions as arguments
        args: Vec<Expr>,
    },
    /// Represents the call of a user-defined scalar function with arguments.
    ScalarUDF {
        /// The function
        fun: Arc<ScalarUDF>,
        /// List of expressions to feed to the functions as arguments
        args: Vec<Expr>,
    },
    /// Represents the call of an aggregate built-in function with arguments.
    AggregateFunction {
        /// Name of the function
        fun: aggregates::AggregateFunction,
        /// List of expressions to feed to the functions as arguments
        args: Vec<Expr>,
        /// Whether this is a DISTINCT aggregation or not
        distinct: bool,
    },
    /// aggregate function
    AggregateUDF {
        /// The function
        fun: Arc<AggregateUDF>,
        /// List of expressions to feed to the functions as arguments
        args: Vec<Expr>,
    },
    /// Returns whether the list contains the expr value.
    InList {
        /// The expression to compare
        expr: Box<Expr>,
        /// A list of values to compare against
        list: Vec<Expr>,
        /// Whether the expression is negated
        negated: bool,
    },
    /// Represents a reference to all fields in a schema.
    Wildcard,
}

impl Expr {
    /// Returns the [arrow::datatypes::DataType] of the expression based on [arrow::datatypes::Schema].
    ///
    /// # Errors
    ///
    /// This function errors when it is not possible to compute its [arrow::datatypes::DataType].
    /// This happens when e.g. the expression refers to a column that does not exist in the schema, or when
    /// the expression is incorrectly typed (e.g. `[utf8] + [bool]`).
    pub fn get_type(&self, schema: &DFSchema) -> Result<DataType> {
        match self {
            Expr::Alias(expr, _) => expr.get_type(schema),
            Expr::Column(name) => Ok(schema
                .field_with_unqualified_name(name)?
                .data_type()
                .clone()),
            Expr::ScalarVariable(_) => Ok(DataType::Utf8),
            Expr::Literal(l) => Ok(l.get_datatype()),
            Expr::Case { when_then_expr, .. } => when_then_expr[0].1.get_type(schema),
            Expr::Cast { data_type, .. } => Ok(data_type.clone()),
            Expr::TryCast { data_type, .. } => Ok(data_type.clone()),
            Expr::ScalarUDF { fun, args } => {
                let data_types = args
                    .iter()
                    .map(|e| e.get_type(schema))
                    .collect::<Result<Vec<_>>>()?;
                Ok((fun.return_type)(&data_types)?.as_ref().clone())
            }
            Expr::ScalarFunction { fun, args } => {
                let data_types = args
                    .iter()
                    .map(|e| e.get_type(schema))
                    .collect::<Result<Vec<_>>>()?;
                functions::return_type(fun, &data_types)
            }
            Expr::AggregateFunction { fun, args, .. } => {
                let data_types = args
                    .iter()
                    .map(|e| e.get_type(schema))
                    .collect::<Result<Vec<_>>>()?;
                aggregates::return_type(fun, &data_types)
            }
            Expr::AggregateUDF { fun, args, .. } => {
                let data_types = args
                    .iter()
                    .map(|e| e.get_type(schema))
                    .collect::<Result<Vec<_>>>()?;
                Ok((fun.return_type)(&data_types)?.as_ref().clone())
            }
            Expr::Not(_) => Ok(DataType::Boolean),
            Expr::Negative(expr) => expr.get_type(schema),
            Expr::IsNull(_) => Ok(DataType::Boolean),
            Expr::IsNotNull(_) => Ok(DataType::Boolean),
            Expr::BinaryExpr {
                ref left,
                ref right,
                ref op,
            } => binary_operator_data_type(
                &left.get_type(schema)?,
                op,
                &right.get_type(schema)?,
            ),
            Expr::Sort { ref expr, .. } => expr.get_type(schema),
            Expr::Between { .. } => Ok(DataType::Boolean),
            Expr::InList { .. } => Ok(DataType::Boolean),
            Expr::Wildcard => Err(DataFusionError::Internal(
                "Wildcard expressions are not valid in a logical query plan".to_owned(),
            )),
        }
    }

    /// Returns the nullability of the expression based on [arrow::datatypes::Schema].
    ///
    /// # Errors
    ///
    /// This function errors when it is not possible to compute its nullability.
    /// This happens when the expression refers to a column that does not exist in the schema.
    pub fn nullable(&self, input_schema: &DFSchema) -> Result<bool> {
        match self {
            Expr::Alias(expr, _) => expr.nullable(input_schema),
            Expr::Column(name) => Ok(input_schema
                .field_with_unqualified_name(name)?
                .is_nullable()),
            Expr::Literal(value) => Ok(value.is_null()),
            Expr::ScalarVariable(_) => Ok(true),
            Expr::Case {
                when_then_expr,
                else_expr,
                ..
            } => {
                // this expression is nullable if any of the input expressions are nullable
                let then_nullable = when_then_expr
                    .iter()
                    .map(|(_, t)| t.nullable(input_schema))
                    .collect::<Result<Vec<_>>>()?;
                if then_nullable.contains(&true) {
                    Ok(true)
                } else if let Some(e) = else_expr {
                    e.nullable(input_schema)
                } else {
                    Ok(false)
                }
            }
            Expr::Cast { expr, .. } => expr.nullable(input_schema),
            Expr::TryCast { .. } => Ok(true),
            Expr::ScalarFunction { .. } => Ok(true),
            Expr::ScalarUDF { .. } => Ok(true),
            Expr::AggregateFunction { .. } => Ok(true),
            Expr::AggregateUDF { .. } => Ok(true),
            Expr::Not(expr) => expr.nullable(input_schema),
            Expr::Negative(expr) => expr.nullable(input_schema),
            Expr::IsNull(_) => Ok(false),
            Expr::IsNotNull(_) => Ok(false),
            Expr::BinaryExpr {
                ref left,
                ref right,
                ..
            } => Ok(left.nullable(input_schema)? || right.nullable(input_schema)?),
            Expr::Sort { ref expr, .. } => expr.nullable(input_schema),
            Expr::Between { ref expr, .. } => expr.nullable(input_schema),
            Expr::InList { ref expr, .. } => expr.nullable(input_schema),
            Expr::Wildcard => Err(DataFusionError::Internal(
                "Wildcard expressions are not valid in a logical query plan".to_owned(),
            )),
        }
    }

    /// Returns the name of this expression based on [arrow::datatypes::Schema].
    ///
    /// This represents how a column with this expression is named when no alias is chosen
    pub fn name(&self, input_schema: &DFSchema) -> Result<String> {
        create_name(self, input_schema)
    }

    /// Returns a [arrow::datatypes::Field] compatible with this expression.
    pub fn to_field(&self, input_schema: &DFSchema) -> Result<DFField> {
        Ok(DFField::new(
            None, //TODO  qualifier
            &self.name(input_schema)?,
            self.get_type(input_schema)?,
            self.nullable(input_schema)?,
        ))
    }

    /// Wraps this expression in a cast to a target [arrow::datatypes::DataType].
    ///
    /// # Errors
    ///
    /// This function errors when it is impossible to cast the
    /// expression to the target [arrow::datatypes::DataType].
    pub fn cast_to(self, cast_to_type: &DataType, schema: &DFSchema) -> Result<Expr> {
        let this_type = self.get_type(schema)?;
        if this_type == *cast_to_type {
            Ok(self)
        } else if can_cast_types(&this_type, cast_to_type) {
            Ok(Expr::Cast {
                expr: Box::new(self),
                data_type: cast_to_type.clone(),
            })
        } else {
            Err(DataFusionError::Plan(format!(
                "Cannot automatically convert {:?} to {:?}",
                this_type, cast_to_type
            )))
        }
    }

    /// Return `self == other`
    pub fn eq(self, other: Expr) -> Expr {
        binary_expr(self, Operator::Eq, other)
    }

    /// Return `self != other`
    pub fn not_eq(self, other: Expr) -> Expr {
        binary_expr(self, Operator::NotEq, other)
    }

    /// Return `self > other`
    pub fn gt(self, other: Expr) -> Expr {
        binary_expr(self, Operator::Gt, other)
    }

    /// Return `self >= other`
    pub fn gt_eq(self, other: Expr) -> Expr {
        binary_expr(self, Operator::GtEq, other)
    }

    /// Return `self < other`
    pub fn lt(self, other: Expr) -> Expr {
        binary_expr(self, Operator::Lt, other)
    }

    /// Return `self <= other`
    pub fn lt_eq(self, other: Expr) -> Expr {
        binary_expr(self, Operator::LtEq, other)
    }

    /// Return `self && other`
    pub fn and(self, other: Expr) -> Expr {
        binary_expr(self, Operator::And, other)
    }

    /// Return `self || other`
    pub fn or(self, other: Expr) -> Expr {
        binary_expr(self, Operator::Or, other)
    }

    /// Return `!self`
    #[allow(clippy::should_implement_trait)]
    pub fn not(self) -> Expr {
        Expr::Not(Box::new(self))
    }

    /// Calculate the modulus of two expressions.
    /// Return `self % other`
    pub fn modulus(self, other: Expr) -> Expr {
        binary_expr(self, Operator::Modulus, other)
    }

    /// Return `self LIKE other`
    pub fn like(self, other: Expr) -> Expr {
        binary_expr(self, Operator::Like, other)
    }

    /// Return `self NOT LIKE other`
    pub fn not_like(self, other: Expr) -> Expr {
        binary_expr(self, Operator::NotLike, other)
    }

    /// Return `self AS name` alias expression
    pub fn alias(self, name: &str) -> Expr {
        Expr::Alias(Box::new(self), name.to_owned())
    }

    /// Return `self IN <list>` if `negated` is false, otherwise
    /// return `self NOT IN <list>`.a
    pub fn in_list(self, list: Vec<Expr>, negated: bool) -> Expr {
        Expr::InList {
            expr: Box::new(self),
            list,
            negated,
        }
    }

    /// Return `IsNull(Box(self))
    #[allow(clippy::wrong_self_convention)]
    pub fn is_null(self) -> Expr {
        Expr::IsNull(Box::new(self))
    }

    /// Return `IsNotNull(Box(self))
    #[allow(clippy::wrong_self_convention)]
    pub fn is_not_null(self) -> Expr {
        Expr::IsNotNull(Box::new(self))
    }

    /// Create a sort expression from an existing expression.
    ///
    /// ```
    /// # use datafusion::logical_plan::col;
    /// let sort_expr = col("foo").sort(true, true); // SORT ASC NULLS_FIRST
    /// ```
    pub fn sort(self, asc: bool, nulls_first: bool) -> Expr {
        Expr::Sort {
            expr: Box::new(self),
            asc,
            nulls_first,
        }
    }

    /// Performs a depth first walk of an expression and
    /// its children, calling [`ExpressionVisitor::pre_visit`] and
    /// `visitor.post_visit`.
    ///
    /// Implements the [visitor pattern](https://en.wikipedia.org/wiki/Visitor_pattern) to
    /// separate expression algorithms from the structure of the
    /// `Expr` tree and make it easier to add new types of expressions
    /// and algorithms that walk the tree.
    ///
    /// For an expression tree such as
    /// ```text
    /// BinaryExpr (GT)
    ///    left: Column("foo")
    ///    right: Column("bar")
    /// ```
    ///
    /// The nodes are visited using the following order
    /// ```text
    /// pre_visit(BinaryExpr(GT))
    /// pre_visit(Column("foo"))
    /// pre_visit(Column("bar"))
    /// post_visit(Column("bar"))
    /// post_visit(Column("bar"))
    /// post_visit(BinaryExpr(GT))
    /// ```
    ///
    /// If an Err result is returned, recursion is stopped immediately
    ///
    /// If `Recursion::Stop` is returned on a call to pre_visit, no
    /// children of that expression are visited, nor is post_visit
    /// called on that expression
    ///
    pub fn accept<V: ExpressionVisitor>(&self, visitor: V) -> Result<V> {
        let visitor = match visitor.pre_visit(self)? {
            Recursion::Continue(visitor) => visitor,
            // If the recursion should stop, do not visit children
            Recursion::Stop(visitor) => return Ok(visitor),
        };

        // recurse (and cover all expression types)
        let visitor = match self {
            Expr::Alias(expr, _) => expr.accept(visitor),
            Expr::Column(..) => Ok(visitor),
            Expr::ScalarVariable(..) => Ok(visitor),
            Expr::Literal(..) => Ok(visitor),
            Expr::BinaryExpr { left, right, .. } => {
                let visitor = left.accept(visitor)?;
                right.accept(visitor)
            }
            Expr::Not(expr) => expr.accept(visitor),
            Expr::IsNotNull(expr) => expr.accept(visitor),
            Expr::IsNull(expr) => expr.accept(visitor),
            Expr::Negative(expr) => expr.accept(visitor),
            Expr::Between {
                expr, low, high, ..
            } => {
                let visitor = expr.accept(visitor)?;
                let visitor = low.accept(visitor)?;
                high.accept(visitor)
            }
            Expr::Case {
                expr,
                when_then_expr,
                else_expr,
            } => {
                let visitor = if let Some(expr) = expr.as_ref() {
                    expr.accept(visitor)
                } else {
                    Ok(visitor)
                }?;
                let visitor = when_then_expr.iter().try_fold(
                    visitor,
                    |visitor, (when, then)| {
                        let visitor = when.accept(visitor)?;
                        then.accept(visitor)
                    },
                )?;
                if let Some(else_expr) = else_expr.as_ref() {
                    else_expr.accept(visitor)
                } else {
                    Ok(visitor)
                }
            }
            Expr::Cast { expr, .. } => expr.accept(visitor),
            Expr::TryCast { expr, .. } => expr.accept(visitor),
            Expr::Sort { expr, .. } => expr.accept(visitor),
            Expr::ScalarFunction { args, .. } => args
                .iter()
                .try_fold(visitor, |visitor, arg| arg.accept(visitor)),
            Expr::ScalarUDF { args, .. } => args
                .iter()
                .try_fold(visitor, |visitor, arg| arg.accept(visitor)),
            Expr::AggregateFunction { args, .. } => args
                .iter()
                .try_fold(visitor, |visitor, arg| arg.accept(visitor)),
            Expr::AggregateUDF { args, .. } => args
                .iter()
                .try_fold(visitor, |visitor, arg| arg.accept(visitor)),
            Expr::InList { expr, list, .. } => {
                let visitor = expr.accept(visitor)?;
                list.iter()
                    .try_fold(visitor, |visitor, arg| arg.accept(visitor))
            }
            Expr::Wildcard => Ok(visitor),
        }?;

        visitor.post_visit(self)
    }

    /// Performs a depth first walk of an expression and its children
    /// to rewrite an expression, consuming `self` producing a new
    /// [`Expr`].
    ///
    /// Implements a modified version of the [visitor
    /// pattern](https://en.wikipedia.org/wiki/Visitor_pattern) to
    /// separate algorithms from the structure of the `Expr` tree and
    /// make it easier to write new, efficient expression
    /// transformation algorithms.
    ///
    /// For an expression tree such as
    /// ```text
    /// BinaryExpr (GT)
    ///    left: Column("foo")
    ///    right: Column("bar")
    /// ```
    ///
    /// The nodes are visited using the following order
    /// ```text
    /// pre_visit(BinaryExpr(GT))
    /// pre_visit(Column("foo"))
    /// mutatate(Column("foo"))
    /// pre_visit(Column("bar"))
    /// mutate(Column("bar"))
    /// mutate(BinaryExpr(GT))
    /// ```
    ///
    /// If an Err result is returned, recursion is stopped immediately
    ///
    /// If [`false`] is returned on a call to pre_visit, no
    /// children of that expression are visited, nor is mutate
    /// called on that expression
    ///
    pub fn rewrite<R>(self, rewriter: &mut R) -> Result<Self>
    where
        R: ExprRewriter,
    {
        if !rewriter.pre_visit(&self)? {
            return Ok(self);
        };

        // recurse into all sub expressions(and cover all expression types)
        let expr = match self {
            Expr::Alias(expr, name) => Expr::Alias(rewrite_boxed(expr, rewriter)?, name),
            Expr::Column(name) => Expr::Column(name),
            Expr::ScalarVariable(names) => Expr::ScalarVariable(names),
            Expr::Literal(value) => Expr::Literal(value),
            Expr::BinaryExpr { left, op, right } => Expr::BinaryExpr {
                left: rewrite_boxed(left, rewriter)?,
                op,
                right: rewrite_boxed(right, rewriter)?,
            },
            Expr::Not(expr) => Expr::Not(rewrite_boxed(expr, rewriter)?),
            Expr::IsNotNull(expr) => Expr::IsNotNull(rewrite_boxed(expr, rewriter)?),
            Expr::IsNull(expr) => Expr::IsNull(rewrite_boxed(expr, rewriter)?),
            Expr::Negative(expr) => Expr::Negative(rewrite_boxed(expr, rewriter)?),
            Expr::Between {
                expr,
                low,
                high,
                negated,
            } => Expr::Between {
                expr: rewrite_boxed(expr, rewriter)?,
                low: rewrite_boxed(low, rewriter)?,
                high: rewrite_boxed(high, rewriter)?,
                negated,
            },
            Expr::Case {
                expr,
                when_then_expr,
                else_expr,
            } => {
                let expr = rewrite_option_box(expr, rewriter)?;
                let when_then_expr = when_then_expr
                    .into_iter()
                    .map(|(when, then)| {
                        Ok((
                            rewrite_boxed(when, rewriter)?,
                            rewrite_boxed(then, rewriter)?,
                        ))
                    })
                    .collect::<Result<Vec<_>>>()?;

                let else_expr = rewrite_option_box(else_expr, rewriter)?;

                Expr::Case {
                    expr,
                    when_then_expr,
                    else_expr,
                }
            }
            Expr::Cast { expr, data_type } => Expr::Cast {
                expr: rewrite_boxed(expr, rewriter)?,
                data_type,
            },
            Expr::TryCast { expr, data_type } => Expr::TryCast {
                expr: rewrite_boxed(expr, rewriter)?,
                data_type,
            },
            Expr::Sort {
                expr,
                asc,
                nulls_first,
            } => Expr::Sort {
                expr: rewrite_boxed(expr, rewriter)?,
                asc,
                nulls_first,
            },
            Expr::ScalarFunction { args, fun } => Expr::ScalarFunction {
                args: rewrite_vec(args, rewriter)?,
                fun,
            },
            Expr::ScalarUDF { args, fun } => Expr::ScalarUDF {
                args: rewrite_vec(args, rewriter)?,
                fun,
            },
            Expr::AggregateFunction {
                args,
                fun,
                distinct,
            } => Expr::AggregateFunction {
                args: rewrite_vec(args, rewriter)?,
                fun,
                distinct,
            },
            Expr::AggregateUDF { args, fun } => Expr::AggregateUDF {
                args: rewrite_vec(args, rewriter)?,
                fun,
            },
            Expr::InList {
                expr,
                list,
                negated,
            } => Expr::InList {
                expr: rewrite_boxed(expr, rewriter)?,
                list,
                negated,
            },
            Expr::Wildcard => Expr::Wildcard,
        };

        // now rewrite this expression itself
        rewriter.mutate(expr)
    }
}

#[allow(clippy::boxed_local)]
fn rewrite_boxed<R>(boxed_expr: Box<Expr>, rewriter: &mut R) -> Result<Box<Expr>>
where
    R: ExprRewriter,
{
    // TODO: It might be possible to avoid an allocation (the
    // Box::new) below by reusing the box.
    let expr: Expr = *boxed_expr;
    let rewritten_expr = expr.rewrite(rewriter)?;
    Ok(Box::new(rewritten_expr))
}

fn rewrite_option_box<R>(
    option_box: Option<Box<Expr>>,
    rewriter: &mut R,
) -> Result<Option<Box<Expr>>>
where
    R: ExprRewriter,
{
    option_box
        .map(|expr| rewrite_boxed(expr, rewriter))
        .transpose()
}

/// rewrite a `Vec` of `Expr`s with the rewriter
fn rewrite_vec<R>(v: Vec<Expr>, rewriter: &mut R) -> Result<Vec<Expr>>
where
    R: ExprRewriter,
{
    v.into_iter().map(|expr| expr.rewrite(rewriter)).collect()
}

/// Controls how the visitor recursion should proceed.
pub enum Recursion<V: ExpressionVisitor> {
    /// Attempt to visit all the children, recursively, of this expression.
    Continue(V),
    /// Do not visit the children of this expression, though the walk
    /// of parents of this expression will not be affected
    Stop(V),
}

/// Encode the traversal of an expression tree. When passed to
/// `Expr::accept`, `ExpressionVisitor::visit` is invoked
/// recursively on all nodes of an expression tree. See the comments
/// on `Expr::accept` for details on its use
pub trait ExpressionVisitor: Sized {
    /// Invoked before any children of `expr` are visisted.
    fn pre_visit(self, expr: &Expr) -> Result<Recursion<Self>>;

    /// Invoked after all children of `expr` are visited. Default
    /// implementation does nothing.
    fn post_visit(self, _expr: &Expr) -> Result<Self> {
        Ok(self)
    }
}

/// Trait for potentially recursively rewriting an [`Expr`] expression
/// tree. When passed to `Expr::rewrite`, `ExpressionVisitor::mutate` is
/// invoked recursively on all nodes of an expression tree. See the
/// comments on `Expr::rewrite` for details on its use
pub trait ExprRewriter: Sized {
    /// Invoked before any children of `expr` are rewritten /
    /// visited. Default implementation returns `Ok(true)`
    fn pre_visit(&mut self, _expr: &Expr) -> Result<bool> {
        Ok(true)
    }

    /// Invoked after all children of `expr` have been mutated and
    /// returns a potentially modified expr.
    fn mutate(&mut self, expr: Expr) -> Result<Expr>;
}

pub struct CaseBuilder {
    expr: Option<Box<Expr>>,
    when_expr: Vec<Expr>,
    then_expr: Vec<Expr>,
    else_expr: Option<Box<Expr>>,
}

impl CaseBuilder {
    pub fn when(&mut self, when: Expr, then: Expr) -> CaseBuilder {
        self.when_expr.push(when);
        self.then_expr.push(then);
        CaseBuilder {
            expr: self.expr.clone(),
            when_expr: self.when_expr.clone(),
            then_expr: self.then_expr.clone(),
            else_expr: self.else_expr.clone(),
        }
    }
    pub fn otherwise(&mut self, else_expr: Expr) -> Result<Expr> {
        self.else_expr = Some(Box::new(else_expr));
        self.build()
    }

    pub fn end(&self) -> Result<Expr> {
        self.build()
    }
}

impl CaseBuilder {
    fn build(&self) -> Result<Expr> {
        // collect all "then" expressions
        let mut then_expr = self.then_expr.clone();
        if let Some(e) = &self.else_expr {
            then_expr.push(e.as_ref().to_owned());
        }

        let then_types: Vec<DataType> = then_expr
            .iter()
            .map(|e| match e {
                Expr::Literal(_) => e.get_type(&DFSchema::empty()),
                _ => Ok(DataType::Null),
            })
            .collect::<Result<Vec<_>>>()?;

        if then_types.contains(&DataType::Null) {
            // cannot verify types until execution type
        } else {
            let unique_types: HashSet<&DataType> = then_types.iter().collect();
            if unique_types.len() != 1 {
                return Err(DataFusionError::Plan(format!(
                    "CASE expression 'then' values had multiple data types: {:?}",
                    unique_types
                )));
            }
        }

        Ok(Expr::Case {
            expr: self.expr.clone(),
            when_then_expr: self
                .when_expr
                .iter()
                .zip(self.then_expr.iter())
                .map(|(w, t)| (Box::new(w.clone()), Box::new(t.clone())))
                .collect(),
            else_expr: self.else_expr.clone(),
        })
    }
}

/// Create a CASE WHEN statement with literal WHEN expressions for comparison to the base expression.
pub fn case(expr: Expr) -> CaseBuilder {
    CaseBuilder {
        expr: Some(Box::new(expr)),
        when_expr: vec![],
        then_expr: vec![],
        else_expr: None,
    }
}

/// Create a CASE WHEN statement with boolean WHEN expressions and no base expression.
pub fn when(when: Expr, then: Expr) -> CaseBuilder {
    CaseBuilder {
        expr: None,
        when_expr: vec![when],
        then_expr: vec![then],
        else_expr: None,
    }
}

/// return a new expression l <op> r
pub fn binary_expr(l: Expr, op: Operator, r: Expr) -> Expr {
    Expr::BinaryExpr {
        left: Box::new(l),
        op,
        right: Box::new(r),
    }
}

/// return a new expression with a logical AND
pub fn and(left: Expr, right: Expr) -> Expr {
    Expr::BinaryExpr {
        left: Box::new(left),
        op: Operator::And,
        right: Box::new(right),
    }
}

/// Combines an array of filter expressions into a single filter expression
/// consisting of the input filter expressions joined with logical AND.
/// Returns None if the filters array is empty.
pub fn combine_filters(filters: &[Expr]) -> Option<Expr> {
    if filters.is_empty() {
        return None;
    }
    let combined_filter = filters
        .iter()
        .skip(1)
        .fold(filters[0].clone(), |acc, filter| and(acc, filter.clone()));
    Some(combined_filter)
}

/// return a new expression with a logical OR
pub fn or(left: Expr, right: Expr) -> Expr {
    Expr::BinaryExpr {
        left: Box::new(left),
        op: Operator::Or,
        right: Box::new(right),
    }
}

/// Create a column expression based on a column name
pub fn col(name: &str) -> Expr {
    Expr::Column(name.to_owned())
}

/// Create an expression to represent the min() aggregate function
pub fn min(expr: Expr) -> Expr {
    Expr::AggregateFunction {
        fun: aggregates::AggregateFunction::Min,
        distinct: false,
        args: vec![expr],
    }
}

/// Create an expression to represent the max() aggregate function
pub fn max(expr: Expr) -> Expr {
    Expr::AggregateFunction {
        fun: aggregates::AggregateFunction::Max,
        distinct: false,
        args: vec![expr],
    }
}

/// Create an expression to represent the sum() aggregate function
pub fn sum(expr: Expr) -> Expr {
    Expr::AggregateFunction {
        fun: aggregates::AggregateFunction::Sum,
        distinct: false,
        args: vec![expr],
    }
}

/// Create an expression to represent the avg() aggregate function
pub fn avg(expr: Expr) -> Expr {
    Expr::AggregateFunction {
        fun: aggregates::AggregateFunction::Avg,
        distinct: false,
        args: vec![expr],
    }
}

/// Create an expression to represent the count() aggregate function
pub fn count(expr: Expr) -> Expr {
    Expr::AggregateFunction {
        fun: aggregates::AggregateFunction::Count,
        distinct: false,
        args: vec![expr],
    }
}

/// Create an expression to represent the count(distinct) aggregate function
pub fn count_distinct(expr: Expr) -> Expr {
    Expr::AggregateFunction {
        fun: aggregates::AggregateFunction::Count,
        distinct: true,
        args: vec![expr],
    }
}

/// Create an in_list expression
pub fn in_list(expr: Expr, list: Vec<Expr>, negated: bool) -> Expr {
    Expr::InList {
        expr: Box::new(expr),
        list,
        negated,
    }
}

/// Trait for converting a type to a [`Literal`] literal expression.
pub trait Literal {
    /// convert the value to a Literal expression
    fn lit(&self) -> Expr;
}

impl Literal for &str {
    fn lit(&self) -> Expr {
        Expr::Literal(ScalarValue::Utf8(Some((*self).to_owned())))
    }
}

impl Literal for String {
    fn lit(&self) -> Expr {
        Expr::Literal(ScalarValue::Utf8(Some((*self).to_owned())))
    }
}

impl Literal for ScalarValue {
    fn lit(&self) -> Expr {
        Expr::Literal(self.clone())
    }
}

macro_rules! make_literal {
    ($TYPE:ty, $SCALAR:ident) => {
        #[allow(missing_docs)]
        impl Literal for $TYPE {
            fn lit(&self) -> Expr {
                Expr::Literal(ScalarValue::$SCALAR(Some(self.clone())))
            }
        }
    };
}

make_literal!(bool, Boolean);
make_literal!(f32, Float32);
make_literal!(f64, Float64);
make_literal!(i8, Int8);
make_literal!(i16, Int16);
make_literal!(i32, Int32);
make_literal!(i64, Int64);
make_literal!(u8, UInt8);
make_literal!(u16, UInt16);
make_literal!(u32, UInt32);
make_literal!(u64, UInt64);

/// Create a literal expression
pub fn lit<T: Literal>(n: T) -> Expr {
    n.lit()
}

/// Create an convenience function representing a unary scalar function
macro_rules! unary_scalar_expr {
    ($ENUM:ident, $FUNC:ident) => {
        #[allow(missing_docs)]
        pub fn $FUNC(e: Expr) -> Expr {
            Expr::ScalarFunction {
                fun: functions::BuiltinScalarFunction::$ENUM,
                args: vec![e],
            }
        }
    };
}

// generate methods for creating the supported unary expressions

// math functions
unary_scalar_expr!(Sqrt, sqrt);
unary_scalar_expr!(Sin, sin);
unary_scalar_expr!(Cos, cos);
unary_scalar_expr!(Tan, tan);
unary_scalar_expr!(Asin, asin);
unary_scalar_expr!(Acos, acos);
unary_scalar_expr!(Atan, atan);
unary_scalar_expr!(Floor, floor);
unary_scalar_expr!(Ceil, ceil);
unary_scalar_expr!(Round, round);
unary_scalar_expr!(Trunc, trunc);
unary_scalar_expr!(Abs, abs);
unary_scalar_expr!(Signum, signum);
unary_scalar_expr!(Exp, exp);
unary_scalar_expr!(Log, ln);
unary_scalar_expr!(Log2, log2);
unary_scalar_expr!(Log10, log10);

// string functions
unary_scalar_expr!(Ascii, ascii);
unary_scalar_expr!(BitLength, bit_length);
unary_scalar_expr!(Btrim, btrim);
unary_scalar_expr!(CharacterLength, character_length);
unary_scalar_expr!(CharacterLength, length);
unary_scalar_expr!(Chr, chr);
unary_scalar_expr!(Concat, concat);
unary_scalar_expr!(ConcatWithSeparator, concat_ws);
unary_scalar_expr!(InitCap, initcap);
unary_scalar_expr!(Left, left);
unary_scalar_expr!(Lower, lower);
unary_scalar_expr!(Lpad, lpad);
unary_scalar_expr!(Ltrim, ltrim);
unary_scalar_expr!(MD5, md5);
unary_scalar_expr!(OctetLength, octet_length);
unary_scalar_expr!(RegexpMatch, regexp_match);
unary_scalar_expr!(RegexpReplace, regexp_replace);
unary_scalar_expr!(Replace, replace);
unary_scalar_expr!(Repeat, repeat);
unary_scalar_expr!(Reverse, reverse);
unary_scalar_expr!(Right, right);
unary_scalar_expr!(Rpad, rpad);
unary_scalar_expr!(Rtrim, rtrim);
unary_scalar_expr!(SHA224, sha224);
unary_scalar_expr!(SHA256, sha256);
unary_scalar_expr!(SHA384, sha384);
unary_scalar_expr!(SHA512, sha512);
unary_scalar_expr!(SplitPart, split_part);
unary_scalar_expr!(StartsWith, starts_with);
unary_scalar_expr!(Strpos, strpos);
unary_scalar_expr!(Substr, substr);
unary_scalar_expr!(ToHex, to_hex);
unary_scalar_expr!(Translate, translate);
unary_scalar_expr!(Trim, trim);
unary_scalar_expr!(Upper, upper);

/// returns an array of fixed size with each argument on it.
pub fn array(args: Vec<Expr>) -> Expr {
    Expr::ScalarFunction {
        fun: functions::BuiltinScalarFunction::Array,
        args,
    }
}

/// Creates a new UDF with a specific signature and specific return type.
/// This is a helper function to create a new UDF.
/// The function `create_udf` returns a subset of all possible `ScalarFunction`:
/// * the UDF has a fixed return type
/// * the UDF has a fixed signature (e.g. [f64, f64])
pub fn create_udf(
    name: &str,
    input_types: Vec<DataType>,
    return_type: Arc<DataType>,
    fun: ScalarFunctionImplementation,
) -> ScalarUDF {
    let return_type: ReturnTypeFunction = Arc::new(move |_| Ok(return_type.clone()));
    ScalarUDF::new(name, &Signature::Exact(input_types), &return_type, &fun)
}

/// Creates a new UDAF with a specific signature, state type and return type.
/// The signature and state type must match the `Acumulator's implementation`.
#[allow(clippy::rc_buffer)]
pub fn create_udaf(
    name: &str,
    input_type: DataType,
    return_type: Arc<DataType>,
    accumulator: AccumulatorFunctionImplementation,
    state_type: Arc<Vec<DataType>>,
) -> AggregateUDF {
    let return_type: ReturnTypeFunction = Arc::new(move |_| Ok(return_type.clone()));
    let state_type: StateTypeFunction = Arc::new(move |_| Ok(state_type.clone()));
    AggregateUDF::new(
        name,
        &Signature::Exact(vec![input_type]),
        &return_type,
        &accumulator,
        &state_type,
    )
}

fn fmt_function(
    f: &mut fmt::Formatter,
    fun: &str,
    distinct: bool,
    args: &[Expr],
) -> fmt::Result {
    let args: Vec<String> = args.iter().map(|arg| format!("{:?}", arg)).collect();
    let distinct_str = match distinct {
        true => "DISTINCT ",
        false => "",
    };
    write!(f, "{}({}{})", fun, distinct_str, args.join(", "))
}

impl fmt::Debug for Expr {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match self {
            Expr::Alias(expr, alias) => write!(f, "{:?} AS {}", expr, alias),
            Expr::Column(name) => write!(f, "#{}", name),
            Expr::ScalarVariable(var_names) => write!(f, "{}", var_names.join(".")),
            Expr::Literal(v) => write!(f, "{:?}", v),
            Expr::Case {
                expr,
                when_then_expr,
                else_expr,
                ..
            } => {
                write!(f, "CASE ")?;
                if let Some(e) = expr {
                    write!(f, "{:?} ", e)?;
                }
                for (w, t) in when_then_expr {
                    write!(f, "WHEN {:?} THEN {:?} ", w, t)?;
                }
                if let Some(e) = else_expr {
                    write!(f, "ELSE {:?} ", e)?;
                }
                write!(f, "END")
            }
            Expr::Cast { expr, data_type } => {
                write!(f, "CAST({:?} AS {:?})", expr, data_type)
            }
            Expr::TryCast { expr, data_type } => {
                write!(f, "TRY_CAST({:?} AS {:?})", expr, data_type)
            }
            Expr::Not(expr) => write!(f, "NOT {:?}", expr),
            Expr::Negative(expr) => write!(f, "(- {:?})", expr),
            Expr::IsNull(expr) => write!(f, "{:?} IS NULL", expr),
            Expr::IsNotNull(expr) => write!(f, "{:?} IS NOT NULL", expr),
            Expr::BinaryExpr { left, op, right } => {
                write!(f, "{:?} {:?} {:?}", left, op, right)
            }
            Expr::Sort {
                expr,
                asc,
                nulls_first,
            } => {
                if *asc {
                    write!(f, "{:?} ASC", expr)?;
                } else {
                    write!(f, "{:?} DESC", expr)?;
                }
                if *nulls_first {
                    write!(f, " NULLS FIRST")
                } else {
                    write!(f, " NULLS LAST")
                }
            }
            Expr::ScalarFunction { fun, args, .. } => {
                fmt_function(f, &fun.to_string(), false, args)
            }
            Expr::ScalarUDF { fun, ref args, .. } => {
                fmt_function(f, &fun.name, false, args)
            }
            Expr::AggregateFunction {
                fun,
                distinct,
                ref args,
                ..
            } => fmt_function(f, &fun.to_string(), *distinct, args),
            Expr::AggregateUDF { fun, ref args, .. } => {
                fmt_function(f, &fun.name, false, args)
            }
            Expr::Between {
                expr,
                negated,
                low,
                high,
            } => {
                if *negated {
                    write!(f, "{:?} NOT BETWEEN {:?} AND {:?}", expr, low, high)
                } else {
                    write!(f, "{:?} BETWEEN {:?} AND {:?}", expr, low, high)
                }
            }
            Expr::InList {
                expr,
                list,
                negated,
            } => {
                if *negated {
                    write!(f, "{:?} NOT IN ({:?})", expr, list)
                } else {
                    write!(f, "{:?} IN ({:?})", expr, list)
                }
            }
            Expr::Wildcard => write!(f, "*"),
        }
    }
}

fn create_function_name(
    fun: &str,
    distinct: bool,
    args: &[Expr],
    input_schema: &DFSchema,
) -> Result<String> {
    let names: Vec<String> = args
        .iter()
        .map(|e| create_name(e, input_schema))
        .collect::<Result<_>>()?;
    let distinct_str = match distinct {
        true => "DISTINCT ",
        false => "",
    };
    Ok(format!("{}({}{})", fun, distinct_str, names.join(",")))
}

/// Returns a readable name of an expression based on the input schema.
/// This function recursively transverses the expression for names such as "CAST(a > 2)".
fn create_name(e: &Expr, input_schema: &DFSchema) -> Result<String> {
    match e {
        Expr::Alias(_, name) => Ok(name.clone()),
        Expr::Column(name) => Ok(name.clone()),
        Expr::ScalarVariable(variable_names) => Ok(variable_names.join(".")),
        Expr::Literal(value) => Ok(format!("{:?}", value)),
        Expr::BinaryExpr { left, op, right } => {
            let left = create_name(left, input_schema)?;
            let right = create_name(right, input_schema)?;
            Ok(format!("{} {:?} {}", left, op, right))
        }
        Expr::Case {
            expr,
            when_then_expr,
            else_expr,
        } => {
            let mut name = "CASE ".to_string();
            if let Some(e) = expr {
                name += &format!("{:?} ", e);
            }
            for (w, t) in when_then_expr {
                name += &format!("WHEN {:?} THEN {:?} ", w, t);
            }
            if let Some(e) = else_expr {
                name += &format!("ELSE {:?} ", e);
            }
            name += "END";
            Ok(name)
        }
        Expr::Cast { expr, data_type } => {
            let expr = create_name(expr, input_schema)?;
            Ok(format!("CAST({} AS {:?})", expr, data_type))
        }
        Expr::TryCast { expr, data_type } => {
            let expr = create_name(expr, input_schema)?;
            Ok(format!("TRY_CAST({} AS {:?})", expr, data_type))
        }
        Expr::Not(expr) => {
            let expr = create_name(expr, input_schema)?;
            Ok(format!("NOT {}", expr))
        }
        Expr::Negative(expr) => {
            let expr = create_name(expr, input_schema)?;
            Ok(format!("(- {})", expr))
        }
        Expr::IsNull(expr) => {
            let expr = create_name(expr, input_schema)?;
            Ok(format!("{} IS NULL", expr))
        }
        Expr::IsNotNull(expr) => {
            let expr = create_name(expr, input_schema)?;
            Ok(format!("{} IS NOT NULL", expr))
        }
        Expr::ScalarFunction { fun, args, .. } => {
            create_function_name(&fun.to_string(), false, args, input_schema)
        }
        Expr::ScalarUDF { fun, args, .. } => {
            create_function_name(&fun.name, false, args, input_schema)
        }
        Expr::AggregateFunction {
            fun,
            distinct,
            args,
            ..
        } => create_function_name(&fun.to_string(), *distinct, args, input_schema),
        Expr::AggregateUDF { fun, args } => {
            let mut names = Vec::with_capacity(args.len());
            for e in args {
                names.push(create_name(e, input_schema)?);
            }
            Ok(format!("{}({})", fun.name, names.join(",")))
        }
        Expr::InList {
            expr,
            list,
            negated,
        } => {
            let expr = create_name(expr, input_schema)?;
            let list = list.iter().map(|expr| create_name(expr, input_schema));
            if *negated {
                Ok(format!("{} NOT IN ({:?})", expr, list))
            } else {
                Ok(format!("{} IN ({:?})", expr, list))
            }
        }
        other => Err(DataFusionError::NotImplemented(format!(
            "Physical plan does not support logical expression {:?}",
            other
        ))),
    }
}

/// Create field meta-data from an expression, for use in a result set schema
pub fn exprlist_to_fields<'a>(
    expr: impl IntoIterator<Item = &'a Expr>,
    input_schema: &DFSchema,
) -> Result<Vec<DFField>> {
    expr.into_iter().map(|e| e.to_field(input_schema)).collect()
}

#[cfg(test)]
mod tests {
    use super::super::{col, lit, when};
    use super::*;

    #[test]
    fn case_when_same_literal_then_types() -> Result<()> {
        let _ = when(col("state").eq(lit("CO")), lit(303))
            .when(col("state").eq(lit("NY")), lit(212))
            .end()?;
        Ok(())
    }

    #[test]
    fn case_when_different_literal_then_types() {
        let maybe_expr = when(col("state").eq(lit("CO")), lit(303))
            .when(col("state").eq(lit("NY")), lit("212"))
            .end();
        assert!(maybe_expr.is_err());
    }

    #[test]
    fn rewriter_visit() {
        let mut rewriter = RecordingRewriter::default();
        col("state").eq(lit("CO")).rewrite(&mut rewriter).unwrap();

        assert_eq!(
            rewriter.v,
            vec![
                "Previsited #state Eq Utf8(\"CO\")",
                "Previsited #state",
                "Mutated #state",
                "Previsited Utf8(\"CO\")",
                "Mutated Utf8(\"CO\")",
                "Mutated #state Eq Utf8(\"CO\")"
            ]
        )
    }

    #[test]
    fn filter_is_null_and_is_not_null() {
        let col_null = Expr::Column("col1".to_string());
        let col_not_null = Expr::Column("col2".to_string());
        assert_eq!(format!("{:?}", col_null.is_null()), "#col1 IS NULL");
        assert_eq!(
            format!("{:?}", col_not_null.is_not_null()),
            "#col2 IS NOT NULL"
        );
    }

    #[derive(Default)]
    struct RecordingRewriter {
        v: Vec<String>,
    }
    impl ExprRewriter for RecordingRewriter {
        fn mutate(&mut self, expr: Expr) -> Result<Expr> {
            self.v.push(format!("Mutated {:?}", expr));
            Ok(expr)
        }

        fn pre_visit(&mut self, expr: &Expr) -> Result<bool> {
            self.v.push(format!("Previsited {:?}", expr));
            Ok(true)
        }
    }

    #[test]
    fn rewriter_rewrite() {
        let mut rewriter = FooBarRewriter {};

        // rewrites "foo" --> "bar"
        let rewritten = col("state").eq(lit("foo")).rewrite(&mut rewriter).unwrap();
        assert_eq!(rewritten, col("state").eq(lit("bar")));

        // doesn't wrewrite
        let rewritten = col("state").eq(lit("baz")).rewrite(&mut rewriter).unwrap();
        assert_eq!(rewritten, col("state").eq(lit("baz")));
    }

    /// rewrites all "foo" string literals to "bar"
    struct FooBarRewriter {}
    impl ExprRewriter for FooBarRewriter {
        fn mutate(&mut self, expr: Expr) -> Result<Expr> {
            match expr {
                Expr::Literal(scalar) => {
                    if let ScalarValue::Utf8(Some(utf8_val)) = scalar {
                        let utf8_val = if utf8_val == "foo" {
                            "bar".to_string()
                        } else {
                            utf8_val
                        };
                        Ok(lit(utf8_val))
                    } else {
                        Ok(Expr::Literal(scalar))
                    }
                }
                // otherwise, return the expression unchanged
                expr => Ok(expr),
            }
        }
    }
}