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// 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.
//! Expression visitor
use crate::{expr::GroupingSet, Expr};
use datafusion_common::Result;
/// 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<E: ExprVisitable = Expr>: Sized {
/// Invoked before any children of `expr` are visisted.
fn pre_visit(self, expr: &E) -> Result<Recursion<Self>>
where
Self: ExpressionVisitor;
/// Invoked after all children of `expr` are visited. Default
/// implementation does nothing.
fn post_visit(self, _expr: &E) -> Result<Self> {
Ok(self)
}
}
/// trait for types that can be visited by [`ExpressionVisitor`]
pub trait ExprVisitable: Sized {
/// accept a visitor, calling `visit` on all children of this
fn accept<V: ExpressionVisitor<Self>>(&self, visitor: V) -> Result<V>;
}
impl ExprVisitable for Expr {
/// 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"))
/// post_visit(Column("foo"))
/// pre_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
///
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::Not(expr)
| Expr::IsNotNull(expr)
| Expr::IsNull(expr)
| Expr::Negative(expr)
| Expr::Cast { expr, .. }
| Expr::TryCast { expr, .. }
| Expr::Sort { expr, .. }
| Expr::InSubquery { expr, .. }
| Expr::GetIndexedField { expr, .. } => expr.accept(visitor),
Expr::GroupingSet(GroupingSet::Rollup(exprs)) => exprs
.iter()
.fold(Ok(visitor), |v, e| v.and_then(|v| e.accept(v))),
Expr::GroupingSet(GroupingSet::Cube(exprs)) => exprs
.iter()
.fold(Ok(visitor), |v, e| v.and_then(|v| e.accept(v))),
Expr::GroupingSet(GroupingSet::GroupingSets(lists_of_exprs)) => {
lists_of_exprs.iter().fold(Ok(visitor), |v, exprs| {
v.and_then(|v| {
exprs.iter().fold(Ok(v), |v, e| v.and_then(|v| e.accept(v)))
})
})
}
Expr::Column(_)
| Expr::ScalarVariable(_, _)
| Expr::Literal(_)
| Expr::Exists { .. }
| Expr::ScalarSubquery(_)
| Expr::Wildcard
| Expr::QualifiedWildcard { .. } => Ok(visitor),
Expr::BinaryExpr { left, right, .. } => {
let visitor = left.accept(visitor)?;
right.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::ScalarFunction { args, .. }
| Expr::ScalarUDF { args, .. }
| Expr::AggregateFunction { args, .. }
| Expr::AggregateUDF { args, .. } => args
.iter()
.try_fold(visitor, |visitor, arg| arg.accept(visitor)),
Expr::WindowFunction {
args,
partition_by,
order_by,
..
} => {
let visitor = args
.iter()
.try_fold(visitor, |visitor, arg| arg.accept(visitor))?;
let visitor = partition_by
.iter()
.try_fold(visitor, |visitor, arg| arg.accept(visitor))?;
let visitor = order_by
.iter()
.try_fold(visitor, |visitor, arg| arg.accept(visitor))?;
Ok(visitor)
}
Expr::InList { expr, list, .. } => {
let visitor = expr.accept(visitor)?;
list.iter()
.try_fold(visitor, |visitor, arg| arg.accept(visitor))
}
}?;
visitor.post_visit(self)
}
}