refactor TreeNode::rewrite()

Author: peter-toth

datafusion-examples/examples/rewrite_expr.rs

@@ -91,7 +91,7 @@ impl AnalyzerRule for MyAnalyzerRule {
 
 impl MyAnalyzerRule {
     fn analyze_plan(plan: LogicalPlan) -> Result<LogicalPlan> {
-        plan.transform(&|plan| {
+        plan.transform_up(&|plan| {
             Ok(match plan {
                 LogicalPlan::Filter(filter) => {
                     let predicate = Self::analyze_expr(filter.predicate.clone())?;
@@ -106,7 +106,7 @@ impl MyAnalyzerRule {
     }
 
     fn analyze_expr(expr: Expr) -> Result<Expr> {
-        expr.transform(&|expr| {
+        expr.transform_up(&|expr| {
             // closure is invoked for all sub expressions
             Ok(match expr {
                 Expr::Literal(ScalarValue::Int64(i)) => {
@@ -161,7 +161,7 @@ impl OptimizerRule for MyOptimizerRule {
 
 /// use rewrite_expr to modify the expression tree.
 fn my_rewrite(expr: Expr) -> Result<Expr> {
-    expr.transform(&|expr| {
+    expr.transform_up(&|expr| {
         // closure is invoked for all sub expressions
         Ok(match expr {
             Expr::Between(Between {

datafusion/common/src/tree_node.rs

@@ -31,12 +31,12 @@ use crate::Result;
 macro_rules! handle_tree_recursion {
     ($EXPR:expr) => {
         match $EXPR {
-            VisitRecursion::Continue => {}
+            TreeNodeRecursion::Continue => {}
             // If the recursion should skip, do not apply to its children, let
             // the recursion continue:
-            VisitRecursion::Skip => return Ok(VisitRecursion::Continue),
+            TreeNodeRecursion::Skip => return Ok(TreeNodeRecursion::Continue),
             // If the recursion should stop, do not apply to its children:
-            VisitRecursion::Stop => return Ok(VisitRecursion::Stop),
+            TreeNodeRecursion::Stop => return Ok(TreeNodeRecursion::Stop),
         }
     };
 }
@@ -58,10 +58,10 @@ pub trait TreeNode: Sized {
     ///
     /// The `op` closure can be used to collect some info from the
     /// tree node or do some checking for the tree node.
-    fn apply<F: FnMut(&Self) -> Result<VisitRecursion>>(
+    fn apply<F: FnMut(&Self) -> Result<TreeNodeRecursion>>(
         &self,
         op: &mut F,
-    ) -> Result<VisitRecursion> {
+    ) -> Result<TreeNodeRecursion> {
         handle_tree_recursion!(op(self)?);
         self.apply_children(&mut |node| node.apply(op))
     }
@@ -88,7 +88,7 @@ pub trait TreeNode: Sized {
     ///
     /// If an Err result is returned, recursion is stopped immediately
     ///
-    /// If [`VisitRecursion::Stop`] is returned on a call to pre_visit, no
+    /// If [`TreeNodeRecursion::Stop`] is returned on a call to pre_visit, no
     /// children of that node will be visited, nor is post_visit
     /// called on that node. Details see [`TreeNodeVisitor`]
     ///
@@ -97,20 +97,53 @@ pub trait TreeNode: Sized {
     fn visit<V: TreeNodeVisitor<N = Self>>(
         &self,
         visitor: &mut V,
-    ) -> Result<VisitRecursion> {
+    ) -> Result<TreeNodeRecursion> {
         handle_tree_recursion!(visitor.pre_visit(self)?);
         handle_tree_recursion!(self.apply_children(&mut |node| node.visit(visitor))?);
         visitor.post_visit(self)
     }
 
-    /// Convenience utils for writing optimizers rule: recursively apply the given `op` to the node tree.
-    /// When `op` does not apply to a given node, it is left unchanged.
-    /// The default tree traversal direction is transform_up(Postorder Traversal).
-    fn transform<F>(self, op: &F) -> Result<Self>
+    /// Transforms the tree using `f_down` while traversing the tree top-down
+    /// (pre-preorder) and using `f_up` while traversing the tree bottom-up (post-order).
+    ///
+    /// E.g. for an tree such as:
+    /// ```text
+    /// ParentNode
+    ///    left: ChildNode1
+    ///    right: ChildNode2
+    /// ```
+    ///
+    /// The nodes are visited using the following order:
+    /// ```text
+    /// f_down(ParentNode)
+    /// f_down(ChildNode1)
+    /// f_up(ChildNode1)
+    /// f_down(ChildNode2)
+    /// f_up(ChildNode2)
+    /// f_up(ParentNode)
+    /// ```
+    ///
+    /// See [`TreeNodeRecursion`] for more details on how the traversal can be controlled.
+    ///
+    /// If `f_down` or `f_up` returns [`Err`], recursion is stopped immediately.
+    fn transform<FD, FU>(self, f_down: &mut FD, f_up: &mut FU) -> Result<Self>
     where
-        F: Fn(Self) -> Result<Transformed<Self>>,
+        FD: FnMut(Self) -> Result<(Transformed<Self>, TreeNodeRecursion)>,
+        FU: FnMut(Self) -> Result<Self>,
     {
-        self.transform_up(op)
+        let (new_node, tnr) = f_down(self).map(|(t, tnr)| (t.into(), tnr))?;
+        match tnr {
+            TreeNodeRecursion::Continue => {}
+            // If the recursion should skip, do not apply to its children. And let the recursion continue
+            TreeNodeRecursion::Skip => return Ok(new_node),
+            // If the recursion should stop, do not apply to its children
+            TreeNodeRecursion::Stop => {
+                panic!("Stop can't be used in TreeNode::transform()")
+            }
+        }
+        let node_with_new_children =
+            new_node.map_children(|node| node.transform(f_down, f_up))?;
+        f_up(node_with_new_children)
     }
 
     /// Convenience utils for writing optimizers rule: recursively apply the given 'op' to the node and all of its
@@ -159,56 +192,50 @@ pub trait TreeNode: Sized {
         Ok(new_node)
     }
 
-    /// Transform the tree node using the given [TreeNodeRewriter]
-    /// It performs a depth first walk of an node and its children.
+    /// Implements the [visitor pattern](https://en.wikipedia.org/wiki/Visitor_pattern) for
+    /// recursively transforming [`TreeNode`]s.
     ///
-    /// For an node tree such as
+    /// E.g. for an tree such as:
     /// ```text
     /// ParentNode
     ///    left: ChildNode1
     ///    right: ChildNode2
     /// ```
     ///
-    /// The nodes are visited using the following order
+    /// The nodes are visited using the following order:
     /// ```text
-    /// pre_visit(ParentNode)
-    /// pre_visit(ChildNode1)
-    /// mutate(ChildNode1)
-    /// pre_visit(ChildNode2)
-    /// mutate(ChildNode2)
-    /// mutate(ParentNode)
+    /// TreeNodeRewriter::f_down(ParentNode)
+    /// TreeNodeRewriter::f_down(ChildNode1)
+    /// TreeNodeRewriter::f_up(ChildNode1)
+    /// TreeNodeRewriter::f_down(ChildNode2)
+    /// TreeNodeRewriter::f_up(ChildNode2)
+    /// TreeNodeRewriter::f_up(ParentNode)
     /// ```
     ///
-    /// If an Err result is returned, recursion is stopped immediately
-    ///
-    /// If [`false`] is returned on a call to pre_visit, no
-    /// children of that node will be visited, nor is mutate
-    /// called on that node
+    /// See [`TreeNodeRecursion`] for more details on how the traversal can be controlled.
     ///
-    /// If using the default [`TreeNodeRewriter::pre_visit`] which
-    /// returns `true`, [`Self::transform`] should be preferred.
-    fn rewrite<R: TreeNodeRewriter<N = Self>>(self, rewriter: &mut R) -> Result<Self> {
-        let need_mutate = match rewriter.pre_visit(&self)? {
-            RewriteRecursion::Mutate => return rewriter.mutate(self),
-            RewriteRecursion::Stop => return Ok(self),
-            RewriteRecursion::Continue => true,
-            RewriteRecursion::Skip => false,
-        };
-
-        let after_op_children = self.map_children(|node| node.rewrite(rewriter))?;
-
-        // now rewrite this node itself
-        if need_mutate {
-            rewriter.mutate(after_op_children)
-        } else {
-            Ok(after_op_children)
+    /// If [`TreeNodeRewriter::f_down()`] or [`TreeNodeRewriter::f_up()`] returns [`Err`],
+    /// recursion is stopped immediately.
+    fn rewrite<R: TreeNodeRewriter<Node = Self>>(self, rewriter: &mut R) -> Result<Self> {
+        let (new_node, tnr) = rewriter.f_down(self)?;
+        match tnr {
+            TreeNodeRecursion::Continue => {}
+            // If the recursion should skip, do not apply to its children. And let the recursion continue
+            TreeNodeRecursion::Skip => return Ok(new_node),
+            // If the recursion should stop, do not apply to its children
+            TreeNodeRecursion::Stop => {
+                panic!("Stop can't be used in TreeNode::rewrite()")
+            }
         }
+        let node_with_new_children =
+            new_node.map_children(|node| node.rewrite(rewriter))?;
+        rewriter.f_up(node_with_new_children)
     }
 
     /// Apply the closure `F` to the node's children
-    fn apply_children<F>(&self, op: &mut F) -> Result<VisitRecursion>
+    fn apply_children<F>(&self, op: &mut F) -> Result<TreeNodeRecursion>
     where
-        F: FnMut(&Self) -> Result<VisitRecursion>;
+        F: FnMut(&Self) -> Result<TreeNodeRecursion>;
 
     /// Apply transform `F` to the node's children, the transform `F` might have a direction(Preorder or Postorder)
     fn map_children<F>(self, transform: F) -> Result<Self>
@@ -231,69 +258,58 @@ pub trait TreeNode: Sized {
 /// If an [`Err`] result is returned, recursion is stopped
 /// immediately.
 ///
-/// If [`VisitRecursion::Stop`] is returned on a call to pre_visit, no
+/// If [`TreeNodeRecursion::Stop`] is returned on a call to pre_visit, no
 /// children of that tree node are visited, nor is post_visit
 /// called on that tree node
 ///
-/// If [`VisitRecursion::Stop`] is returned on a call to post_visit, no
+/// If [`TreeNodeRecursion::Stop`] is returned on a call to post_visit, no
 /// siblings of that tree node are visited, nor is post_visit
 /// called on its parent tree node
 ///
-/// If [`VisitRecursion::Skip`] is returned on a call to pre_visit, no
+/// If [`TreeNodeRecursion::Skip`] is returned on a call to pre_visit, no
 /// children of that tree node are visited.
 pub trait TreeNodeVisitor: Sized {
     /// The node type which is visitable.
     type N: TreeNode;
 
     /// Invoked before any children of `node` are visited.
-    fn pre_visit(&mut self, node: &Self::N) -> Result<VisitRecursion>;
+    fn pre_visit(&mut self, node: &Self::N) -> Result<TreeNodeRecursion>;
 
     /// Invoked after all children of `node` are visited. Default
     /// implementation does nothing.
-    fn post_visit(&mut self, _node: &Self::N) -> Result<VisitRecursion> {
-        Ok(VisitRecursion::Continue)
+    fn post_visit(&mut self, _node: &Self::N) -> Result<TreeNodeRecursion> {
+        Ok(TreeNodeRecursion::Continue)
     }
 }
 
-/// Trait for potentially recursively transform an [`TreeNode`] node
-/// tree. When passed to `TreeNode::rewrite`, `TreeNodeRewriter::mutate` is
-/// invoked recursively on all nodes of a tree.
+/// Trait for potentially recursively transform a [`TreeNode`] node tree.
 pub trait TreeNodeRewriter: Sized {
     /// The node type which is rewritable.
-    type N: TreeNode;
+    type Node: TreeNode;
 
-    /// Invoked before (Preorder) any children of `node` are rewritten /
-    /// visited. Default implementation returns `Ok(Recursion::Continue)`
-    fn pre_visit(&mut self, _node: &Self::N) -> Result<RewriteRecursion> {
-        Ok(RewriteRecursion::Continue)
+    /// Invoked while traversing down the tree before any children are rewritten /
+    /// visited.
+    /// Default implementation returns the node unmodified and continues recursion.
+    fn f_down(&mut self, node: Self::Node) -> Result<(Self::Node, TreeNodeRecursion)> {
+        Ok((node, TreeNodeRecursion::Continue))
     }
 
-    /// Invoked after (Postorder) all children of `node` have been mutated and
-    /// returns a potentially modified node.
-    fn mutate(&mut self, node: Self::N) -> Result<Self::N>;
-}
-
-/// Controls how the [`TreeNode`] recursion should proceed for [`TreeNode::rewrite`].
-#[derive(Debug)]
-pub enum RewriteRecursion {
-    /// Continue rewrite this node tree.
-    Continue,
-    /// Call 'op' immediately and return.
-    Mutate,
-    /// Do not rewrite the children of this node.
-    Stop,
-    /// Keep recursive but skip apply op on this node
-    Skip,
+    /// Invoked while traversing up the tree after all children have been rewritten /
+    /// visited.
+    /// Default implementation returns the node unmodified.
+    fn f_up(&mut self, node: Self::Node) -> Result<Self::Node> {
+        Ok(node)
+    }
 }
 
-/// Controls how the [`TreeNode`] recursion should proceed for [`TreeNode::visit`].
+/// Controls how [`TreeNode`] recursions should proceed.
 #[derive(Debug)]
-pub enum VisitRecursion {
-    /// Continue the visit to this node tree.
+pub enum TreeNodeRecursion {
+    /// Continue recursion with the next node.
     Continue,
-    /// Keep recursive but skip applying op on the children
+    /// Skip the current subtree.
     Skip,
-    /// Stop the visit to this node tree.
+    /// Stop recursion.
     Stop,
 }
 
@@ -340,14 +356,14 @@ pub trait DynTreeNode {
 /// [`DynTreeNode`] (such as [`Arc<dyn PhysicalExpr>`])
 impl<T: DynTreeNode + ?Sized> TreeNode for Arc<T> {
     /// Apply the closure `F` to the node's children
-    fn apply_children<F>(&self, op: &mut F) -> Result<VisitRecursion>
+    fn apply_children<F>(&self, op: &mut F) -> Result<TreeNodeRecursion>
     where
-        F: FnMut(&Self) -> Result<VisitRecursion>,
+        F: FnMut(&Self) -> Result<TreeNodeRecursion>,
     {
         for child in self.arc_children() {
             handle_tree_recursion!(op(&child)?)
         }
-        Ok(VisitRecursion::Continue)
+        Ok(TreeNodeRecursion::Continue)
     }
 
     fn map_children<F>(self, transform: F) -> Result<Self>
@@ -382,14 +398,14 @@ pub trait ConcreteTreeNode: Sized {
 
 impl<T: ConcreteTreeNode> TreeNode for T {
     /// Apply the closure `F` to the node's children
-    fn apply_children<F>(&self, op: &mut F) -> Result<VisitRecursion>
+    fn apply_children<F>(&self, op: &mut F) -> Result<TreeNodeRecursion>
     where
-        F: FnMut(&Self) -> Result<VisitRecursion>,
+        F: FnMut(&Self) -> Result<TreeNodeRecursion>,
     {
         for child in self.children() {
             handle_tree_recursion!(op(child)?)
         }
-        Ok(VisitRecursion::Continue)
+        Ok(TreeNodeRecursion::Continue)
     }
 
     fn map_children<F>(self, transform: F) -> Result<Self>