[GitHub] [spark] cloud-fan commented on a change in pull request #35534: [SPARK-38240][SQL] Improve RuntimeReplaceable and add a guideline for adding new functions

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cloud-fan commented on a change in pull request #35534:
URL: https://github.com/apache/spark/pull/35534#discussion_r809118029



##########
File path: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Expression.scala
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@@ -352,34 +352,44 @@ trait Unevaluable extends Expression {
  * An expression that gets replaced at runtime (currently by the optimizer) into a different
  * expression for evaluation. This is mainly used to provide compatibility with other databases.
  * For example, we use this to support "nvl" by replacing it with "coalesce".
- *
- * A RuntimeReplaceable should have the original parameters along with a "child" expression in the
- * case class constructor, and define a normal constructor that accepts only the original
- * parameters. For an example, see [[Nvl]]. To make sure the explain plan and expression SQL
- * works correctly, the implementation should also override flatArguments method and sql method.
  */
-trait RuntimeReplaceable extends UnaryExpression with Unevaluable {
-  override def nullable: Boolean = child.nullable
-  override def dataType: DataType = child.dataType
+trait RuntimeReplaceable extends Expression {
+  def replacement: Expression
+
+  override val nodePatterns: Seq[TreePattern] = Seq(RUNTIME_REPLACEABLE)
+  override def nullable: Boolean = replacement.nullable
+  override def dataType: DataType = replacement.dataType
   // As this expression gets replaced at optimization with its `child" expression,
   // two `RuntimeReplaceable` are considered to be semantically equal if their "child" expressions
   // are semantically equal.
-  override lazy val preCanonicalized: Expression = child.preCanonicalized
-
-  /**
-   * Only used to generate SQL representation of this expression.
-   *
-   * Implementations should override this with original parameters
-   */
-  def exprsReplaced: Seq[Expression]
-
-  override def sql: String = mkString(exprsReplaced.map(_.sql))
+  override lazy val preCanonicalized: Expression = replacement.preCanonicalized
 
-  final override val nodePatterns: Seq[TreePattern] = Seq(RUNTIME_REPLACEABLE)
+  final override def eval(input: InternalRow = null): Any =
+    throw QueryExecutionErrors.cannotEvaluateExpressionError(this)
+  final override protected def doGenCode(ctx: CodegenContext, ev: ExprCode): ExprCode =
+    throw QueryExecutionErrors.cannotGenerateCodeForExpressionError(this)
+}
 
-  def mkString(childrenString: Seq[String]): String = {
-    prettyName + childrenString.mkString("(", ", ", ")")
+/**
+ * A special variant of [[RuntimeReplaceable]]. It makes `replacement` the child of the expression,
+ * to inherit the type coercion rules for it. The implementation should put `replacement` in the
+ * case class constructor, and define a normal constructor that accepts only the original
+ * parameters, or use `ExpressionBuilder`. For an example, see [[TryAdd]]. To make sure the explain
+ * plan and expression SQL works correctly, the implementation should also implement the
+ * `actualInputs` method, which should extracts the actual inputs after type coercion from
+ * `replacement`.
+ */
+trait RuntimeReplaceableInheritingTypeCoercion

Review comment:
       This is the same as the old `RuntimeReplaceable` before this PR.




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