[GitHub] [spark] maropu commented on a change in pull request #28420: [SPARK-31615][SQL] Pretty string output for sql method of RuntimeReplaceable expressions

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maropu commented on a change in pull request #28420:
URL: https://github.com/apache/spark/pull/28420#discussion_r418346518



##########
File path: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Expression.scala
##########
@@ -323,6 +323,20 @@ trait RuntimeReplaceable extends UnaryExpression with Unevaluable {
   // two `RuntimeReplaceable` are considered to be semantically equal if their "child" expressions
   // are semantically equal.
   override lazy val canonicalized: Expression = child.canonicalized
+
+  override def innerChildren: Seq[Expression] = sys.error("RuntimeReplaceable must implement" +
+    " innerChildren with the original parameters")
+
+  protected val sqlStrSeparator: String = ", "

Review comment:
       We need this variable? It seems only `Extract` uses this.

##########
File path: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Expression.scala
##########
@@ -323,6 +323,20 @@ trait RuntimeReplaceable extends UnaryExpression with Unevaluable {
   // two `RuntimeReplaceable` are considered to be semantically equal if their "child" expressions
   // are semantically equal.
   override lazy val canonicalized: Expression = child.canonicalized
+
+  override def innerChildren: Seq[Expression] = sys.error("RuntimeReplaceable must implement" +

Review comment:
       Since compilers cannot catch the case where one forget to override this, how about it like this?
   ```
     // RuntimeReplaceable
     def exprsReplaced: Seq[Expression]
     override def innerChildren: Seq[Expression] = exprsReplaced
   
     // e.g., ParseToTimestamp
     override def exprsReplaced: Seq[Expression] = Seq(Some(left), format).flatten
   
   ```

##########
File path: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Expression.scala
##########
@@ -323,6 +323,20 @@ trait RuntimeReplaceable extends UnaryExpression with Unevaluable {
   // two `RuntimeReplaceable` are considered to be semantically equal if their "child" expressions
   // are semantically equal.
   override lazy val canonicalized: Expression = child.canonicalized
+
+  override def innerChildren: Seq[Expression] = sys.error("RuntimeReplaceable must implement" +
+    " innerChildren with the original parameters")
+
+  protected val sqlStrSeparator: String = ", "
+
+  override def sql: String = RuntimeReplaceable.this.prettyName +
+    prettyChildren.map(_.sql).mkString("(", sqlStrSeparator, ")")
+
+  protected var prettyChildren: Seq[Expression] = innerChildren

Review comment:
       I think its better to user `var` where possible, so how about it like this?
   ```
     // RuntimeReplaceable
     def newInstance(innerChildren: Seq[Expression]): RuntimeReplaceable
   
     // e.g., ParseToTimestamp
     override def newInstance(innerChildren: Seq[Expression]): ParseToTimestamp = {
       innerChildren match {
         case Seq(left) => ParseToTimestamp(left, None, child)
         case Seq(left, format) => ParseToTimestamp(left, Some(format), child)
       }
     }
   ```
   Then, update the `toPrettySQL` in `utils.package`;
   ```
     def toPrettySQL(e: Expression): String = e match {
       case r: RuntimeReplaceable =>
         r.newInstance(r.innerChildren.map(usePrettyExpression)).sql
       case _ => usePrettyExpression(e).sql
     }
   ```




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