Github user wzhfy commented on a diff in the pull request:
https://github.com/apache/spark/pull/18323#discussion_r123401878
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Expression.scala
---
@@ -631,3 +631,109 @@ abstract class TernaryExpression extends Expression {
}
}
}
+
+/**
+ * An expression with four inputs and one output. The output is by default
evaluated to null
+ * if any input is evaluated to null.
+ */
+abstract class QuaternaryExpression extends Expression {
+
+ override def foldable: Boolean = children.forall(_.foldable)
+
+ override def nullable: Boolean = children.exists(_.nullable)
+
+ /**
+ * Default behavior of evaluation according to the default nullability
of TernaryExpression.
--- End diff --
`TernaryExpression` -> `QuaternaryExpression`, please also fix other places
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