Github user cloud-fan commented on a diff in the pull request:
https://github.com/apache/spark/pull/6710#discussion_r32029310
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Expression.scala
---
@@ -23,6 +23,15 @@ import org.apache.spark.sql.catalyst.trees
import org.apache.spark.sql.catalyst.trees.TreeNode
import org.apache.spark.sql.types._
+
+/**
+ * For Catalyst to work correctly, concrete implementations of
[[Expression]]s must be case classes
+ * whose constructor arguments are all Expressions types. In addition, if
we want to support more
--- End diff --
ah, I see. So this is a restriction for function-like expressions(need to
be registered at `FunctionRegistry`), not for all expressions. Should we make
the doc more concise about it?
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