Github user aokolnychyi commented on a diff in the pull request: https://github.com/apache/spark/pull/18692#discussion_r154164912 --- Diff: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/ExpressionSet.scala --- @@ -27,6 +27,8 @@ object ExpressionSet { expressions.foreach(set.add) set } + + val empty: ExpressionSet = ExpressionSet(Nil) --- End diff -- I thought that writing ``ExpressionSet.empty`` would be more readable than ``ExpressionSet(Nil)``. Usually, mutable collections have ``def empty()`` and immutable ones have separate objects that represent empty collections (e.g., ``Nil``, ``Stream.Empty``). I defined ``val empty`` since ``ExpressionSet`` is immutable.
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