Github user viirya commented on a diff in the pull request:

    https://github.com/apache/spark/pull/21053#discussion_r181622879
  
    --- Diff: 
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/collectionOperations.scala
 ---
    @@ -287,3 +287,106 @@ case class ArrayContains(left: Expression, right: 
Expression)
     
       override def prettyName: String = "array_contains"
     }
    +
    +/**
    + * Returns the value of index `right` in Array `left` or the value for key 
`right` in Map `left`.
    + */
    +@ExpressionDescription(
    +  usage = """
    +    _FUNC_(array, index) - Returns element of array at given index. If 
index < 0, accesses elements
    +      from the last to the first.
    +
    +    _FUNC_(map, key) - Returns value for given key, or NULL if the key is 
not contained in the map
    +  """,
    +  examples = """
    +    Examples:
    +      > SELECT _FUNC_(array(1, 2, 3), 2);
    +       2
    +      > SELECT _FUNC_(map(1, 'a', 2, 'b'), 2);
    +       "b"
    +  """,
    +  since = "2.4.0")
    +case class ElementAt(left: Expression, right: Expression) extends 
GetMapValueUtil {
    +
    +  override def dataType: DataType = left.dataType match {
    +    case _: ArrayType => left.dataType.asInstanceOf[ArrayType].elementType
    +    case _: MapType => left.dataType.asInstanceOf[MapType].valueType
    +  }
    +
    +  override def inputTypes: Seq[AbstractDataType] = {
    +    Seq(TypeCollection(ArrayType, MapType),
    +      left.dataType match {
    +        case _: ArrayType => IntegerType
    +        case _: MapType => left.dataType.asInstanceOf[MapType].keyType
    +      }
    +    )
    +  }
    +
    +  override def nullable: Boolean = true
    --- End diff --
    
    Maybe?
    ```scala
    override def nullable: Boolean = left.dataType match {
        case a: ArrayType => a.containsNull
        case m: MapType => m.valueContainsNull
    } ||  left.nullable || right.nullable
    ```


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