case class A(s:String, i:Int)

spark.read.parquet(file).as[A]

sealed trait E
case object A extends E
case object B extends E

spark.read.parquet(file).as[E]
// java.lang.UnsupportedOperationException: No Encoder found for E

implicit val eEncoder = new org.apache.spark.sql.Encoder[E] {
 def clsTag = ClassTag(classOf[E])
 def schema = StructType(StructField("e", IntegerType, nullable = false)::Nil)
}