Read NullVector as Column<Nothing?>

Author: Kopilov

dataframe-arrow/src/main/kotlin/org/jetbrains/kotlinx/dataframe/io/arrowReadingImpl.kt

@@ -173,8 +173,8 @@ private fun StructVector.values(range: IntRange): List<Map<String, Any?>?> = ran
     getObject(it)
 }
 
-private fun NullVector.values(range: IntRange): List<Any?> = range.map {
-    getObject(it)
+private fun NullVector.values(range: IntRange): List<Nothing?> = range.map {
+    getObject(it) as Nothing?
 }
 
 private fun VarCharVector.values(range: IntRange): List<String?> = range.map {
@@ -209,6 +209,12 @@ private fun LargeVarCharVector.values(range: IntRange): List<String?> = range.ma
     }
 }
 
+internal fun nothingType(nullable: Boolean): KType = if (nullable) {
+    typeOf<List<Nothing?>>()
+} else {
+    typeOf<List<Nothing>>()
+}.arguments.first().type!!
+
 private inline fun <reified T> List<T?>.withTypeNullable(
     expectedNulls: Boolean,
     nullabilityOptions: NullabilityOptions,
@@ -250,7 +256,7 @@ private fun readField(root: VectorSchemaRoot, field: Field, nullability: Nullabi
             is TimeStampMilliVector -> vector.values(range).withTypeNullable(field.isNullable, nullability)
             is TimeStampSecVector -> vector.values(range).withTypeNullable(field.isNullable, nullability)
             is StructVector -> vector.values(range).withTypeNullable(field.isNullable, nullability)
-            is NullVector -> vector.values(range).withTypeNullable(field.isNullable, nullability)
+            is NullVector -> vector.values(range) to nothingType(field.isNullable)
             else -> {
                 throw NotImplementedError("reading from ${vector.javaClass.canonicalName} is not implemented")
             }

dataframe-arrow/src/test/kotlin/org/jetbrains/kotlinx/dataframe/io/exampleEstimatesAssertions.kt

@@ -160,6 +160,7 @@ internal fun assertEstimations(exampleFrame: AnyFrame, expectedNullable: Boolean
     }
 
     exampleFrame.getColumnOrNull("nulls")?.let { nullCol ->
+        nullCol.type() shouldBe nothingType(hasNulls)
         assert(hasNulls)
         nullCol.values().forEach {
             assert(it == null)