Merge pull request #801 from Kotlin/implodeFix

Small implode fix

Author: Jolanrensen

core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/impl/api/implode.kt

@@ -30,8 +30,14 @@ internal fun <T, C> DataFrame<T>.implodeImpl(dropNA: Boolean = false, columns: C
                     first = false
                     value
                 } else {
-                    null
+                    // these rows will not be taken into account,
+                    // but we cannot leave them empty, as `map` creates a full column
+                    when (column.kind()) {
+                        ColumnKind.Value -> emptyList<Any?>()
+                        ColumnKind.Group -> DataFrame.empty()
+                        ColumnKind.Frame -> emptyList<AnyFrame>()
+                    }
                 }
             }
-        }[0..0]
+        }[0..0] // takes only the first row
     }.concat()

core/src/main/kotlin/org/jetbrains/kotlinx/dataframe/impl/columns/DataColumnImpl.kt

@@ -5,6 +5,7 @@ import org.jetbrains.kotlinx.dataframe.DataColumn
 import org.jetbrains.kotlinx.dataframe.api.dataFrameOf
 import org.jetbrains.kotlinx.dataframe.impl.isArray
 import org.jetbrains.kotlinx.dataframe.impl.isPrimitiveArray
+import org.jetbrains.kotlinx.dataframe.kind
 import kotlin.reflect.KClass
 import kotlin.reflect.KType
 import kotlin.reflect.full.isSubclassOf
@@ -37,7 +38,7 @@ internal abstract class DataColumnImpl<T>(
         if (BuildConfig.DEBUG) {
             require(values.all { it matches type }) {
                 val types = values.map { if (it == null) "Nothing?" else it!!::class.simpleName }.distinct()
-                "Values of column '$name' have types '$types' which are not compatible given with column type '$type'"
+                "Values of $kind '$name' have types '$types' which are not compatible given with column type '$type'"
             }
         }
     }