Support string to enum conversion in convert operation.

Author: nikitinas

docs/StardustDocs/topics/convert.md

@@ -34,6 +34,7 @@ df.convert { dfsOf<String>() }.with { it.toCharArray().toList() }
 * `LocalDateTime`
 * `LocalDate`
 * `LocalTime`
+* `Duration`
 
 <!---FUN convertTo-->
 
@@ -45,3 +46,18 @@ df.convert { weight }.toFloat()
 ```
 
 <!---END-->
+
+Automatic conversion from `String` into enum class is also supported:
+
+```kotlin
+enum class Direction { NORTH, SOUTH, WEST, EAST }
+```
+
+<!---FUN convertToEnum-->
+
+```kotlin
+dataFrameOf("direction")("NORTH", "WEST")
+   .convert("direction").to<Direction>()
+```
+
+<!---END-->

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

@@ -105,6 +105,8 @@ internal fun Any.convertTo(type: KType): Any? {
 
 internal inline fun <T> convert(crossinline converter: (T) -> Any?): TypeConverter = { converter(it as T) }
 
+private enum class DummyEnum
+
 internal fun createConverter(from: KType, to: KType, options: ParserOptions? = null): TypeConverter? {
     if (from.arguments.isNotEmpty() || to.arguments.isNotEmpty()) return null
     if (from.isMarkedNullable) {
@@ -117,7 +119,11 @@ internal fun createConverter(from: KType, to: KType, options: ParserOptions? = n
     if (fromClass == toClass) return { it }
 
     return when {
-        fromClass == String::class -> Parsers[to.withNullability(false)]?.toConverter(options)
+        fromClass == String::class -> {
+            Parsers[to.withNullability(false)]?.toConverter(options)
+                ?: if (toClass.isSubclassOf(Enum::class)) convert<String> { java.lang.Enum.valueOf(toClass.java as Class<DummyEnum>, it) }
+                else null
+        }
         toClass == String::class -> convert<Any> { it.toString() }
         else -> when (fromClass) {
             Boolean::class -> when (toClass) {

tests/src/test/kotlin/org/jetbrains/kotlinx/dataframe/api/convert.kt

@@ -33,4 +33,11 @@ class ConvertTests {
         val df = listOf(Schema(Clock.System.now())).toDataFrame()
         df.convert { time }.toLocalDateTime()
     }
+
+    enum class Value { A, B }
+
+    @Test
+    fun `convert string to enum`() {
+        columnOf("A", "B").convertTo<Value>() shouldBe columnOf(Value.A, Value.B)
+    }
 }

tests/src/test/kotlin/org/jetbrains/kotlinx/dataframe/samples/api/Modify.kt

@@ -74,6 +74,18 @@ class Modify : TestBase() {
         // SampleEnd
     }
 
+    enum class Direction {
+        NORTH, SOUTH, WEST, EAST
+    }
+
+    @Test
+    fun convertToEnum() {
+        // SampleStart
+        dataFrameOf("direction")("NORTH", "WEST")
+            .convert("direction").to<Direction>()
+        // SampleEnd
+    }
+
     @Test
     fun parseAll() {
         // SampleStart