implicit conversion from Char to String in DataColumn.convertTo and DataFrame.convert()

Author: Jolanrensen

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

@@ -367,7 +367,13 @@ internal fun createConverter(from: KType, to: KType, options: ParserOptions? = n
 
             Char::class -> when (toClass) {
                 Int::class -> convert<Char> { it.code }
-                else -> null
+
+                else -> // convert char to string and then to target type
+                    getConverter(typeOf<String>(), to, options)?.let { stringConverter ->
+                        convert<Char> {
+                            stringConverter(it.toString())
+                        }
+                    }
             }
 
             Int::class -> when (toClass) {

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

@@ -3,6 +3,7 @@ package org.jetbrains.kotlinx.dataframe.api
 import io.kotest.assertions.throwables.shouldNotThrow
 import io.kotest.assertions.throwables.shouldThrow
 import io.kotest.matchers.shouldBe
+import io.kotest.matchers.shouldNotBe
 import kotlinx.datetime.Clock
 import kotlinx.datetime.Instant
 import kotlinx.datetime.LocalTime
@@ -69,6 +70,20 @@ class ConvertTests {
     @Test
     fun `convert string to enum`() {
         columnOf("A", "B").convertTo<EnumClass>() shouldBe columnOf(EnumClass.A, EnumClass.B)
+
+        dataFrameOf(columnOf("A", "B") named "colA")
+            .convert("colA").to<EnumClass>()
+            .getColumn("colA") shouldBe columnOf(EnumClass.A, EnumClass.B).named("colA")
+    }
+
+    @Test
+    fun `convert char to enum`() {
+        // Char -> String -> Enum
+        columnOf('A', 'B').convertTo<EnumClass>() shouldBe columnOf(EnumClass.A, EnumClass.B)
+
+        dataFrameOf(columnOf('A', 'B') named "colA")
+            .convert("colA").to<EnumClass>()
+            .getColumn("colA") shouldBe columnOf(EnumClass.A, EnumClass.B).named("colA")
     }
 
     @JvmInline
@@ -199,6 +214,13 @@ class ConvertTests {
         val col = columnOf(65, 66)
         col.convertTo<Char>() shouldBe columnOf('A', 'B')
         col.convertTo<Char>().convertTo<Int>() shouldBe col
+
+        // this means
+        columnOf('1', '2').convertToInt() shouldNotBe columnOf(1, 2)
+        columnOf('1', '2').convertToInt() shouldBe columnOf(49, 50)
+
+        // but
+        columnOf('1', '2').convertToString().convertToInt() shouldBe columnOf(1, 2)
     }
 
     @Test