introducing parsing of Char? columns. It works the same as String par…

…sing, but can never result in Char and can never fail (since it can parse to String)

core/api/core.api

@@ -6501,8 +6501,12 @@ public final class org/jetbrains/kotlinx/dataframe/api/ParseKt {
 	public static synthetic fun parse$default (Lorg/jetbrains/kotlinx/dataframe/DataFrame;[Lorg/jetbrains/kotlinx/dataframe/columns/ColumnReference;Lorg/jetbrains/kotlinx/dataframe/api/ParserOptions;ILjava/lang/Object;)Lorg/jetbrains/kotlinx/dataframe/DataFrame;
 	public static final fun parseAnyFrameNullable (Lorg/jetbrains/kotlinx/dataframe/DataColumn;Lorg/jetbrains/kotlinx/dataframe/api/ParserOptions;)Lorg/jetbrains/kotlinx/dataframe/DataColumn;
 	public static synthetic fun parseAnyFrameNullable$default (Lorg/jetbrains/kotlinx/dataframe/DataColumn;Lorg/jetbrains/kotlinx/dataframe/api/ParserOptions;ILjava/lang/Object;)Lorg/jetbrains/kotlinx/dataframe/DataColumn;
+	public static final fun parseChar (Lorg/jetbrains/kotlinx/dataframe/DataColumn;Lorg/jetbrains/kotlinx/dataframe/api/ParserOptions;)Lorg/jetbrains/kotlinx/dataframe/DataColumn;
+	public static synthetic fun parseChar$default (Lorg/jetbrains/kotlinx/dataframe/DataColumn;Lorg/jetbrains/kotlinx/dataframe/api/ParserOptions;ILjava/lang/Object;)Lorg/jetbrains/kotlinx/dataframe/DataColumn;
 	public static final fun tryParse (Lorg/jetbrains/kotlinx/dataframe/DataColumn;Lorg/jetbrains/kotlinx/dataframe/api/ParserOptions;)Lorg/jetbrains/kotlinx/dataframe/DataColumn;
 	public static synthetic fun tryParse$default (Lorg/jetbrains/kotlinx/dataframe/DataColumn;Lorg/jetbrains/kotlinx/dataframe/api/ParserOptions;ILjava/lang/Object;)Lorg/jetbrains/kotlinx/dataframe/DataColumn;
+	public static final fun tryParseChar (Lorg/jetbrains/kotlinx/dataframe/DataColumn;Lorg/jetbrains/kotlinx/dataframe/api/ParserOptions;)Lorg/jetbrains/kotlinx/dataframe/DataColumn;
+	public static synthetic fun tryParseChar$default (Lorg/jetbrains/kotlinx/dataframe/DataColumn;Lorg/jetbrains/kotlinx/dataframe/api/ParserOptions;ILjava/lang/Object;)Lorg/jetbrains/kotlinx/dataframe/DataColumn;
 }
 
 public final class org/jetbrains/kotlinx/dataframe/api/ParserOptions {

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

@@ -18,6 +18,7 @@ import java.time.format.DateTimeFormatter
 import java.util.Locale
 import kotlin.reflect.KProperty
 import kotlin.reflect.KType
+import kotlin.reflect.typeOf
 
 /**
  * ### Global Parser Options
@@ -197,6 +198,28 @@ public class ParserOptions(
 /** @include [tryParseImpl] */
 public fun DataColumn<String?>.tryParse(options: ParserOptions? = null): DataColumn<*> = tryParseImpl(options)
 
+/**
+ * Tries to parse a column of chars into a column of a different type.
+ * Each parser in [Parsers] is run in order until a valid parser is found,
+ * a.k.a. that parser was able to parse all values in the column successfully. If a parser
+ * fails to parse any value, the next parser is tried. If all the others fail, the final parser
+ * returns strings.
+ *
+ * Parsers that are [covered by][StringParser.coveredBy] other parsers are skipped.
+ *
+ * @param options options for parsing, like providing a locale or a custom date-time formatter
+ * @throws IllegalStateException if no valid parser is found (unlikely, unless the `String` parser is disabled)
+ * @return a new column with parsed values
+ */
+@JvmName("tryParseChar")
+public fun DataColumn<Char?>.tryParse(options: ParserOptions? = null): DataColumn<*> {
+    // skip the Char parser, as we're trying to parse away from Char
+    val providedSkipTypes = options?.skipTypes ?: DataFrame.parser.skipTypes
+    val parserOptions = (options ?: ParserOptions()).copy(skipTypes = providedSkipTypes + typeOf<Char>())
+
+    return map { it?.toString() }.tryParse(parserOptions)
+}
+
 public fun <T> DataFrame<T>.parse(options: ParserOptions? = null): DataFrame<T> =
     parse(options) {
         colsAtAnyDepth { !it.isColumnGroup() }
@@ -220,6 +243,23 @@ public fun <T> DataFrame<T>.parse(options: ParserOptions? = null): DataFrame<T>
 public fun DataColumn<String?>.parse(options: ParserOptions? = null): DataColumn<*> =
     tryParse(options).also { if (it.typeClass == String::class) error("Can't guess column type") }
 
+/**
+ * Tries to parse a column of chars as strings into a column of a different type.
+ * Each parser in [Parsers] is run in order until a valid parser is found,
+ * a.k.a. that parser was able to parse all values in the column successfully. If a parser
+ * fails to parse any value, the next parser is tried.
+ *
+ * If all fail, the column is returned as `String`, this can never fail.
+ *
+ * Parsers that are [covered by][StringParser.coveredBy] other parsers are skipped.
+ *
+ * @param options options for parsing, like providing a locale or a custom date-time formatter
+ * @return a new column with parsed values
+ */
+@JvmName("parseChar")
+public fun DataColumn<Char?>.parse(options: ParserOptions? = null): DataColumn<*> =
+    tryParse(options) // no need to throw an exception, as Char can always be parsed as String
+
 @JvmName("parseAnyFrameNullable")
 public fun DataColumn<AnyFrame?>.parse(options: ParserOptions? = null): DataColumn<AnyFrame?> =
     map { it?.parse(options) }

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

@@ -564,29 +564,28 @@ internal fun <T> DataFrame<T>.parseImpl(options: ParserOptions?, columns: Column
         when {
             // when a frame column is requested to be parsed,
             // parse each value/frame column at any depth inside each DataFrame in the frame column
-            col.isFrameColumn() -> {
+            col.isFrameColumn() ->
                 col.map {
                     it.parseImpl(options) {
                         colsAtAnyDepth { !it.isColumnGroup() }
                     }
                 }
-            }
 
             // when a column group is requested to be parsed,
             // parse each column in the group
-            col.isColumnGroup() -> {
+            col.isColumnGroup() ->
                 col.parseImpl(options) { all() }
                     .asColumnGroup(col.name())
                     .asDataColumn()
-            }
+
+            // Base case, parse the column as String if it's a `Char?` column
+            col.isSubtypeOf<Char?>() ->
+                col.cast<Char?>().map { it?.toString() }.tryParseImpl(options)
 
             // Base case, parse the column if it's a `String?` column
-            col.isSubtypeOf<String?>() -> {
+            col.isSubtypeOf<String?>() ->
                 col.cast<String?>().tryParseImpl(options)
-            }
 
-            else -> {
-                col
-            }
+            else -> col
         }
     }

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

@@ -221,6 +221,8 @@ class ConvertTests {
 
         // but
         columnOf('1', '2').convertToString().convertToInt() shouldBe columnOf(1, 2)
+        // or
+        columnOf('1', '2').parse() shouldBe columnOf(1, 2)
     }
 
     @Test

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

@@ -32,6 +32,22 @@ import java.time.Duration as JavaDuration
 import java.time.Instant as JavaInstant
 
 class ParseTests {
+
+    @Test
+    fun `parse chars to string`() {
+        val char = columnOf('a', 'b', 'c')
+        char.parse() shouldBe columnOf("a", "b", "c")
+        char.tryParse() shouldBe columnOf("a", "b", "c")
+        char.parse().cast<String>().parse() shouldBe char
+    }
+
+    @Test
+    fun `parse chars to int`() {
+        val char = columnOf('1', '2', '3')
+        char.parse() shouldBe columnOf(1, 2, 3)
+        char.tryParse() shouldBe columnOf(1, 2, 3)
+    }
+
     @Test
     fun parseDate() {
         val currentLocale = Locale.getDefault()

core/src/test/kotlin/org/jetbrains/kotlinx/dataframe/io/ParserTests.kt

@@ -46,6 +46,12 @@ class ParserTests {
         DataFrame.parser.resetToDefault()
     }
 
+    @Test
+    fun `parse to Char`() {
+        val col by columnOf("a", "b")
+        col.parse().type() shouldBe typeOf<Char>()
+    }
+
     @Test(expected = IllegalStateException::class)
     fun `parse should throw`() {
         val col by columnOf("a", "bc")