Fix NPE in guess type for values of functional types

From my quick research, reflection doesn't know anything about these values. They don't have invoke methods, nor any supertypes. So for now i decided to simply fix NPE by falling back to generic Function type for such columns. It will then at least work together with compiler plugin

Author: koperagen

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

@@ -436,6 +436,8 @@ internal fun guessValueType(values: Sequence<Any?>, upperBound: KType? = null, l
                 collectionClasses.add(it.javaClass.kotlin)
             }
 
+            is Function<*> -> classes.add(Function::class)
+
             else -> classes.add(it.javaClass.kotlin)
         }
     }

core/src/test/kotlin/org/jetbrains/kotlinx/dataframe/testSets/person/DataFrameTests.kt

@@ -217,6 +217,13 @@ class DataFrameTests : BaseTest() {
         dataFrameOf("name", "age", "city", "weight")(c1, c2, c3, c4) shouldBe df
     }
 
+    @Test
+    fun `guess column type for type without classifier`() {
+        val df = dataFrameOf("a", "b")({ 1 }, 2)
+        df["a"].type() shouldBe typeOf<Function<*>>()
+        (df["a"][0] as () -> Int).invoke() shouldBe 1
+    }
+
     @Test
     fun `create with columnOf`() {
         val col = columnOf("Alice", "Bob")

plugins/kotlin-dataframe/testData/box/dataFrameOf_vararg.kt

@@ -6,5 +6,8 @@ import org.jetbrains.kotlinx.dataframe.io.*
 fun box(): String {
     val df = dataFrameOf("a")(1, 2, 3)
     val i: Int = df.a[0]
+
+    val df1 = dataFrameOf("a", "b")({ 1 }, 2)
+    val i1: Int = df1.a[0].invoke()
     return "OK"
 }