Fix reading Excel file that has cells with formulas that return string

dataframe-excel/src/main/kotlin/org/jetbrains/kotlinx/dataframe/io/xlsx.kt

@@ -278,8 +278,9 @@ private fun repairNameIfRequired(
     }
 }
 
-private fun Cell?.cellValue(sheetName: String): Any? =
-    when (this?.cellType) {
+private fun Cell?.cellValue(sheetName: String): Any? {
+    if (this == null) return null
+    fun getValueFromType(type: CellType?): Any? = when (type) {
         CellType._NONE -> error("Cell $address of sheet $sheetName has a CellType that should only be used internally. This is a bug, please report https://github.com/Kotlin/dataframe/issues")
         CellType.NUMERIC -> {
             val number = numericCellValue
@@ -290,12 +291,14 @@ private fun Cell?.cellValue(sheetName: String): Any? =
         }
 
         CellType.STRING -> stringCellValue
-        CellType.FORMULA -> numericCellValue
+        CellType.FORMULA -> getValueFromType(cachedFormulaResultType)
         CellType.BLANK -> stringCellValue
         CellType.BOOLEAN -> booleanCellValue
         CellType.ERROR -> errorCellValue
         null -> null
     }
+    return getValueFromType(cellType)
+}
 
 public fun <T> DataFrame<T>.writeExcel(
     path: String,

dataframe-excel/src/test/kotlin/org/jetbrains/kotlinx/dataframe/io/XlsxTest.kt

@@ -121,4 +121,10 @@ class XlsxTest {
         df.columnNames() shouldBe listOf("Sepal.Length", "Sepal.Width", "C",
             "Petal.Length", "Petal.Width", "Species", "Other.Width", "Species1", "I", "Other.Width1", "Species2")
     }
+
+    @Test
+    fun `read xlsx file that has cells with formulas that return numbers and strings`() {
+        val df = DataFrame.readExcel(testResource("formula_cell.xlsx"))
+        df.columnNames() shouldBe listOf("Number", "Greater than 5", "Multiplied by 10", "Divided by 5")
+    }
 }