add suugested extended enum puzzler (which also applies to any class-…

…level extensions) #23

src/types/extendedEnum/Rationale.md

@@ -0,0 +1,14 @@
+Correct answer: ***d) will not compile***
+
+* Extension functions on Color apply to instances of Color, e.g. `Color.Blue.from()`
+* Extension function on the enum itself can be made only if it has a Companion object
+```
+    enum class Color {
+      Red, Green, Blue;
+      companion object 
+    }
+
+    fun Color.Companion.from(...)
+```
+
+Puzzler by Dmitry Kandalov @dkandalov

src/types/extendedEnum/extendedEnum.kts

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+package types.extendedEnum
+
+enum class Color {
+  Red, Green, Blue
+}
+
+fun Color.from(s: String) = when (s) {
+  "#FF0000" -> Color.Red
+  "#00FF00" -> Color.Green
+  "#0000FF" -> Color.Blue
+  else -> null
+}
+
+println(Color.from("#00FF00"))
+
+// What will it print?
+// a) Green
+// b) Color.Green
+// c) null
+// d) will not compile