Improved typing

mappie-api/src/commonMain/kotlin/tech/mappie/api/Mappie.kt

@@ -2,7 +2,7 @@
 
 package tech.mappie.api
 
-public abstract class Mappie<FROM, TO> {
+public abstract class Mappie<in FROM, out TO> {
 
     /**
      * Map [from] to an instance of [TO].

testing/src/main/kotlin/testing/ListToNullableListMapper.kt

@@ -0,0 +1,17 @@
+package testing
+
+import tech.mappie.api.ObjectMappie
+
+data class SourceList(val value: List<String>)
+
+data class TargetList(val value: List<String>?)
+
+object NullableListFromListMapper : ObjectMappie<SourceList, TargetList>() {
+    override fun map(from: SourceList): TargetList = mapping {
+        TargetList::value fromProperty SourceList::value via StringIdentityMapper.forList
+    }
+}
+
+object StringIdentityMapper : ObjectMappie<String, String>() {
+    override fun map(from: String): String = from
+}

testing/src/test/kotlin/testing/ListToNullableListMapperTest.kt

@@ -0,0 +1,15 @@
+package testing
+
+import org.junit.jupiter.api.Test
+import kotlin.test.assertEquals
+
+class ListToNullableListMapperTest {
+
+    @Test
+    fun `map List to nullable List via NullableListFromList`() {
+        assertEquals(
+            TargetList(listOf("A", "B")),
+            NullableListFromListMapper.map(SourceList(listOf("A", "B"))),
+        )
+    }
+}