Improve type constraint on toErrorIfNull

Closes #84 and closes #86

Author: Kirill Zhukov

build.gradle.kts

@@ -223,6 +223,11 @@ subprojects {
                             name.set("Matthias Geisler")
                             url.set("https://github.com/bitPogo")
                         }
+
+                        contributor {
+                            name.set("Kirill Zhukov")
+                            url.set("https://github.com/kirillzh")
+                        }
                     }
 
                     scm {

kotlin-result/src/commonMain/kotlin/com/github/michaelbull/result/Map.kt

@@ -195,15 +195,19 @@ public inline fun <V, E> Result<V, E>.toErrorIf(predicate: (V) -> Boolean, trans
  *
  * @see [toErrorIf]
  */
-public inline fun <V, E> Result<V, E>.toErrorIfNull(error: () -> E): Result<V, E> {
+public inline fun <V, E> Result<V?, E>.toErrorIfNull(error: () -> E): Result<V, E> {
     contract {
         callsInPlace(error, InvocationKind.AT_MOST_ONCE)
     }
 
-    return toErrorIf(
-        { it == null },
-        { error() }
-    )
+    return when (this) {
+        is Ok -> if(value == null) {
+            Err(error())
+        } else {
+            Ok(value)
+        }
+        is Err -> this
+    }
 }
 
 /**
@@ -239,8 +243,12 @@ public inline fun <V, E> Result<V, E>.toErrorUnlessNull(error: () -> E): Result<
         callsInPlace(error, InvocationKind.AT_MOST_ONCE)
     }
 
-    return toErrorUnless(
-        { it == null },
-        { error() }
-    )
+    return when (this) {
+        is Ok -> if (value == null) {
+            this
+        } else {
+            Err(error())
+        }
+        is Err -> Err(error())
+    }
 }

kotlin-result/src/commonTest/kotlin/com/github/michaelbull/result/MapTest.kt

@@ -192,7 +192,7 @@ class MapTest {
 
         @Test
         fun returnsTransformedErrorIfNull() {
-            val result = Ok(null).toErrorIfNull { "a" }
+            val result: Result<Nothing, String> = Ok(null).toErrorIfNull { "a" }
 
             result as Err
 
@@ -245,7 +245,7 @@ class MapTest {
             result as Err
 
             assertEquals(
-                expected = "a",
+                expected = "b",
                 actual = result.error
             )
         }