Add toErrorIfNull & toErrorUnlessNull

Closes #84

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

@@ -189,6 +189,23 @@ public inline fun <V, E> Result<V, E>.toErrorIf(predicate: (V) -> Boolean, trans
     }
 }
 
+/**
+ * Returns the supplied [error] if this [Result] is [Ok] and the [value][Ok.value] is `null`,
+ * otherwise this [Result].
+ *
+ * @see [toErrorIf]
+ */
+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() }
+    )
+}
+
 /**
  * Returns the [transformation][transform] of the [value][Ok.value] if this [Result] is [Ok]
  * and _does not_ satisfy the given [predicate], otherwise this [Result].
@@ -210,3 +227,20 @@ public inline fun <V, E> Result<V, E>.toErrorUnless(predicate: (V) -> Boolean, t
         is Err -> this
     }
 }
+
+/**
+ * Returns the supplied [error] unless this [Result] is [Ok] and the [value][Ok.value] is `null`,
+ * otherwise this [Result].
+ *
+ * @see [toErrorUnless]
+ */
+public inline fun <V, E> Result<V, E>.toErrorUnlessNull(error: () -> E): Result<V, E> {
+    contract {
+        callsInPlace(error, InvocationKind.AT_MOST_ONCE)
+    }
+
+    return toErrorUnless(
+        { it == null },
+        { error() }
+    )
+}

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

@@ -2,6 +2,7 @@ package com.github.michaelbull.result
 
 import kotlin.test.Test
 import kotlin.test.assertEquals
+import kotlin.test.assertNull
 import kotlin.test.assertSame
 
 class MapTest {
@@ -174,4 +175,79 @@ class MapTest {
             )
         }
     }
+
+    class ToErrorIfNull {
+
+        @Test
+        fun returnsValueIfOk() {
+            val result = Ok("a").toErrorIfNull { "b" }
+
+            result as Ok
+
+            assertEquals(
+                expected = "a",
+                actual = result.value
+            )
+        }
+
+        @Test
+        fun returnsTransformedErrorIfNull() {
+            val result = Ok(null).toErrorIfNull { "a" }
+
+            result as Err
+
+            assertEquals(
+                expected = "a",
+                actual = result.error
+            )
+        }
+
+        @Test
+        fun returnsErrorIfErr() {
+            val result = Err("a").toErrorIfNull { "b" }
+
+            result as Err
+
+            assertEquals(
+                expected = "a",
+                actual = result.error
+            )
+        }
+    }
+
+    class ToErrorUnlessNull {
+
+        @Test
+        fun returnsTransformedErrorIfNotNull() {
+            val result = Ok("a").toErrorUnlessNull { "b" }
+
+            result as Err
+
+            assertEquals(
+                expected = "b",
+                actual = result.error
+            )
+        }
+
+        @Test
+        fun returnsValueIfNull() {
+            val result = Ok(null).toErrorUnlessNull { "a" }
+
+            result as Ok
+
+            assertNull(result.value)
+        }
+
+        @Test
+        fun returnsErrorIfErr() {
+            val result = Err("a").toErrorUnlessNull { "b" }
+
+            result as Err
+
+            assertEquals(
+                expected = "a",
+                actual = result.error
+            )
+        }
+    }
 }