Add Result#{throwIf,throwUnless}

Closes #64, #66

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

@@ -105,3 +105,45 @@ public inline fun <V, E> Result<V, E>.recoverUnless(predicate: (E) -> Boolean, t
         }
     }
 }
+
+/**
+ * Throws the [error][Err.error] if this [Result] is an [Err]
+ * and satisfies the given [predicate], otherwise this [Result].
+ *
+ * @see [takeIf]
+ */
+public inline fun <V, E : Throwable> Result<V, E>.throwIf(predicate: (E) -> Boolean): Result<V, E> {
+    contract {
+        callsInPlace(predicate, InvocationKind.AT_MOST_ONCE)
+    }
+
+    return when (this) {
+        is Ok -> this
+        is Err -> if (predicate(error)) {
+            throw error
+        } else {
+            this
+        }
+    }
+}
+
+/**
+ * Throws the [error][Err.error] if this [Result] is an [Err]
+ * and _does not_ satisfy the given [predicate], otherwise this [Result].
+ *
+ * @see [takeUnless]
+ */
+public inline fun <V, E : Throwable> Result<V, E>.throwUnless(predicate: (E) -> Boolean): Result<V, E> {
+    contract {
+        callsInPlace(predicate, InvocationKind.AT_MOST_ONCE)
+    }
+
+    return when (this) {
+        is Ok -> this
+        is Err -> if (!predicate(error)) {
+            throw error
+        } else {
+            this
+        }
+    }
+}

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

@@ -181,4 +181,62 @@ class OrTest {
             )
         }
     }
+
+    class ThrowIf {
+        @Test
+        fun returnsValueIfOk() {
+            assertEquals(
+                expected = Ok(200),
+                actual = Ok(200).throwIf { true }
+            )
+        }
+
+        @Test
+        fun returnsErrIfPredicateDoesNotMatch() {
+            val error = RuntimeException("throw if")
+
+            assertEquals(
+                expected = Err(error),
+                actual = Err(error).throwIf { false }
+            )
+        }
+
+        @Test
+        fun throwsErrIfPredicateMatches() {
+            val error = RuntimeException("throw if")
+
+            assertFailsWith<RuntimeException>(error.message) {
+                Err(error).throwIf { true }
+            }
+        }
+    }
+
+    class ThrowUnless {
+        @Test
+        fun returnsValueIfOk() {
+            assertEquals(
+                expected = Ok(500),
+                actual = Ok(500).throwUnless { false }
+            )
+        }
+
+        @Test
+        fun returnsErrIfPredicateMatches() {
+            val error = RuntimeException("example")
+
+            assertEquals(
+                expected = Err(error),
+                actual = Err(error).throwUnless { true }
+            )
+        }
+
+        @Test
+        fun throwsErrIfPredicateDoesNotMatch() {
+            val error = RuntimeException("throw unless")
+
+            assertFailsWith<RuntimeException>(error.message) {
+                Err(error).throwUnless { false }
+            }
+        }
+    }
 }