Add flatten

Converts from Result<Result<V, E>, E> to Result<V, E>

See: https://doc.rust-lang.org/std/result/enum.Result.html#method.flatten

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

@@ -167,6 +167,15 @@ public inline infix fun <V, E, U> Result<V, E>.flatMap(transform: (V) -> Result<
     return andThen(transform)
 }
 
+/**
+ * Maps this [Result<Result<V, E>, E>][Result] to [Result<V, E>][Result].
+ *
+ * - Rust: [Result.flatten](https://doc.rust-lang.org/std/result/enum.Result.html#method.flatten)
+ */
+public inline fun <V, E> Result<Result<V, E>, E>.flatten(): Result<V, E> {
+    return andThen { it }
+}
+
 /**
  * Returns the [transformation][transform] of the [value][Ok.value] if this [Result] is [Ok]
  * and satisfies the given [predicate], otherwise this [Result].

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

@@ -176,6 +176,54 @@ class MapTest {
         }
     }
 
+    class Flatten {
+
+        @Test
+        fun returnsFlattenedValueIfOk() {
+            val result = Ok(Ok("hello"))
+
+            assertEquals(
+                expected = Ok("hello"),
+                actual = result.flatten()
+            )
+        }
+
+        @Test
+        fun returnsFlattenedErrIfErr() {
+            val result = Ok(Err(6))
+
+            assertEquals(
+                expected = Err(6),
+                actual = result.flatten()
+            )
+        }
+
+        @Test
+        fun returnsErrIfFlatErr() {
+            val result = Err(6)
+
+            assertEquals(
+                expected = Err(6),
+                actual = result.flatten()
+            )
+        }
+
+        @Test
+        fun returnsFlattenNestedResult() {
+            val result = Ok(Ok(Ok("hello")))
+
+            assertEquals(
+                expected = Ok(Ok("hello")),
+                actual = result.flatten()
+            )
+
+            assertEquals(
+                expected = Ok("hello"),
+                actual = result.flatten().flatten()
+            )
+        }
+    }
+
     class ToErrorIfNull {
 
         @Test