Add transpose

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

Author: michaelbull

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

@@ -45,6 +45,25 @@ public inline infix fun <V, U> Result<V, Throwable>.mapCatching(transform: (V) -
     }
 }
 
+/**
+ * Transposes this [Result<V?, E>][Result] to [Result<V, E>][Result].
+ *
+ * Returns null if this [Result] is [Ok] and the [value][Ok.value] is `null`, otherwise this [Result].
+ *
+ * - Rust: [Result.transpose][https://doc.rust-lang.org/std/result/enum.Result.html#method.transpose]
+ */
+public inline fun <V, E> Result<V?, E>.transpose(): Result<V, E>? {
+    return when (this) {
+        is Ok -> if (value == null) {
+            null
+        } else {
+            Ok(value)
+        }
+
+        is Err -> this
+    }
+}
+
 /**
  * Maps this [Result<Result<V, E>, E>][Result] to [Result<V, E>][Result].
  *

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
 
 class MapTest {
     private sealed interface MapErr {
@@ -76,6 +77,36 @@ class MapTest {
         }
     }
 
+    class Transpose {
+
+        @Test
+        fun returnsNullIfValueIsNull() {
+            val result = Ok(null)
+
+            assertNull(result.transpose())
+        }
+
+        @Test
+        fun returnsOkIfValueIsNotNull() {
+            val result = Ok("non null")
+
+            assertEquals(
+                expected = Ok("non null"),
+                actual = result.transpose()
+            )
+        }
+
+        @Test
+        fun returnsErrIfErr() {
+            val result = Err("non null error")
+
+            assertEquals(
+                expected = Err("non null error"),
+                actual = result.transpose()
+            )
+        }
+    }
+
     class Flatten {
 
         @Test