From 15fc1ff0139306b640a38f00922748b34e2e1d5b Mon Sep 17 00:00:00 2001
From: Michael Bull <mikebull94@gmail.com>
Date: Fri, 8 Mar 2024 20:36:21 +0000
Subject: [PATCH] Add mapCatching

See: https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/map-catching.html
---
 .../com/github/michaelbull/result/Map.kt      | 23 ++++++++++++
 .../com/github/michaelbull/result/MapTest.kt  | 35 +++++++++++++++++++
 2 files changed, 58 insertions(+)

diff --git a/kotlin-result/src/commonMain/kotlin/com/github/michaelbull/result/Map.kt b/kotlin-result/src/commonMain/kotlin/com/github/michaelbull/result/Map.kt
index ade93e5..67690e3 100644
--- a/kotlin-result/src/commonMain/kotlin/com/github/michaelbull/result/Map.kt
+++ b/kotlin-result/src/commonMain/kotlin/com/github/michaelbull/result/Map.kt
@@ -22,6 +22,29 @@ public inline infix fun <V, E, U> Result<V, E>.map(transform: (V) -> U): Result<
     }
 }
 
+/**
+ * Maps this [Result<V, Throwable>][Result] to [Result<U, Throwable>][Result] by either applying
+ * the [transform] function to the [value][Ok.value] if this [Result] is [Ok], or returning this
+ * [Err].
+ *
+ * This function catches any [Throwable] exception thrown by [transform] function and encapsulates
+ * it as a failure.
+ *
+ * - Elm: [Result.map](http://package.elm-lang.org/packages/elm-lang/core/latest/Result#map)
+ * - Haskell: [Data.Bifunctor.first](https://hackage.haskell.org/package/base-4.10.0.0/docs/Data-Bifunctor.html#v:first)
+ * - Rust: [Result.map](https://doc.rust-lang.org/std/result/enum.Result.html#method.map)
+ */
+public inline infix fun <V, U> Result<V, Throwable>.mapCatching(transform: (V) -> U): Result<U, Throwable> {
+    contract {
+        callsInPlace(transform, InvocationKind.AT_MOST_ONCE)
+    }
+
+    return when (this) {
+        is Ok -> runCatching { transform(value) }
+        is Err -> this
+    }
+}
+
 /**
  * Maps this [Result<Result<V, E>, E>][Result] to [Result<V, E>][Result].
  *
diff --git a/kotlin-result/src/commonTest/kotlin/com/github/michaelbull/result/MapTest.kt b/kotlin-result/src/commonTest/kotlin/com/github/michaelbull/result/MapTest.kt
index 3afd768..31cabd1 100644
--- a/kotlin-result/src/commonTest/kotlin/com/github/michaelbull/result/MapTest.kt
+++ b/kotlin-result/src/commonTest/kotlin/com/github/michaelbull/result/MapTest.kt
@@ -41,6 +41,41 @@ class MapTest {
         }
     }
 
+    class MapCatching {
+
+        private object MapException : Throwable()
+
+        @Test
+        fun returnsTransformedValueIfOk() {
+            val value: Result<Int, Throwable> = Ok(10)
+
+            assertEquals(
+                expected = Ok(30),
+                actual = value.mapCatching { it + 20 },
+            )
+        }
+
+        @Test
+        fun returnsErrIfTransformationThrows() {
+            val value: Result<Int, Throwable> = Ok(10)
+
+            assertEquals(
+                expected = Err(MapException),
+                actual = value.mapCatching { throw MapException },
+            )
+        }
+
+        @Test
+        fun returnsErrorIfErr() {
+            val value: Result<Int, Throwable> = Err(MapException)
+
+            assertEquals(
+                expected = Err(MapException),
+                actual = value.mapCatching { "hello $it" },
+            )
+        }
+    }
+
     class Flatten {
 
         @Test