Adds Result tap and flatTap (#90)

Author: Synesso

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 ### Added
 * Adds `Result<T>.unit(): Result<Unit>` as alias for `.map { }` (Jem Mawson)
+* Adds `Result<T>.tap` and `Result<T>.flatTap` (Jem Mawson)
 
 ## [0.5.5] - 2024-06-20
 

lib/api/lib.api

@@ -1,5 +1,8 @@
 public final class app/cash/quiver/Absent : app/cash/quiver/Outcome {
 	public static final field INSTANCE Lapp/cash/quiver/Absent;
+	public fun equals (Ljava/lang/Object;)Z
+	public fun hashCode ()I
+	public fun toString ()Ljava/lang/String;
 }
 
 public final class app/cash/quiver/Failure : app/cash/quiver/Outcome {
@@ -272,10 +275,12 @@ public final class app/cash/quiver/extensions/OptionKt {
 
 public final class app/cash/quiver/extensions/ResultKt {
 	public static final fun failure (Ljava/lang/Throwable;)Ljava/lang/Object;
+	public static final fun flatTap (Ljava/lang/Object;Lkotlin/jvm/functions/Function1;)Ljava/lang/Object;
 	public static final fun flatten (Ljava/lang/Object;)Ljava/lang/Object;
 	public static final fun mapFailure (Ljava/lang/Object;Lkotlin/jvm/functions/Function1;)Ljava/lang/Object;
 	public static final fun orThrow (Ljava/lang/Object;)Ljava/lang/Object;
 	public static final fun success (Ljava/lang/Object;)Ljava/lang/Object;
+	public static final fun tap (Ljava/lang/Object;Lkotlin/jvm/functions/Function1;)Ljava/lang/Object;
 	public static final fun toEither (Ljava/lang/Object;)Larrow/core/Either;
 	public static final fun toResult (Ljava/lang/Object;Lkotlin/jvm/functions/Function0;)Ljava/lang/Object;
 	public static final fun tryCatch (Lkotlin/Result$Companion;Lkotlin/jvm/functions/Function0;)Ljava/lang/Object;

lib/src/main/kotlin/app/cash/quiver/Outcome.kt

@@ -108,7 +108,7 @@ sealed class Outcome<out E, out A> constructor(val inner: Either<E, Option<A>>)
  */
 data class Present<A>(val value: A) : Outcome<Nothing, A>(value.some().right())
 data class Failure<E>(val error: E) : Outcome<E, Nothing>(error.left())
-object Absent : Outcome<Nothing, Nothing>(None.right())
+data object Absent : Outcome<Nothing, Nothing>(None.right())
 
 fun <A> A.present(): Outcome<Nothing, A> = Present(this)
 fun <E> E.failure(): Outcome<E, Nothing> = Failure(this)

lib/src/main/kotlin/app/cash/quiver/extensions/Result.kt

@@ -63,3 +63,20 @@ fun <T> Result<Result<T>>.flatten(): Result<T> = flatMap(::identity)
  * Map success to Unit, included for consistency with Either.
  */
 fun <T> Result<T>.unit() = map { }
+
+/**
+ * Performs an effect over successes but maps the original value back into
+ * the Result.
+ */
+inline fun <A, B> Result<A>.tap(f: (A) -> B): Result<A> = this.map { a ->
+  f(a)
+  a
+}
+
+/**
+ * Performs an effect over successes but maps the original value back into
+ * the Result.  This is useful for mixing with validation functions.
+ */
+inline fun <A> Result<A>.flatTap(f: (A) -> Result<Any>): Result<A> = this.flatMap { a ->
+  f(a).map { a }
+}

lib/src/test/kotlin/app/cash/quiver/extensions/ResultTest.kt

@@ -1,5 +1,6 @@
 package app.cash.quiver.extensions
 
+import arrow.core.raise.result
 import io.kotest.assertions.arrow.core.shouldBeLeft
 import io.kotest.assertions.arrow.core.shouldBeRight
 import io.kotest.assertions.throwables.shouldThrow
@@ -79,4 +80,24 @@ class ResultTest : StringSpec({
     e.failure<Int>().unit() shouldBe e.failure()
   }
 
+  "tap performs computation but keeps original value" {
+    val right: Result<String> = "hello".success()
+    var sideEffect: String? = null
+
+    right.tap { a -> sideEffect = "$a world" }.shouldBeSuccess("hello")
+    sideEffect shouldBe "hello world"
+  }
+
+  "flatTap performs computation but keeps original value" {
+    val e = RuntimeException("banana")
+    val right: Result<String> = "hello".success()
+    var sideEffect: String? = null
+
+    right.flatTap { a -> result { sideEffect = "$a world" } }.shouldBeSuccess("hello")
+    sideEffect shouldBe "hello world"
+
+    right.flatTap { Result.failure<Int>(e) }.shouldBeFailure(e)
+    e.failure<Int>().flatTap { Result.failure(Exception("broken")) }.shouldBeFailure(e)
+  }
+
 })