Merge pull request #1769 from tpolecat/kleisli-tap

Kleisli tap, tapWith
typelevel · Jul 21, 2017 · 0df2821 · 0df2821
2 parents 16ea2ed + 11988af
commit 0df2821
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diff --git a/core/src/main/scala/cats/data/Kleisli.scala b/core/src/main/scala/cats/data/Kleisli.scala
@@ -60,6 +60,14 @@ final case class Kleisli[F[_], A, B](run: A => F[B]) { self =>
   def second[C](implicit F: Functor[F]): Kleisli[F, (C, A), (C, B)] =
     Kleisli{ case (c, a) => F.map(run(a))(c -> _)}
 
+  /** Discard computed B and yield the input value. */
+  def tap(implicit F: Functor[F]): Kleisli[F, A, A] =
+    Kleisli(a => F.map(run(a))(_ => a))
+
+  /** Yield computed B combined with input value. */
+  def tapWith[C](f: (A, B) => C)(implicit F: Functor[F]): Kleisli[F, A, C] =
+    Kleisli(a => F.map(run(a))(b => f(a, b)))
+
   def apply(a: A): F[B] = run(a)
 }
 

diff --git a/tests/src/test/scala/cats/tests/KleisliTests.scala b/tests/src/test/scala/cats/tests/KleisliTests.scala
@@ -95,7 +95,7 @@ class KleisliTests extends CatsSuite {
     checkAll("Kleisli[Option, Int, Int]", FunctorTests[Kleisli[Option, Int, ?]].functor[Int, Int, Int])
     checkAll("Functor[Kleisli[Option, Int, ?]]", SerializableTests.serializable(Functor[Kleisli[Option, Int, ?]]))
   }
-  
+
   {
     implicit val catsDataMonoidForKleisli = Kleisli.catsDataMonoidForKleisli[Option, Int, String]
     checkAll("Kleisli[Option, Int, String]", GroupLaws[Kleisli[Option, Int, String]].monoid)
@@ -159,6 +159,18 @@ class KleisliTests extends CatsSuite {
     }
   }
 
+  test("tap") {
+    forAll { (f: Kleisli[List, Int, String], i: Int) =>
+      f.run(i).as(i) should === (f.tap.run(i))
+    }
+  }
+
+  test("tapWith") {
+    forAll { (f: Kleisli[List, Int, String], g: (Int, String) => Boolean, i: Int) =>
+      f.run(i).map(s => g(i, s)) should === (f.tapWith(g).run(i))
+    }
+  }
+
   test("apply") {
     forAll { (f: Kleisli[List, Int, Int], i: Int) =>
       f.run(i) should === (f(i))