Reversed order of evaluation of Applicative.ap - will reverse order o…

…f params to ap in 1.0
typelevel · Nov 2, 2016 · 38f31cd · 38f31cd · pchlupacek · Nov 2, 2016
1 parent 1edf3fd
commit 38f31cd
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diff --git a/core/shared/src/main/scala/fs2/util/Monad.scala b/core/shared/src/main/scala/fs2/util/Monad.scala
@@ -10,7 +10,7 @@ trait Monad[F[_]] extends Applicative[F] {
   def flatMap[A,B](a: F[A])(f: A => F[B]): F[B]
 
   def map[A,B](a: F[A])(f: A => B): F[B] = flatMap(a)(f andThen pure)
-  def ap[A,B](fa: F[A])(f: F[A => B]): F[B] = flatMap(f)(map(fa))
+  def ap[A,B](fa: F[A])(f: F[A => B]): F[B] = flatMap(fa)(a => map(f)(_(a)))
 }
 
 object Monad {

diff --git a/core/shared/src/test/scala/fs2/util/TraverseSpec.scala b/core/shared/src/test/scala/fs2/util/TraverseSpec.scala
@@ -0,0 +1,14 @@
+package fs2
+package util
+
+class TraverseSpec extends Fs2Spec {
+
+  "Traverse" - {
+    "evaluate effects in left-to-right order" in {
+      var acc = collection.mutable.ListBuffer[Int]()
+      val result = Traverse[List].traverse((1 to 5).toList)(n => Task.delay(acc += n))
+      result.unsafeRunSync()
+      acc.toList shouldBe List(1, 2, 3, 4, 5)
+    }
+  }
+}