Change MonadErrorOps#reject so it no longer runs effects twice (#2810)

* Add regression test for MonadErrorOps bug * Change MonadErrorOps#reject so it no longer re-runs the effect it is called on. Fixes #2809
typelevel · Apr 25, 2019 · 0d6aafc · 0d6aafc
1 parent a6723ac
commit 0d6aafc
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diff --git a/core/src/main/scala/cats/syntax/monadError.scala b/core/src/main/scala/cats/syntax/monadError.scala
@@ -24,7 +24,7 @@ final class MonadErrorOps[F[_], E, A](private val fa: F[A]) extends AnyVal {
    */
   def reject(pf: PartialFunction[A, E])(implicit F: MonadError[F, E]): F[A] =
     F.flatMap(fa) { a =>
-      pf.andThen(F.raiseError[A] _).applyOrElse(a, (_: A) => fa)
+      pf.andThen(F.raiseError[A] _).applyOrElse(a, F.pure)
     }
 
   def adaptError(pf: PartialFunction[E, E])(implicit F: MonadError[F, E]): F[A] =

diff --git a/tests/src/test/scala/cats/tests/RegressionSuite.scala b/tests/src/test/scala/cats/tests/RegressionSuite.scala
@@ -1,7 +1,7 @@
 package cats
 package tests
 
-import cats.data.{Const, NonEmptyList}
+import cats.data.{Const, NonEmptyList, StateT}
 import scala.collection.mutable
 import scala.collection.immutable.SortedMap
 class RegressionSuite extends CatsSuite {
@@ -157,4 +157,8 @@ class RegressionSuite extends CatsSuite {
 
   }
 
+  test("#2809 MonadErrorOps.reject runs effects only once") {
+    val program = StateT.modify[Either[Throwable, ?], Int](_ + 1).reject { case _ if false => new Throwable }
+    program.runS(0).right.get should ===(1)
+  }
 }