fix #2940 (#2942)

typelevel · Jul 12, 2019 · e131e53 · e131e53
1 parent 07b2c21
commit e131e53
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diff --git a/free/src/main/scala/cats/free/Cofree.scala b/free/src/main/scala/cats/free/Cofree.scala
@@ -80,7 +80,7 @@ object Cofree extends CofreeInstances {
    * A stack-safe algebraic recursive fold out of the cofree comonad.
    */
   def cata[F[_], A, B](cof: Cofree[F, A])(folder: (A, F[B]) => Eval[B])(implicit F: Traverse[F]): Eval[B] =
-    F.traverse(cof.tailForced)(cata(_)(folder)).flatMap(folder(cof.head, _))
+    F.traverse(cof.tailForced)(c => Eval.defer(cata(c)(folder))).flatMap(folder(cof.head, _))
 
   /**
    * A monadic recursive fold out of the cofree comonad into a monad which can express Eval's stack-safety.

diff --git a/free/src/test/scala/cats/free/CofreeSuite.scala b/free/src/test/scala/cats/free/CofreeSuite.scala
@@ -108,6 +108,19 @@ class CofreeSuite extends CatsSuite {
     cata should ===(nelUnfoldedHundred)
   }
 
+  test("Cofree.cata is stack-safe") {
+    val unfolded = Cofree.unfold[Option, Int](0)(i => if (i == 50000) None else Some(i + 1))
+    val sum = List.tabulate(50000)(identity).sum
+    val cata =
+      Cofree
+        .cata[Option, Int, Int](unfolded)(
+          (i, lb) => Eval.now(lb.fold(0)(_ + i))
+        )
+        .value
+
+    cata should ===(sum)
+  }
+
   test("Cofree.cataM") {
 
     type EvalOption[A] = OptionT[Eval, A]