From fe3d57cb0d6b92e891037a4f8c5c58907878a6cd Mon Sep 17 00:00:00 2001
From: Aleksey Troitskiy <amtroitskiy@gmail.com>
Date: Fri, 12 Jul 2019 18:02:30 +0300
Subject: [PATCH] fix #2940

---
 free/src/main/scala/cats/free/Cofree.scala      |  2 +-
 free/src/test/scala/cats/free/CofreeSuite.scala | 13 +++++++++++++
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/free/src/main/scala/cats/free/Cofree.scala b/free/src/main/scala/cats/free/Cofree.scala
index 36c4893a11..051c916d1a 100644
--- 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
index 6bbf13c6d9..14fe7f94a2 100644
--- 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]