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fix Cofree.cata stack-safety #2940 #2942

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Jul 12, 2019
Merged

fix Cofree.cata stack-safety #2940 #2942

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2 changes: 1 addition & 1 deletion free/src/main/scala/cats/free/Cofree.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -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.
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13 changes: 13 additions & 0 deletions free/src/test/scala/cats/free/CofreeSuite.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -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]
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