Add Defer.recursiveFn to aid in recursion

core/src/main/scala/cats/Defer.scala

@@ -72,6 +72,29 @@ trait Defer[F[_]] extends Serializable {
  lazy val res: F[A] = fn(defer(res))
  res
  }
+
+ /**
+ * Useful when you want a recursive function that returns F where
+ * F[_]: Defer. Examples include IO, Eval, or transformers such
+ * as EitherT or OptionT.
+ * 
+ * example:
+ *
+ * val sumTo: Int => Eval[Int] =
+ * Defer[Eval].recursiveFn[Int, Int] { recur =>
+ * 
+ * { i =>
+ * if (i > 0) recur(i - 1).map(_ + i)
+ * else Eval.now(0)
+ * }
+ * }
+ */
+ def recursiveFn[A, B](fn: (A => F[B]) => (A => F[B])): A => F[B] =
+ new Function1[A, F[B]] { self =>
+ val loopFn: A => F[B] = fn(self)
+
+ def apply(a: A): F[B] = defer(loopFn(a))
+ }
 }
 
 object Defer {

tests/shared/src/test/scala/cats/tests/EvalSuite.scala

@@ -297,4 +297,17 @@ class EvalSuite extends CatsSuite {
  assert(n2 == 1)
  }
  }
+
+ test("test Defer.recursiveFn example") {
+ val sumTo: Int => Eval[Int] =
+ cats.Defer[Eval].recursiveFn[Int, Int] { recur =>
+
+ { i =>
+ if (i > 0) recur(i - 1).map(_ + i)
+ else Eval.now(0)
+ }
+ }
+
+ assert(sumTo(300000).value == (0 to 300000).sum)
+ }
 }