Merge pull request #1193 from benhutchison/topic/reduceM

Implement Reducible.reduceM

core/src/main/scala/cats/Reducible.scala

@@ -59,6 +59,12 @@ import simulacrum.typeclass
    */
   def reduceLeftTo[A, B](fa: F[A])(f: A => B)(g: (B, A) => B): B
 
+  /**
+   *  Monadic variant of [[reduceLeftTo]]
+   */
+  def reduceLeftM[G[_], A, B](fa: F[A])(f: A => G[B])(g: (B, A) => G[B])(implicit G: FlatMap[G]): G[B] =
+    reduceLeftTo(fa)(f)((gb, a) => G.flatMap(gb)(g(_, a)))
+
   /**
    * Overriden from Foldable[_] for efficiency.
    */

tests/src/test/scala/cats/tests/ReducibleTests.scala

@@ -0,0 +1,18 @@
+package cats
+package tests
+
+class ReducibleTestsAdditional extends CatsSuite {
+
+  test("Reducible[NonEmptyList].reduceLeftM stack safety") {
+    def nonzero(acc: Long, x: Long): Option[Long] =
+      if (x == 0) None else Some(acc + x)
+
+    val n = 100000L
+    val expected = n*(n+1)/2
+    val actual = (1L to n).toList.toNel.flatMap(_.reduceLeftM(Option.apply)(nonzero))
+    actual should === (Some(expected))
+  }
+
+}
+
+