Merge e197d58 into e592e31

core/src/main/scala/cats/Foldable.scala

@@ -79,6 +79,17 @@ import simulacrum._
   def foldMap[A, B](fa: F[A])(f: A => B)(implicit B: Monoid[B]): B =
     foldLeft(fa, B.empty)((b, a) => B.combine(b, f(a)))
 
+  /**
+   * Left associative monadic folding on `F`.
+   */
+  def foldM[G[_], A, B](fa: F[A], z: B)(f: (B, A) => G[B])
+    (implicit G: Monad[G]): G[B] =
+    partialFold[A, B => G[B]](fa)({a: A =>
+      Fold.Continue({ b =>
+        (w: B) => G.flatMap(f(w, a))(b)
+      })
+    }).complete(Lazy { b: B => G.pure(b) })(z)
+
   /**
    * Traverse `F[A]` using `Applicative[G]`.
    *

tests/src/test/scala/cats/tests/FoldableTests.scala

@@ -26,6 +26,12 @@ class FoldableTestsAdditional extends CatsSuite {
     // more basic checks
     val names = List("Aaron", "Betty", "Calvin", "Deirdra")
     assert(F.foldMap(names)(_.length) == names.map(_.length).sum)
+    val sumM = F.foldM(names, "") { (acc, x) => (Some(acc + x): Option[String]) }
+    assert(sumM == Some("AaronBettyCalvinDeirdra"))
+    val notCalvin = F.foldM(names, "") { (acc, x) =>
+      if (x == "Calvin") (None: Option[String])
+      else (Some(acc + x): Option[String]) }
+    assert(notCalvin == None)
 
     // test trampolining
     val large = (1 to 10000).toList