add foldMapA to Foldable and add a test for sufficient short-circuiti…

…ng (#3130)

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

@@ -333,6 +333,14 @@ import Foldable.sentinel
   def foldMapM[G[_], A, B](fa: F[A])(f: A => G[B])(implicit G: Monad[G], B: Monoid[B]): G[B] =
     foldM(fa, B.empty)((b, a) => G.map(f(a))(B.combine(b, _)))
 
+  /**
+   * Equivalent to foldMapM.
+   * The difference is that foldMapA only requires G to be an Applicative
+   * rather than a Monad. It is also slower due to use of Eval.
+   */
+  def foldMapA[G[_], A, B](fa: F[A])(f: A => G[B])(implicit G: Applicative[G], B: Monoid[B]): G[B] =
+    foldRight(fa, Eval.now(G.pure(B.empty)))((a, egb) => G.map2Eval(f(a), egb)(B.combine)).value
+
   /**
    * Traverse `F[A]` using `Applicative[G]`.
    *

tests/src/test/scala/cats/tests/FoldableSuite.scala

@@ -280,6 +280,18 @@ class FoldableSuiteAdditional extends CatsSuite with ScalaVersionSpecificFoldabl
       (Some(x): Option[String])
     }
     assert(sumMapM == Some("AaronBettyCalvinDeirdra"))
+
+    // foldMapM should short-circuit and not call the function when not necessary
+    val f = (_: String) match {
+      case "Calvin" => None
+      case "Deirdra" =>
+        fail: Unit // : Unit ascription suppresses unreachable code warning
+        None
+      case x => Some(x)
+    }
+    names.foldMapM(f)
+    names.foldMapA(f)
+
     val isNotCalvin: String => Option[String] =
       x => if (x == "Calvin") None else Some(x)
     val notCalvin = F.foldM(names, "") { (acc, x) =>