Added flatMapN

Author: domaspoliakas

project/Boilerplate.scala

@@ -522,6 +522,12 @@ object Boilerplate {
         else
           s"def traverseN[G[_]: Applicative, Z](f: (${`A..N`}) => G[Z])(implicit traverse: Traverse[F], semigroupal: Semigroupal[F]): G[F[Z]] = Semigroupal.traverse$arity($tupleArgs)(f)"
 
+      val flatMap =
+        if (arity == 1)
+          s"def flatMap[Z](f: (${`A..N`}) => F[Z])(implicit flatMap: FlatMap[F]): F[Z] = flatMap.flatMap($tupleArgs)(f)"
+        else
+          s"def flatMapN[Z](f: (${`A..N`}) => F[Z])(implicit flatMap: FlatMap[F], semigroupal: Semigroupal[F]): F[Z] = flatMap.flatten(Semigroupal.map$arity($tupleArgs)(f))"
+
       block"""
       |package cats
       |package syntax
@@ -534,6 +540,7 @@ object Boilerplate {
         -  $map
         -  $contramap
         -  $imap
+        -  $flatMap
         -  $tupled
         -  $traverse
         -  def apWith[Z](f: F[(${`A..N`}) => Z])(implicit apply: Apply[F]): F[Z] = apply.ap$n(f)($tupleArgs)

tests/src/test/scala/cats/tests/SyntaxSuite.scala

@@ -372,10 +372,14 @@ object SyntaxSuite {
     val fa = a.pure[F]
   }
 
-  def testFlatMap[F[_]: FlatMap, A, B, C, D]: Unit = {
+  def testFlatMap[F[_]: FlatMap, A, B, C, D, Z]: Unit = {
     val a = mock[A]
     val returnValue = mock[F[Either[A, B]]]
     val done = a.tailRecM[F, B](a => returnValue)
+    val tfabc = mock[(F[A], F[B], F[C])]
+    val ff = mock[(A, B, C) => F[Z]]
+
+    tfabc.flatMapN(ff)
 
     val x = mock[Function[A, F[B]]]
     val y = mock[Function[B, F[C]]]