Add foldMapK, reduceMapK (#2292)

* Add foldMapK, reduceMapK

* Fix doctests

core/src/main/scala/cats/syntax/all.scala

@@ -79,4 +79,8 @@ trait AllSyntaxBinCompat2
 
 trait AllSyntaxBinCompat3 extends UnorderedFoldableSyntax with Function1Syntax
 
-trait AllSyntaxBinCompat4 extends TraverseFilterSyntaxBinCompat0 with ParallelApplySyntax
+trait AllSyntaxBinCompat4
+    extends TraverseFilterSyntaxBinCompat0
+    with ParallelApplySyntax
+    with FoldableSyntaxBinCompat0
+    with ReducibleSyntaxBinCompat0

core/src/main/scala/cats/syntax/foldable.scala

@@ -9,6 +9,11 @@ trait FoldableSyntax extends Foldable.ToFoldableOps with UnorderedFoldable.ToUno
     new FoldableOps[F, A](fa)
 }
 
+trait FoldableSyntaxBinCompat0 {
+  implicit final def catsSyntaxFoldableOps0[F[_], A](fa: F[A]): FoldableOps0[F, A] =
+    new FoldableOps0[F, A](fa)
+}
+
 final class NestedFoldableOps[F[_], G[_], A](private val fga: F[G[A]]) extends AnyVal {
   def sequence_(implicit F: Foldable[F], G: Applicative[G]): G[Unit] = F.sequence_(fga)
 
@@ -192,3 +197,21 @@ final class FoldableOps[F[_], A](private val fa: F[A]) extends AnyVal {
     )
 
 }
+
+final class FoldableOps0[F[_], A](val fa: F[A]) extends AnyVal {
+
+  /**
+   * Fold implemented by mapping `A` values into `B` in a context `G` and then
+   * combining them using the `MonoidK[G]` instance.
+   *
+   * {{{
+   * scala> import cats._, cats.implicits._
+   * scala> val f: Int => Endo[String] = i => (s => s + i)
+   * scala> val x: Endo[String] = List(1, 2, 3).foldMapK(f)
+   * scala> val a = x("foo")
+   * a: String = "foo321"
+   * }}}
+   * */
+  def foldMapK[G[_], B](f: A => G[B])(implicit F: Foldable[F], G: MonoidK[G]): G[B] =
+    F.foldMap(fa)(f)(G.algebra)
+}

core/src/main/scala/cats/syntax/package.scala

@@ -26,7 +26,7 @@ package object syntax {
   object either extends EitherSyntax with EitherSyntaxBinCompat0
   object eq extends EqSyntax
   object flatMap extends FlatMapSyntax
-  object foldable extends FoldableSyntax
+  object foldable extends FoldableSyntax with FoldableSyntaxBinCompat0
   object functor extends FunctorSyntax
   object functorFilter extends FunctorFilterSyntax
   object group extends GroupSyntax
@@ -42,7 +42,7 @@ package object syntax {
   object parallel extends ParallelSyntax with ParallelTraverseSyntax with ParallelFlatSyntax with ParallelApplySyntax
   object partialOrder extends PartialOrderSyntax
   object profunctor extends ProfunctorSyntax
-  object reducible extends ReducibleSyntax
+  object reducible extends ReducibleSyntax with ReducibleSyntaxBinCompat0
   object representable extends RepresentableSyntax
   object semigroup extends SemigroupSyntax
   object semigroupal extends SemigroupalSyntax

core/src/main/scala/cats/syntax/reducible.scala

@@ -9,3 +9,26 @@ trait ReducibleSyntax extends Reducible.ToReducibleOps {
 final class NestedReducibleOps[F[_], G[_], A](private val fga: F[G[A]]) extends AnyVal {
   def reduceK(implicit F: Reducible[F], G: SemigroupK[G]): G[A] = F.reduceK(fga)
 }
+
+trait ReducibleSyntaxBinCompat0 {
+  implicit final def catsSyntaxReducibleOps0[F[_], A](fa: F[A]): ReducibleOps0[F, A] =
+    new ReducibleOps0[F, A](fa)
+}
+
+final class ReducibleOps0[F[_], A](val fa: F[A]) extends AnyVal {
+
+  /**
+   * Apply `f` to each element of `fa` and combine them using the
+   * given `SemigroupK[G]`.
+   *
+   * {{{
+   * scala> import cats._, cats.data._, cats.implicits._
+   * scala> val f: Int => Endo[String] = i => (s => s + i)
+   * scala> val x: Endo[String] = NonEmptyList.of(1, 2, 3).reduceMapK(f)
+   * scala> val a = x("foo")
+   * a: String = "foo321"
+   * }}}
+   * */
+  def reduceMapK[G[_], B](f: A => G[B])(implicit F: Reducible[F], G: SemigroupK[G]): G[B] =
+    F.reduceLeftTo(fa)(f)((b, a) => G.combineK(b, f(a)))
+}