Add FunctorFilter and TraverseFilter

core/src/main/scala/cats/FunctorEmpty.scala

@@ -0,0 +1,28 @@
+package cats
+
+import simulacrum.typeclass
+
+/**
+ * `FunctorEmpty[F]` allows you to `map` and filter out elements simultaneously.
+ */
+@typeclass
+trait FunctorEmpty[F[_]] extends Serializable {
+  def functor: Functor[F]
+
+  def mapFilter[A, B](fa: F[A])(f: A => Option[B]): F[B]
+
+  def collect[A, B](fa: F[A])(f: PartialFunction[A, B]): F[B] =
+    mapFilter(fa)(f.lift)
+
+  def flattenOption[A](fa: F[Option[A]]): F[A] =
+    mapFilter(fa)(identity)
+
+  def filter[A](fa: F[A])(f: A => Boolean): F[A] =
+    mapFilter(fa)(a => if (f(a)) Some(a) else None)
+}
+
+object FunctorEmpty {
+  implicit def catsFunctorForFunctorEmpty[F[_]](fe: FunctorEmpty[F]): Functor[F] =
+    fe.functor
+}
+

core/src/main/scala/cats/TraverseEmpty.scala

@@ -0,0 +1,31 @@
+package cats
+
+import simulacrum.typeclass
+
+/**
+ * `TraverseEmpty`, also known as `Witherable`, represents list-like structures
+ * that can essentially have a `traverse` and a `filter` applied as a single
+ * combined operation (`traverseFilter`).
+ *
+ * Based on Haskell's [[https://hackage.haskell.org/package/witherable-0.1.3.3/docs/Data-Witherable.html Data.Witherable]]
+ */
+
+@typeclass
+trait TraverseEmpty[F[_]] extends FunctorEmpty[F] {
+  def traverse: Traverse[F]
+
+  override def functor: Functor[F] = traverse
+
+  def traverseFilter[G[_], A, B](fa: F[A])(f: A => G[Option[B]])(implicit G: Applicative[G]): G[F[B]]
+
+  def filterA[G[_], A](fa: F[A])(f: A => G[Boolean])(implicit G: Applicative[G]): G[F[A]] =
+    traverseFilter(fa)(a => G.map(f(a))(if (_) Some(a) else None))
+
+  override def mapFilter[A, B](fa: F[A])(f: A => Option[B]): F[B] =
+    traverseFilter[Id, A, B](fa)(f)
+}
+
+object TraverseEmpty {
+  def catsTraverseForTraverseEmpty[F[_]](te: TraverseEmpty[F]): Traverse[F] =
+    te.traverse
+}

core/src/main/scala/cats/data/Chain.scala

@@ -617,6 +617,29 @@ private[data] sealed abstract class ChainInstances extends ChainInstances1 {
       }
     }
 
+  implicit val catsDataTraverseEmptyForChain: TraverseEmpty[Chain] = new TraverseEmpty[Chain] {
+    def traverse: Traverse[Chain] = Chain.catsDataInstancesForChain
+
+    override def filter[A](fa: Chain[A])(f: A => Boolean): Chain[A] = fa.filter(f)
+
+    override def collect[A, B](fa: Chain[A])(f: PartialFunction[A, B]): Chain[B] = fa.collect(f)
+
+    override def mapFilter[A, B](fa: Chain[A])(f: A => Option[B]): Chain[B] = fa.collect(Function.unlift(f))
+
+    override def flattenOption[A](fa: Chain[Option[A]]): Chain[A] = fa.collect { case Some(a) => a }
+
+    def traverseFilter[G[_], A, B](fa: Chain[A])(f: A => G[Option[B]])(implicit G: Applicative[G]): G[Chain[B]] =
+      fa.foldRight(G.pure(Chain.empty[B]))(
+        (a, gcb) => G.map2(f(a), gcb)((ob, cb) => ob.fold(cb)(_ +: cb))
+      )
+
+    override def filterA[G[_], A](fa: Chain[A])(f: A => G[Boolean])(implicit G: Applicative[G]): G[Chain[A]] =
+      fa.foldRight(G.pure(Chain.empty[A]))(
+        (a, gca) =>
+          G.map2(f(a), gca)((b, chain) => if (b) a +: chain else chain))
+
+  }
+
 }
 
 private[data] sealed abstract class ChainInstances1 extends ChainInstances2 {

core/src/main/scala/cats/data/Const.scala

@@ -87,6 +87,25 @@ private[data] sealed abstract class ConstInstances extends ConstInstances0 {
       fa.traverse(f)
   }
 
+  implicit def catsDataTraverseEmptyForConst[C]: TraverseEmpty[Const[C, ?]] = new TraverseEmpty[Const[C, ?]] {
+
+    override def mapFilter[A, B](fa: Const[C, A])(f: (A) => Option[B]): Const[C, B] = fa.retag
+
+    override def collect[A, B](fa: Const[C, A])(f: PartialFunction[A, B]): Const[C, B] = fa.retag
+
+    override def flattenOption[A](fa: Const[C, Option[A]]): Const[C, A] = fa.retag
+
+    override def filter[A](fa: Const[C, A])(f: (A) => Boolean): Const[C, A] = fa.retag
+
+    def traverseFilter[G[_], A, B](fa: Const[C, A])(f: (A) => G[Option[B]])(implicit G: Applicative[G]): G[Const[C, B]] =
+      G.pure(fa.retag[B])
+
+    override def filterA[G[_], A](fa: Const[C, A])(f: (A) => G[Boolean])(implicit G: Applicative[G]): G[Const[C, A]] =
+      G.pure(fa)
+
+    val traverse: Traverse[Const[C, ?]] = Const.catsDataTraverseForConst[C]
+  }
+
   implicit def catsDataMonoidForConst[A: Monoid, B]: Monoid[Const[A, B]] = new Monoid[Const[A, B]]{
     def empty: Const[A, B] =
       Const.empty

core/src/main/scala/cats/data/EitherT.scala

@@ -3,6 +3,7 @@ package data
 
 import cats.Bifunctor
 import cats.instances.either._
+import cats.instances.option._
 import cats.syntax.either._
 
 /**
@@ -505,6 +506,28 @@ private[data] abstract class EitherTInstances extends EitherTInstances1 {
       def defer[A](fa: => EitherT[F, L, A]): EitherT[F, L, A] =
         EitherT(F.defer(fa.value))
     }
+
+  implicit def catsDataTraverseEmptyForEitherT[F[_], L](implicit F0: TraverseEmpty[F]): TraverseEmpty[EitherT[F, L, ?]] =
+    new EitherTFunctorEmpty[F, L] with TraverseEmpty[EitherT[F, L, ?]] {
+      implicit def F: FunctorEmpty[F] = F0
+      def traverse: Traverse[EitherT[F, L, ?]] = catsDataTraverseForEitherT[F, L](F0.traverse)
+
+      def traverseFilter[G[_], A, B]
+      (fa: EitherT[F, L, A])
+      (f: A => G[Option[B]])
+      (implicit G: Applicative[G]): G[EitherT[F, L, B]] =
+        G.map(
+          F0.traverseFilter[G, Either[L, A], Either[L, B]](fa.value) {
+            case l@Left(_) => G.pure(Option(l.rightCast[B]))
+            case Right(a) => G.map(f(a))(_.map(Either.right))
+          })(EitherT(_))
+
+      override def filterA[G[_], A]
+      (fa: EitherT[F, L, A])
+      (f: A => G[Boolean])
+      (implicit G: Applicative[G]): G[EitherT[F, L, A]] =
+        G.map(F0.filterA(fa.value)(_.fold(_ => G.pure(true), f)))(EitherT[F, L, A])
+    }
 }
 
 private[data] abstract class EitherTInstances1 extends EitherTInstances2 {
@@ -536,6 +559,9 @@ private[data] abstract class EitherTInstances1 extends EitherTInstances2 {
       override def ensureOr[A](fa: EitherT[F, L, A])(error: (A) => L)(predicate: (A) => Boolean): EitherT[F, L, A] =
         fa.ensureOr(error)(predicate)(F)
     }
+
+  implicit def catsDataFunctorEmptyForEitherT[F[_], L](implicit F0: FunctorEmpty[F]): FunctorEmpty[EitherT[F, L, ?]] =
+    new EitherTFunctorEmpty[F, L] { implicit def F = F0 }
 }
 
 private[data] abstract class EitherTInstances2 extends EitherTInstances3 {
@@ -695,3 +721,24 @@ private[data] sealed trait EitherTOrder[F[_], L, A] extends Order[EitherT[F, L,
 
   override def compare(x: EitherT[F, L, A], y: EitherT[F, L, A]): Int = x compare y
 }
+
+private[data] sealed trait EitherTFunctorEmpty[F[_], E] extends FunctorEmpty[EitherT[F, E, ?]] {
+  implicit def F: FunctorEmpty[F]
+
+  override def functor: Functor[EitherT[F, E, ?]] = EitherT.catsDataFunctorForEitherT[F, E](F.functor)
+
+  def mapFilter[A, B](fa: EitherT[F, E, A])(f: (A) => Option[B]): EitherT[F, E, B] =
+    EitherT[F, E, B](F.mapFilter(fa.value)(_.traverse(f)))
+
+  override def collect[A, B](fa: EitherT[F, E, A])(f: PartialFunction[A, B]): EitherT[F, E, B] = {
+    EitherT[F, E, B](F.mapFilter(fa.value)(_.traverse(f.lift)))
+  }
+
+  override def flattenOption[A](fa: EitherT[F, E, Option[A]]): EitherT[F, E, A] = {
+    EitherT[F, E, A](F.flattenOption[Either[E, A]](F.map(fa.value)(Traverse[Either[E, ?]].sequence[Option, A])))
+  }
+
+  override def filter[A](fa: EitherT[F, E, A])(f: (A) => Boolean): EitherT[F, E, A] = {
+    EitherT[F, E, A](F.filter(fa.value)(_.forall(f)))
+  }
+}

core/src/main/scala/cats/data/Nested.scala

@@ -2,7 +2,6 @@ package cats
 package data
 
 
-
 /** Similar to [[cats.data.Tuple2K]], but for nested composition.
  *
  * For instance, since both `List` and `Option` have a `Functor`, then so does
@@ -54,6 +53,14 @@ private[data] sealed abstract class NestedInstances extends NestedInstances0 {
       def defer[A](fa: => Nested[F, G, A]): Nested[F, G, A] =
         Nested(F.defer(fa.value))
     }
+
+  implicit def catsDataTraverseEmptyForNested[F[_], G[_]](implicit F0: Traverse[F], G0: TraverseEmpty[G]): TraverseEmpty[Nested[F, G, ?]] =
+    new NestedTraverseEmpty[F, G] {
+      implicit val F: Traverse[F] = F0
+      implicit val G: TraverseEmpty[G] = G0
+    }
+
+
 }
 
 private[data] sealed abstract class NestedInstances0 extends NestedInstances1 {
@@ -315,3 +322,35 @@ private[data] trait NestedInvariantSemigroupalApply[F[_], G[_]] extends Invarian
   def product[A, B](fa: Nested[F, G, A], fb: Nested[F, G, B]): Nested[F, G, (A, B)] =
     Nested(FG.product(fa.value, fb.value))
 }
+
+private[data] abstract class NestedTraverseEmpty[F[_], G[_]] extends TraverseEmpty[Nested[F, G, ?]] {
+  implicit val F: Traverse[F]
+
+  implicit val G: TraverseEmpty[G]
+
+  def traverse: Traverse[Nested[F, G, ?]] = Nested.catsDataTraverseForNested(F, G.traverse)
+
+  override def mapFilter[A, B](fa: Nested[F, G, A])(f: (A) => Option[B]): Nested[F, G, B] =
+    Nested[F, G, B](F.map(fa.value)(G.mapFilter(_)(f)))
+
+  override def collect[A, B](fa: Nested[F, G, A])(f: PartialFunction[A, B]): Nested[F, G, B] =
+    Nested[F, G, B](F.map(fa.value)(G.collect(_)(f)))
+
+  override def flattenOption[A](fa: Nested[F, G, Option[A]]): Nested[F, G, A] =
+    Nested[F, G, A](F.map(fa.value)(G.flattenOption))
+
+  override def filter[A](fa: Nested[F, G, A])(f: (A) => Boolean): Nested[F, G, A] =
+    Nested[F, G, A](F.map(fa.value)(G.filter(_)(f)))
+
+  override def filterA[H[_], A]
+  (fa: Nested[F, G, A])
+  (f: A => H[Boolean])
+  (implicit H: Applicative[H]): H[Nested[F, G, A]] =
+    H.map(F.traverse(fa.value)(G.filterA[H, A](_)(f)))(Nested[F, G, A])
+
+  def traverseFilter[H[_], A, B]
+  (fga: Nested[F, G, A])
+  (f: A => H[Option[B]])
+  (implicit H: Applicative[H]): H[Nested[F, G, B]] =
+    H.map(F.traverse[H, G[A], G[B]](fga.value)(ga => G.traverseFilter(ga)(f)))(Nested[F, G, B])
+}

core/src/main/scala/cats/data/OptionT.scala

@@ -231,6 +231,20 @@ private[data] sealed abstract class OptionTInstances extends OptionTInstances0 {
       def defer[A](fa: => OptionT[F, A]): OptionT[F, A] =
         OptionT(F.defer(fa.value))
     }
+
+  implicit def optionTFunctorEmpty[F[_]: Functor]: FunctorEmpty[OptionT[F, ?]] = {
+    new FunctorEmpty[OptionT[F, ?]] {
+      override val functor: Functor[OptionT[F, ?]] = OptionT.catsDataFunctorForOptionT[F]
+
+      override def mapFilter[A, B](fa: OptionT[F, A])(f: (A) => Option[B]): OptionT[F, B] = fa.subflatMap(f)
+
+      override def collect[A, B](fa: OptionT[F, A])(f: PartialFunction[A, B]): OptionT[F, B] = fa.subflatMap(f.lift)
+
+      override def flattenOption[A](fa: OptionT[F, Option[A]]): OptionT[F, A] = fa.subflatMap(identity)
+
+      override def filter[A](fa: OptionT[F, A])(f: (A) => Boolean): OptionT[F, A] = fa.filter(f)
+    }
+  }
 }
 
 private[data] sealed abstract class OptionTInstances0 extends OptionTInstances1 {

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

@@ -38,4 +38,9 @@ trait AllInstancesBinCompat0
     with    Tuple2InstancesBinCompat0
 
 trait AllInstancesBinCompat1
-    extends MapInstancesBinCompat0
+    extends OptionInstancesBinCompat0
+    with ListInstancesBinCompat0
+    with VectorInstancesBinCompat0
+    with StreamInstancesBinCompat0
+    with MapInstancesBinCompat0
+    with SortedMapInstancesBinCompat0

core/src/main/scala/cats/instances/list.scala

@@ -143,3 +143,29 @@ trait ListInstances extends cats.kernel.instances.ListInstances {
         fa.iterator.map(_.show).mkString("List(", ", ", ")")
     }
 }
+
+trait ListInstancesBinCompat0 {
+  implicit val catsStdTraverseEmptyForList: TraverseEmpty[List] = new TraverseEmpty[List] {
+    val traverse: Traverse[List] = cats.instances.list.catsStdInstancesForList
+
+    override def mapFilter[A, B](fa: List[A])(f: (A) => Option[B]): List[B] = fa.collect(Function.unlift(f))
+
+    override def filter[A](fa: List[A])(f: (A) => Boolean): List[A] = fa.filter(f)
+
+    override def collect[A, B](fa: List[A])(f: PartialFunction[A, B]): List[B] = fa.collect(f)
+
+    override def flattenOption[A](fa: List[Option[A]]): List[A] = fa.flatten
+
+    def traverseFilter[G[_], A, B](fa: List[A])(f: (A) => G[Option[B]])(implicit G: Applicative[G]): G[List[B]] =
+      fa.foldRight(Eval.now(G.pure(List.empty[B])))(
+        (x, xse) =>
+          G.map2Eval(f(x), xse)((i, o) => i.fold(o)(_ :: o))
+      ).value
+
+    override def filterA[G[_], A](fa: List[A])(f: (A) => G[Boolean])(implicit G: Applicative[G]): G[List[A]] =
+      fa.foldRight(Eval.now(G.pure(List.empty[A])))(
+        (x, xse) =>
+          G.map2Eval(f(x), xse)((b, list) => if (b) x :: list else list)
+      ).value
+  }
+}

core/src/main/scala/cats/instances/map.scala

@@ -107,7 +107,26 @@ trait MapInstancesBinCompat0 {
           }
       }
     }
+  }
+
+  implicit def catsStdFunctorEmptyForMap[K]: FunctorEmpty[Map[K, ?]] = {
+    new FunctorEmpty[Map[K, ?]] {
+
+      val functor: Functor[Map[K, ?]] = cats.instances.map.catsStdInstancesForMap[K]
+
+      def mapFilter[A, B](fa: Map[K, A])(f: A => Option[B]) =
+        fa.collect(scala.Function.unlift(t => f(t._2).map(t._1 -> _)))
 
+      override def collect[A, B](fa: Map[K, A])(f: PartialFunction[A, B]) =
+        fa.collect(scala.Function.unlift(t => f.lift(t._2).map(t._1 -> _)))
+
+      override def flattenOption[A](fa: Map[K, Option[A]]) =
+        fa.collect(scala.Function.unlift(t => t._2.map(t._1 -> _)))
+
+      override def filter[A](fa: Map[K, A])(f: A => Boolean) =
+        fa.filter { case (_, v) => f(v) }
+
+    }
   }
 
 }

core/src/main/scala/cats/instances/option.scala

@@ -121,3 +121,30 @@ trait OptionInstances extends cats.kernel.instances.OptionInstances {
       }
     }
 }
+
+trait OptionInstancesBinCompat0 {
+  implicit val catsStdTraverseEmptyForOption: TraverseEmpty[Option] = new TraverseEmpty[Option] {
+    val traverse: Traverse[Option] = cats.instances.option.catsStdInstancesForOption
+
+    override def mapFilter[A, B](fa: Option[A])(f: (A) => Option[B]): Option[B] = fa.flatMap(f)
+
+    override def filter[A](fa: Option[A])(f: (A) => Boolean): Option[A] = fa.filter(f)
+
+    override def collect[A, B](fa: Option[A])(f: PartialFunction[A, B]): Option[B] = fa.collect(f)
+
+    override def flattenOption[A](fa: Option[Option[A]]): Option[A] = fa.flatten
+
+    def traverseFilter[G[_], A, B](fa: Option[A])(f: (A) => G[Option[B]])(implicit G: Applicative[G]): G[Option[B]] =
+      fa match {
+        case _: None.type => G.pure(Option.empty[B])
+        case Some(a) => f(a)
+      }
+
+    override def filterA[G[_], A](fa: Option[A])(f: (A) => G[Boolean])(implicit G: Applicative[G]): G[Option[A]] =
+      fa match {
+        case _: None.type => G.pure(Option.empty[A])
+        case Some(a) => G.map(f(a))(b => if (b) Some(a) else None)
+      }
+
+  }
+}