Add FunctorFilter and TraverseFilter

core/src/main/scala/cats/Composed.scala

@@ -63,14 +63,30 @@ private[cats] trait ComposedFoldable[F[_], G[_]] extends Foldable[λ[α => F[G[
     F.foldRight(fga, lb)((ga, lb) => G.foldRight(ga, lb)(f))
 }
 
-private[cats] trait ComposedTraverse[F[_], G[_]] extends Traverse[λ[α => F[G[α]]]] with ComposedFoldable[F, G] with ComposedFunctor[F, G] { outer =>
+private[cats] trait ComposedTraverse[F[_], G[_]] extends Traverse[λ[α => F[G[α]]]] with ComposedFoldable[F, G] with ComposedFunctor[F, G] {
   def F: Traverse[F]
   def G: Traverse[G]
 
   override def traverse[H[_]: Applicative, A, B](fga: F[G[A]])(f: A => H[B]): H[F[G[B]]] =
     F.traverse(fga)(ga => G.traverse(ga)(f))
 }
 
+private[cats] trait ComposedTraverseFilter[F[_], G[_]] extends TraverseFilter[λ[α => F[G[α]]]] with ComposedTraverse[F, G] {
+  def F: Traverse[F]
+  def G: TraverseFilter[G]
+
+  override def traverseFilter[H[_]: Applicative, A, B](fga: F[G[A]])(f: A => H[Option[B]]): H[F[G[B]]] =
+    F.traverse[H, G[A], G[B]](fga)(ga => G.traverseFilter(ga)(f))
+}
+
+private[cats] trait ComposedFunctorFilter[F[_], G[_]] extends FunctorFilter[λ[α => F[G[α]]]] with ComposedFunctor[F, G] {
+  def F: Functor[F]
+  def G: FunctorFilter[G]
+
+  override def mapFilter[A, B](fga: F[G[A]])(f: A => Option[B]): F[G[B]] =
+    F.map(fga)(G.mapFilter(_)(f))
+}
+
 private[cats] trait ComposedReducible[F[_], G[_]] extends Reducible[λ[α => F[G[α]]]] with ComposedFoldable[F, G] { outer =>
   def F: Reducible[F]
   def G: Reducible[G]

core/src/main/scala/cats/Functor.scala

@@ -58,6 +58,12 @@ import simulacrum.typeclass
       val G = Functor[G]
     }
 
+  def composeFilter[G[_]: FunctorFilter]: FunctorFilter[λ[α => F[G[α]]]] =
+    new ComposedFunctorFilter[F, G] {
+      val F = self
+      val G = FunctorFilter[G]
+    }
+
   override def composeContravariant[G[_]: Contravariant]: Contravariant[λ[α => F[G[α]]]] =
     new ComposedCovariantContravariant[F, G] {
       val F = self

core/src/main/scala/cats/FunctorFilter.scala

@@ -0,0 +1,62 @@
+package cats
+
+import simulacrum.typeclass
+
+@typeclass trait FunctorFilter[F[_]] extends Functor[F] {
+
+  /**
+   * A combined [[map]] and [[filter]]. Filtering is handled via `Option`
+   * instead of `Boolean` such that the output type `B` can be different than
+   * the input type `A`.
+   *
+   * Example:
+   * {{{
+   * scala> import cats.implicits._
+   * scala> val m: Map[Int, String] = Map(1 -> "one", 3 -> "three")
+   * scala> val l: List[Int] = List(1, 2, 3, 4)
+   * scala> def asString(i: Int): Option[String] = m.get(i)
+   * scala> l.mapFilter(i => m.get(i))
+   * res0: List[String] = List(one, three)
+   * }}}
+   */
+  def mapFilter[A, B](fa: F[A])(f: A => Option[B]): F[B]
+
+  /**
+   * Similar to [[mapFilter]] but uses a partial function instead of a function
+   * that returns an `Option`.
+   *
+   * Example:
+   * {{{
+   * scala> import cats.implicits._
+   * scala> val l: List[Int] = List(1, 2, 3, 4)
+   * scala> FunctorFilter[List].collect(l){
+   *      |   case 1 => "one"
+   *      |   case 3 => "three"
+   *      | }
+   * res0: List[String] = List(one, three)
+   * }}}
+   */
+  def collect[A, B](fa: F[A])(f: PartialFunction[A, B]): F[B] =
+    mapFilter(fa)(f.lift)
+
+  /**
+   * "Flatten" out a structure by collapsing `Option`s.
+   *
+   * Example:
+   * {{{
+   * scala> import cats.implicits._
+   * scala> val l: List[Option[Int]] = List(Some(1), None, Some(3), None)
+   * scala> l.flattenOption
+   * res0: List[Int] = List(1, 3)
+   * }}}
+   */
+  def flattenOption[A](fa: F[Option[A]]): F[A] = mapFilter(fa)(identity)
+
+  /**
+   * Apply a filter to a structure such that the output structure contains all
+   * `A` elements in the input structure that satisfy the predicate `f` but none
+   * that don't.
+   */
+  def filter[A](fa: F[A])(f: A => Boolean): F[A] =
+    mapFilter(fa)(a => if (f(a)) Some(a) else None)
+}

core/src/main/scala/cats/MonadFilter.scala

@@ -4,15 +4,15 @@ import simulacrum.typeclass
 
 /**
  * a Monad equipped with an additional method which allows us to
- * create an "Empty" value for the Monad (for whatever "empty" makes
+ * create an "empty" value for the Monad (for whatever "empty" makes
  * sense for that particular monad). This is of particular interest to
  * us since it allows us to add a `filter` method to a Monad, which is
  * used when pattern matching or using guards in for comprehensions.
  */
-@typeclass trait MonadFilter[F[_]] extends Monad[F] {
+@typeclass trait MonadFilter[F[_]] extends Monad[F] with FunctorFilter[F] {
 
   def empty[A]: F[A]
 
-  def filter[A](fa: F[A])(f: A => Boolean): F[A] =
-    flatMap(fa)(a => if (f(a)) pure(a) else empty[A])
+  override def mapFilter[A, B](fa: F[A])(f: A => Option[B]): F[B] =
+    flatMap(fa)(a => f(a).fold(empty[B])(pure))
 }

core/src/main/scala/cats/Traverse.scala

@@ -110,6 +110,12 @@ import simulacrum.typeclass
       val G = Traverse[G]
     }
 
+  def composeFilter[G[_]: TraverseFilter]: TraverseFilter[λ[α => F[G[α]]]] =
+    new ComposedTraverseFilter[F, G] {
+      val F = self
+      val G = TraverseFilter[G]
+    }
+
   override def map[A, B](fa: F[A])(f: A => B): F[B] =
     traverse[Id, A, B](fa)(f)
 }

core/src/main/scala/cats/TraverseFilter.scala

@@ -0,0 +1,65 @@
+package cats
+
+import simulacrum.typeclass
+
+/**
+ * `TraverseFilter`, also known as `Witherable`, represents list-like structures
+ * that can essentially have a [[traverse]] and a [[filter]] applied as a single
+ * combined operation ([[traverseFilter]]).
+ *
+ * Must obey the laws defined in cats.laws.TraverseFilterLaws.
+ *
+ * Based on Haskell's [[https://hackage.haskell.org/package/witherable-0.1.3.3/docs/Data-Witherable.html Data.Witherable]]
+ */
+@typeclass trait TraverseFilter[F[_]] extends Traverse[F] with FunctorFilter[F] { self =>
+
+  /**
+   * A combined [[traverse]] and [[filter]]. Filtering is handled via `Option`
+   * instead of `Boolean` such that the output type `B` can be different than
+   * the input type `A`.
+   *
+   * Example:
+   * {{{
+   * scala> import cats.implicits._
+   * scala> val m: Map[Int, String] = Map(1 -> "one", 3 -> "three")
+   * scala> val l: List[Int] = List(1, 2, 3, 4)
+   * scala> def asString(i: Int): Eval[Option[String]] = Now(m.get(i))
+   * scala> val result: Eval[List[String]] = l.traverseFilter(asString)
+   * scala> result.value
+   * res0: List[String] = List(one, three)
+   * }}}
+   */
+  def traverseFilter[G[_]: Applicative, A, B](fa: F[A])(f: A => G[Option[B]]): G[F[B]]
+
+  override def mapFilter[A, B](fa: F[A])(f: A => Option[B]): F[B] =
+    traverseFilter[Id, A, B](fa)(f)
+
+  /**
+   *
+   * Filter values inside a `G` context.
+   *
+   * This is a generalized version of Haskell's [[http://hackage.haskell.org/package/base-4.9.0.0/docs/Control-Monad.html#v:filterM filterM]].
+   * [[http://stackoverflow.com/questions/28872396/haskells-filterm-with-filterm-x-true-false-1-2-3 This StackOverflow question]] about `filterM` may be helpful in understanding how it behaves.
+   *
+   * Example:
+   * {{{
+   * scala> import cats.implicits._
+   * scala> val l: List[Int] = List(1, 2, 3, 4)
+   * scala> def odd(i: Int): Eval[Boolean] = Now(i % 2 == 1)
+   * scala> val res: Eval[List[Int]] = l.filterA(odd)
+   * scala> res.value
+   * res0: List[Int] = List(1, 3)
+   *
+   * scala> List(1, 2, 3).filterA(_ => List(true, false))
+   * res1: List[List[Int]] = List(List(1, 2, 3), List(1, 2), List(1, 3), List(1), List(2, 3), List(2), List(3), List())
+   * }}}
+   */
+  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 filter[A](fa: F[A])(f: A => Boolean): F[A] =
+    filterA[Id, A](fa)(f)
+
+  override def traverse[G[_], A, B](fa: F[A])(f: A => G[B])(implicit G: Applicative[G]): G[F[B]] =
+    traverseFilter(fa)(a => G.map(f(a))(Some(_)))
+}

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

@@ -17,6 +17,9 @@ final case class Const[A, B](getConst: A) {
   def combine(that: Const[A, B])(implicit A: Semigroup[A]): Const[A, B] =
     Const(A.combine(getConst, that.getConst))
 
+  def traverseFilter[F[_], C](f: B => F[Option[C]])(implicit F: Applicative[F]): F[Const[A, C]] =
+    F.pure(retag[C])
+
   def traverse[F[_], C](f: B => F[C])(implicit F: Applicative[F]): F[Const[A, C]] =
     F.pure(retag[C])
 
@@ -53,13 +56,16 @@ private[data] sealed abstract class ConstInstances extends ConstInstances0 {
       fa.retag[B]
   }
 
-  implicit def catsDataTraverseForConst[C]: Traverse[Const[C, ?]] = new Traverse[Const[C, ?]] {
-    def traverse[G[_]: Applicative, A, B](fa: Const[C, A])(f: A => G[B]): G[Const[C, B]] =
-      fa.traverse(f)
+  implicit def catsDataTraverseFilterForConst[C]: TraverseFilter[Const[C, ?]] = new TraverseFilter[Const[C, ?]] {
+    def traverseFilter[G[_]: Applicative, A, B](fa: Const[C, A])(f: A => G[Option[B]]): G[Const[C, B]] =
+      fa.traverseFilter(f)
 
     def foldLeft[A, B](fa: Const[C, A], b: B)(f: (B, A) => B): B = b
 
     def foldRight[A, B](fa: Const[C, A], lb: Eval[B])(f: (A, Eval[B]) => Eval[B]): Eval[B] = lb
+
+    override def traverse[G[_]: Applicative, A, B](fa: Const[C, A])(f: A => G[B]): G[Const[C, B]] =
+      fa.traverse(f)
   }
 
   implicit def catsDataMonoidForConst[A: Monoid, B]: Monoid[Const[A, B]] = new Monoid[Const[A, B]]{

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

@@ -27,10 +27,18 @@ final case class Nested[F[_], G[_], A](value: F[G[A]])
 
 object Nested extends NestedInstances
 
-private[data] sealed abstract class NestedInstances extends NestedInstances1 {
+private[data] sealed abstract class NestedInstances extends NestedInstances0 {
   implicit def catsDataEqForNested[F[_], G[_], A](implicit FGA: Eq[F[G[A]]]): Eq[Nested[F, G, A]] =
     FGA.on(_.value)
 
+  implicit def catsDataTraverseFilterForNested[F[_]: Traverse, G[_]: TraverseFilter]: TraverseFilter[Nested[F, G, ?]] =
+    new NestedTraverseFilter[F, G] {
+      val FG: TraverseFilter[λ[α => F[G[α]]]] = Traverse[F].composeFilter[G]
+    }
+}
+
+private[data] sealed abstract class NestedInstances0 extends NestedInstances1 {
+
   implicit def catsDataTraverseForNested[F[_]: Traverse, G[_]: Traverse]: Traverse[Nested[F, G, ?]] =
     new NestedTraverse[F, G] {
       val FG: Traverse[λ[α => F[G[α]]]] = Traverse[F].compose[G]
@@ -118,7 +126,14 @@ private[data] sealed abstract class NestedInstances8 extends NestedInstances9 {
     }
 }
 
-private[data] sealed abstract class NestedInstances9 {
+private[data] sealed abstract class NestedInstances9 extends NestedInstances10 {
+  implicit def catsDataFunctorFilterForNested[F[_]: Functor, G[_]: FunctorFilter]: FunctorFilter[Nested[F, G, ?]] =
+    new NestedFunctorFilter[F, G] {
+      val FG: FunctorFilter[λ[α => F[G[α]]]] = Functor[F].composeFilter[G]
+    }
+}
+
+private[data] sealed abstract class NestedInstances10 {
   implicit def catsDataInvariantForNestedContravariant[F[_]: Invariant, G[_]: Contravariant]: Invariant[Nested[F, G, ?]] =
     new NestedInvariant[F, G] {
       val FG: Invariant[λ[α => F[G[α]]]] = Invariant[F].composeContravariant[G]
@@ -139,6 +154,26 @@ private[data] trait NestedFunctor[F[_], G[_]] extends Functor[Nested[F, G, ?]] w
     Nested(FG.map(fga.value)(f))
 }
 
+private[data] trait NestedFunctorFilter[F[_], G[_]] extends FunctorFilter[Nested[F, G, ?]] with NestedFunctor[F, G] {
+  override def FG: FunctorFilter[λ[α => F[G[α]]]]
+
+  override def mapFilter[A, B](fga: Nested[F, G, A])(f: A => Option[B]): Nested[F, G, B] =
+    Nested(FG.mapFilter(fga.value)(f))
+
+  override def collect[A, B](fga: Nested[F, G, A])(f: PartialFunction[A, B]): Nested[F, G, B] =
+    Nested(FG.collect(fga.value)(f))
+
+  override def filter[A](fga: Nested[F, G, A])(f: A => Boolean): Nested[F, G, A] =
+    Nested(FG.filter(fga.value)(f))
+}
+
+private[data] trait NestedTraverseFilter[F[_], G[_]] extends TraverseFilter[Nested[F, G, ?]] with NestedFunctorFilter[F, G] with NestedTraverse[F, G] {
+  override def FG: TraverseFilter[λ[α => F[G[α]]]]
+
+  override def traverseFilter[H[_]: Applicative, A, B](fga: Nested[F, G, A])(f: A => H[Option[B]]): H[Nested[F, G, B]] =
+    Applicative[H].map(FG.traverseFilter(fga.value)(f))(Nested(_))
+}
+
 private[data] trait NestedApply[F[_], G[_]] extends Apply[Nested[F, G, ?]] with NestedFunctor[F, G] {
   override def FG: Apply[λ[α => F[G[α]]]]
 

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

@@ -30,6 +30,9 @@ final case class OptionT[F[_], A](value: F[Option[A]]) {
   def semiflatMap[B](f: A => F[B])(implicit F: Monad[F]): OptionT[F, B] =
     flatMap(a => OptionT.liftF(f(a)))
 
+  def mapFilter[B](f: A => Option[B])(implicit F: Functor[F]): OptionT[F, B] =
+    OptionT(F.map(value)(_.flatMap(f)))
+
   def flatMap[B](f: A => OptionT[F, B])(implicit F: Monad[F]): OptionT[F, B] =
     flatMapF(a => f(a).value)
 
@@ -99,6 +102,9 @@ final case class OptionT[F[_], A](value: F[Option[A]]) {
   def ===(that: OptionT[F, A])(implicit eq: Eq[F[Option[A]]]): Boolean =
     eq.eqv(value, that.value)
 
+  def traverseFilter[G[_], B](f: A => G[Option[B]])(implicit F: Traverse[F], G: Applicative[G]): G[OptionT[F, B]] =
+    G.map(F.composeFilter(optionInstance).traverseFilter(value)(f))(OptionT.apply)
+
   def traverse[G[_], B](f: A => G[B])(implicit F: Traverse[F], G: Applicative[G]): G[OptionT[F, B]] =
     G.map(F.compose(optionInstance).traverse(value)(f))(OptionT.apply)
 
@@ -219,19 +225,22 @@ private[data] sealed trait OptionTInstances1 extends OptionTInstances2 {
 }
 
 private[data] sealed trait OptionTInstances2 extends OptionTInstances3 {
-  implicit def catsDataTraverseForOptionT[F[_]](implicit F0: Traverse[F]): Traverse[OptionT[F, ?]] =
-    new OptionTTraverse[F] { implicit val F = F0 }
+  implicit def catsDataTraverseForOptionT[F[_]](implicit F0: Traverse[F]): TraverseFilter[OptionT[F, ?]] =
+    new OptionTTraverseFilter[F] { implicit val F = F0 }
 }
 
 private[data] sealed trait OptionTInstances3 {
-  implicit def catsDataFunctorForOptionT[F[_]](implicit F0: Functor[F]): Functor[OptionT[F, ?]] =
+  implicit def catsDataFunctorFilterForOptionT[F[_]](implicit F0: Functor[F]): FunctorFilter[OptionT[F, ?]] =
     new OptionTFunctor[F] { implicit val F = F0 }
 }
 
-private[data] trait OptionTFunctor[F[_]] extends Functor[OptionT[F, ?]] {
+private[data] trait OptionTFunctor[F[_]] extends FunctorFilter[OptionT[F, ?]] {
   implicit def F: Functor[F]
 
   override def map[A, B](fa: OptionT[F, A])(f: A => B): OptionT[F, B] = fa.map(f)
+
+  override def mapFilter[A, B](fa: OptionT[F, A])(f: A => Option[B]): OptionT[F, B] =
+    fa.mapFilter(f)
 }
 
 private[data] trait OptionTMonad[F[_]] extends Monad[OptionT[F, ?]] {
@@ -273,9 +282,12 @@ private[data] trait OptionTFoldable[F[_]] extends Foldable[OptionT[F, ?]] {
     fa.foldRight(lb)(f)
 }
 
-private[data] sealed trait OptionTTraverse[F[_]] extends Traverse[OptionT[F, ?]] with OptionTFoldable[F] {
+private[data] sealed trait OptionTTraverseFilter[F[_]] extends TraverseFilter[OptionT[F, ?]] with OptionTFoldable[F] {
   implicit def F: Traverse[F]
 
+  def traverseFilter[G[_]: Applicative, A, B](fa: OptionT[F, A])(f: A => G[Option[B]]): G[OptionT[F, B]] =
+    fa traverseFilter f
+
   override def traverse[G[_]: Applicative, A, B](fa: OptionT[F, A])(f: A => G[B]): G[OptionT[F, B]] =
     fa traverse f
 }

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

@@ -10,8 +10,8 @@ import cats.data.Xor
 
 trait ListInstances extends cats.kernel.instances.ListInstances {
 
-  implicit val catsStdInstancesForList: Traverse[List] with MonadCombine[List] with MonadRec[List] with CoflatMap[List] =
-    new Traverse[List] with MonadCombine[List] with MonadRec[List] with CoflatMap[List] {
+  implicit val catsStdInstancesForList: TraverseFilter[List] with MonadCombine[List] with MonadRec[List] with CoflatMap[List] =
+    new TraverseFilter[List] with MonadCombine[List] with MonadRec[List] with CoflatMap[List] {
 
       def empty[A]: List[A] = Nil
 
@@ -63,7 +63,12 @@ trait ListInstances extends cats.kernel.instances.ListInstances {
         Eval.defer(loop(fa))
       }
 
-      def traverse[G[_], A, B](fa: List[A])(f: A => G[B])(implicit G: Applicative[G]): G[List[B]] =
+      def traverseFilter[G[_], A, B](fa: List[A])(f: A => G[Option[B]])(implicit G: Applicative[G]): G[List[B]] =
+        foldRight[A, G[List[B]]](fa, Always(G.pure(List.empty))){ (a, lglb) =>
+          G.map2Eval(f(a), lglb)((ob, l) => ob.fold(l)(_ :: l))
+        }.value
+
+      override def traverse[G[_], A, B](fa: List[A])(f: A => G[B])(implicit G: Applicative[G]): G[List[B]] =
         foldRight[A, G[List[B]]](fa, Always(G.pure(List.empty))){ (a, lglb) =>
           G.map2Eval(f(a), lglb)(_ :: _)
         }.value
@@ -75,6 +80,8 @@ trait ListInstances extends cats.kernel.instances.ListInstances {
         fa.forall(p)
 
       override def isEmpty[A](fa: List[A]): Boolean = fa.isEmpty
+
+      override def filter[A](fa: List[A])(f: A => Boolean): List[A] = fa.filter(f)
     }
 
   implicit def catsStdShowForList[A:Show]: Show[List[A]] =