Add sequenceFilter syntax

core/src/main/scala/cats/implicits.scala

@@ -6,6 +6,7 @@ object implicits
     with syntax.AllSyntaxBinCompat1
     with syntax.AllSyntaxBinCompat2
     with syntax.AllSyntaxBinCompat3
+    with syntax.AllSyntaxBinCompat4
     with instances.AllInstances
     with instances.AllInstancesBinCompat0
     with instances.AllInstancesBinCompat1

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

@@ -7,6 +7,7 @@ abstract class AllSyntaxBinCompat
     with AllSyntaxBinCompat1
     with AllSyntaxBinCompat2
     with AllSyntaxBinCompat3
+    with AllSyntaxBinCompat4
 
 trait AllSyntax
     extends AlternativeSyntax
@@ -77,3 +78,5 @@ trait AllSyntaxBinCompat2
     with ValidatedSyntaxBincompat0
 
 trait AllSyntaxBinCompat3 extends UnorderedFoldableSyntax with Function1Syntax
+
+trait AllSyntaxBinCompat4 extends TraverseFilterSyntaxBinCompat0

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

@@ -51,7 +51,7 @@ package object syntax {
   object strong extends StrongSyntax
   object try_ extends TrySyntax
   object traverse extends TraverseSyntax
-  object traverseFilter extends TraverseFilterSyntax
+  object traverseFilter extends TraverseFilterSyntax with TraverseFilterSyntaxBinCompat0
   object nonEmptyTraverse extends NonEmptyTraverseSyntax
   object unorderedFoldable extends UnorderedFoldableSyntax
   object unorderedTraverse extends UnorderedTraverseSyntax

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

@@ -2,3 +2,21 @@ package cats
 package syntax
 
 trait TraverseFilterSyntax extends TraverseFilter.ToTraverseFilterOps
+
+trait TraverseFilterSyntaxBinCompat0 {
+  implicit def toSequenceFilterOps[F[_], G[_], A](fgoa: F[G[Option[A]]]): SequenceFilterOps[F, G, A] =
+    new SequenceFilterOps(fgoa)
+}
+
+final class SequenceFilterOps[F[_], G[_], A](val fgoa: F[G[Option[A]]]) extends AnyVal {
+
+  /**
+   * {{{
+   * scala> import cats.implicits._
+   * scala> val a: List[Either[String, Option[Int]]] = List(Right(Some(1)), Right(Some(5)), Right(Some(3)))
+   * scala> val b: Either[String, List[Int]] = a.sequenceFilter
+   * b: Either[String, List[Int]] = Right(List(1, 5, 3))
+   * }}}
+   * */
+  def sequenceFilter(implicit F: TraverseFilter[F], G: Applicative[G]): G[F[A]] = F.traverseFilter(fgoa)(identity)
+}

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

@@ -5,8 +5,8 @@ import scala.collection.immutable.SortedSet
 import scala.collection.immutable.SortedMap
 import cats.arrow.Compose
 import cats.data.{Binested, Nested, NonEmptyChain, NonEmptyList, NonEmptySet}
-import cats.instances.AllInstances
-import cats.syntax.{AllSyntax, AllSyntaxBinCompat}
+import cats.instances.{AllInstances, AllInstancesBinCompat0, AllInstancesBinCompat1, AllInstancesBinCompat2}
+import cats.syntax.AllSyntaxBinCompat
 
 /**
  * Test that our syntax implicits are working.
@@ -26,7 +26,12 @@ import cats.syntax.{AllSyntax, AllSyntaxBinCompat}
  *
  * None of these tests should ever run, or do any runtime checks.
  */
-object SyntaxSuite extends AllSyntaxBinCompat with AllInstances with AllSyntax {
+object SyntaxSuite
+    extends AllSyntaxBinCompat
+    with AllInstances
+    with AllInstancesBinCompat0
+    with AllInstancesBinCompat1
+    with AllInstancesBinCompat2 {
 
   // pretend we have a value of type A
   def mock[A]: A = ???
@@ -387,4 +392,9 @@ object SyntaxSuite extends AllSyntaxBinCompat with AllInstances with AllSyntax {
     val grouped: SortedMap[B, NonEmptyChain[A]] = list.groupByNec(f)
   }
 
+  def testSequenceFilter[A, B]: Unit = {
+    val f = mock[List[Either[A, Option[B]]]]
+
+    val result: Either[A, List[B]] = f.sequenceFilter
+  }
 }