splitWhen & splitWhenM for Foldable

core/src/main/scala/cats/Foldable.scala

@@ -23,6 +23,7 @@ package cats
 
 import scala.collection.mutable
 import cats.kernel.CommutativeMonoid
+import cats.data.NonEmptyList
 
 import Foldable.{sentinel, Source}
 
@@ -960,6 +961,73 @@ trait Foldable[F[_]] extends UnorderedFoldable[F] with FoldableNFunctions[F] { s
     import cats.instances.either.*
     partitionBifoldM[G, Either, A, B, C](fa)(f)(A, M, Bifoldable[Either])
   }
+
+  /**
+   * Split this Foldable into a NonEmptyList of Lists based on a predicate.
+   * The behaviour is aimed to be identical to that of haskell's `splitWhen`
+   * 
+   * {{{
+   * scala> import cats.syntax.all._, cats.Foldable, cats.data.NonEmptyList
+   * scala> Foldable[List].splitWhen(List(1,1))(_ == 1)
+   * res0: NonEmptyList[List[Int]] = NonEmptyList(List(), List(), List())
+   * scala> Foldable[List].splitWhen(Nil)(_ == 1)
+   * res1: NonEmptyList[List[Nothing]] = NonEmptyList(List())
+   * scala> Foldable[List].splitWhen(List(1, 2, 3, 1, 4, 5))(_ == 1)
+   * res2: NonEmptyList[List[Int]] = NonEmptyList(List(), List(2, 3), List(4, 5))
+   * }}}
+   */
+
+  def splitWhen[A](fa: F[A])(f: A => Boolean)(implicit
+    FA: Alternative[F]
+  ): NonEmptyList[F[A]] = {
+    foldRight(fa, Eval.now(NonEmptyList.one(FA.empty[A]))) {
+      case (a, acc) if f(a) => acc.map(FA.empty[A] :: _)
+      case (a, acc)         => acc.map(nel => NonEmptyList(FA.prependK(a, nel.head), nel.tail))
+    }.value
+  }
+
+  /**
+   * Split this Foldable into a NonEmptyList of Lists based on the effectufl predicate. Monadic version of `splitWhen`
+   * 
+   * {{{
+   * scala> import cats.syntax.all._, cats.Foldable, cats.Eval, cats.data.NonEmptyList
+   * scala> Foldable[List].splitWhenM(List(1,1))(x => Eval.now(x == 1)).value
+   * res0: NonEmptyList[List[Int]] = NonEmptyList(List(), List(), List())
+   * scala> Foldable[List].splitWhenM(List.empty[Int])(x => Eval.now(x == 1)).value
+   * res1: NonEmptyList[List[Int]] = NonEmptyList(List())
+   * scala> Foldable[List].splitWhenM(List(1, 2, 3, 1, 4, 5))(x => Eval.now(x == 1)).value
+   * val res2: NonEmptyList[List[Int]] = NonEmptyList(List(), List(2, 3), List(4, 5))
+   * }}}
+   */
+
+  def splitWhenM[G[_], A](fa: F[A])(f: A => G[Boolean])(implicit
+    FF: Foldable[F],
+    FA: Alternative[F],
+    GM: Monad[G]
+  ): G[NonEmptyList[F[A]]] = {
+    type Acc = NonEmptyList[F[A]]
+    type State = (Foldable.Source[A], Acc)
+
+    def step(state: State): G[Either[State, Acc]] =
+      (state._1.uncons, state._2) match {
+        case (Some((a, rest)), acc) =>
+          GM.map(f(a)) { shouldSplit =>
+            val nextAcc =
+              if (shouldSplit)
+                FA.empty[A] :: acc
+              else
+                NonEmptyList(FA.appendK(acc.head, a), acc.tail)
+
+            Left((rest.value, nextAcc))
+          }
+
+        case (_, acc) =>
+          GM.pure(Right(acc))
+      }
+
+    val state0: State = (Foldable.Source.fromFoldable(fa), NonEmptyList.one(FA.empty[A]))
+    GM.map(GM.tailRecM(state0)(step))(_.reverse)
+  }
 }
 
 object Foldable {

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

@@ -22,6 +22,8 @@
 package cats
 package syntax
 
+import cats.data.NonEmptyList
+
 trait FoldableSyntax extends Foldable.ToFoldableOps with UnorderedFoldable.ToUnorderedFoldableOps {
 
   implicit final def catsSyntaxNestedFoldable[F[_]: Foldable, G[_], A](fga: F[G[A]]): NestedFoldableOps[F, G, A] =
@@ -304,6 +306,16 @@ final class FoldableOps0[F[_], A](private val fa: F[A]) extends AnyVal {
   )(implicit A: Alternative[F], F: Foldable[F], M: Monad[G]): G[(F[B], F[C])] =
     F.partitionEitherM[G, A, B, C](fa)(f)(A, M)
 
+  def splitWhen(f: A => Boolean)(implicit FF: Foldable[F], FA: Alternative[F]): NonEmptyList[F[A]] = {
+    FF.splitWhen[A](fa)(f)(FA)
+  }
+
+  def splitWhenM[G[_]](
+    f: A => G[Boolean]
+  )(implicit FF: Foldable[F], FA: Alternative[F], G: Monad[G]): G[NonEmptyList[F[A]]] = {
+    FF.splitWhenM[G, A](fa)(f)(FF, FA, G)
+  }
+
   def sliding2(implicit F: Foldable[F]): List[(A, A)] =
     F.sliding2(fa)
   def sliding3(implicit F: Foldable[F]): List[(A, A, A)] =

tests/shared/src/test/scala/cats/tests/FoldableSuite.scala

@@ -333,6 +333,33 @@ abstract class FoldableSuite[F[_]: Foldable](name: String)(implicit
     }
   }
 
+  test(s"Foldable[$name].splitWhen") {
+    forAll { (fi: F[Int], x: Int) =>
+      val pred = (y: Int) => x == y
+      val li = fi.toList
+      val res = li.splitWhen(pred)
+      val expectedFiltered = li.filterNot(pred)
+      val expectedSize = li.size - expectedFiltered.size + 1
+      assert(res.size === expectedSize)
+      assert(res.reduce === expectedFiltered)
+      assert(Reducible[NonEmptyList].nonEmptyIntercalate(res, List(x)) == li)
+    }
+  }
+
+  test(s"Foldable[$name].splitWhenM") {
+    forAll { (fi: F[Int], x: Int) =>
+      val pred = (y: Int) => x == y
+      val predM = (y: Int) => Eval.now(pred(y))
+      val li = fi.toList
+      val res = li.splitWhenM(predM).value
+      val expectedFiltered = li.filterNot(pred)
+      val expectedSize = li.size - expectedFiltered.size + 1
+      assert(res.size === expectedSize)
+      assert(res.reduce === expectedFiltered)
+      assert(Reducible[NonEmptyList].nonEmptyIntercalate(res, List(x)) == li)
+    }
+  }
+
   test(s"Foldable[$name].sliding2 consistent with List#sliding(2)") {
     forAll { (fi: F[Int]) =>
       val n = 2