splitWhen & splitWhenM for Foldable

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

@@ -139,6 +139,53 @@ final class ListOps[A](private val la: List[A]) extends AnyVal {
    */
   def scanRightNel[B](b: B)(f: (A, B) => B): NonEmptyList[B] =
     NonEmptyList.fromListUnsafe(la.scanRight(b)(f))
+
+  /**
+   * Split this List 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._
+   * scala> List(1, 1).splitWhen(_ == 1)
+   * val res0: cats.data.NonEmptyList[List[Int]] = NonEmptyList(List(), List(), List())
+   * 
+   * scala> List.empty[Int].splitWhen(_ == 1)
+   * val res1: cats.data.NonEmptyList[List[Int]] = NonEmptyList(List())
+   * 
+   * scala> List(1, 2, 3, 1, 4, 5).splitWhen(_ == 1)
+   * val res2: cats.data.NonEmptyList[List[Int]] = NonEmptyList(List(), List(2, 3), List(4, 5))
+   * }}}
+   */
+
+  def splitWhen(f: A => Boolean): NonEmptyList[List[A]] = {
+    la.reverse.foldLeft(NonEmptyList.one(List.empty[A])) { case (lst, e) =>
+      if (f(e)) Nil :: lst else NonEmptyList(e :: lst.head, lst.tail)
+    }
+  }
+
+  /**
+   * Split this List into a NonEmptyList of Lists based on the effectufl predicate. Monadic version of `splitWhen`
+   * 
+   * {{{
+   * scala> import cats.syntax.all._, cats.Eval
+   * scala> List(1, 1).splitWhenM(x => Eval.now(x == 1)).value
+   * val res0: cats.data.NonEmptyList[List[Int]] = NonEmptyList(List(), List(), List())
+   * 
+   * scala> List.empty[Int].splitWhenM(x => Eval.now(x == 1)).value
+   * val res1: cats.data.NonEmptyList[List[Int]] = NonEmptyList(List())
+   * 
+   * scala> List(1, 2, 3, 1, 4, 5).splitWhenM(x => Eval.now(x == 1)).value
+   * val res2: cats.data.NonEmptyList[List[Int]] = NonEmptyList(List(), List(2, 3), List(4, 5))
+   * }}}
+   */
+
+  def splitWhenM[G[_]](f: A => G[Boolean])(implicit M: Monad[G]): G[NonEmptyList[List[A]]] = {
+    la.reverse.foldLeft(M.pure(NonEmptyList.one(List.empty[A]))) { case (acc, e) =>
+      M.flatMap(acc) { case lst =>
+        M.map(f(e))(if (_) Nil :: lst else NonEmptyList(e :: lst.head, lst.tail))
+      }
+    }
+  }
 }
 
 private[syntax] trait ListSyntaxBinCompat0 {

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

@@ -137,6 +137,29 @@ class ListSuite extends CatsSuite {
 
     assert(sumAll == lst.sum)
   }
+
+  test(s"splitWhen") {
+    forAll { (li: List[Int]) =>
+      val pred = (x: Int) => x > 0
+      val res = li.splitWhen(pred)
+      val expectedFiltered = li.filterNot(pred)
+      val expectedSize = li.size - expectedFiltered.size + 1
+      assert(res.size === expectedSize)
+      assert(res.toList.flatten === expectedFiltered)
+    }
+  }
+
+  test(s"splitWhenM") {
+    forAll { (li: List[Int]) =>
+      val pred = (x: Int) => x > 0
+      val predM = (x: Int) => Eval.now(pred(x))
+      val res = li.splitWhenM(predM)
+      val expectedFiltered = li.filterNot(pred)
+      val expectedSize = li.size - expectedFiltered.size + 1
+      assert(res.value.size === expectedSize)
+      assert(res.value.toList.flatten === expectedFiltered)
+    }
+  }
 }
 
 final class ListInstancesSuite extends munit.FunSuite {